FCES19: International Conference on Frontiers of Correlated Electron Sciences

May 29-31, 2019 Koshiba Hall, Hongo Campus, University of Tokyo

FCES19: International Conference on Frontiers of Correlated Electron Sciences

May 29-31, 2019 Koshiba Hall, Hongo Campus, University of Tokyo

FCES19
International Conference on Frontiers of Correlated Electron Sciences
Date:
May 29-31, 2019
Venue:
Koshiba Hall, Hongo Campus, University of Tokyo
Scope:
This conference (FCES19) will highlight recent advances in sciences of strongly correlated electron systems
and complex quantum matter. The aim of the conference is to provide a platform for members of the community
in correlated electron and quantum complex matter researches including approaches with theoretical, andcomputational tools for new horizons of experimental developments. We encourage report on latest results,
exchange information and ideas, and foster collaborations. The conference will cover the following topics:• High-Tc superconductors including cuprate, iron-based and organic superconductors.
• Topological materials• Efficient and functional materials such as strong permanent magnets, solar cells and thermoelectric materials• Non-periodic systems such as thin films and interfaces as well as quasicrystals• Nonequilibrium phenomena• Computational methods including first-principles and correlated electron tools• Metal-insulator transitions• Competing orders• Quantum criticality• Quantum spin liquids and topological order in frustrated systems

Organizers:
Yukitoshi Motome (co-chair), Ryotaro Arita (co-chair), Takashi Miyake, Synge Todo, Shiro Sakai, andMasatoshi Imada
Sponsored by:• “Materials Design and Exploration of Functions for Strongly Correlated Materials - topological quantum

criticality, quantum multi-Challenges to Non-Equilibrium and Non-Periodic Systems –”, Grant-in-Aid for
Scientific Research S from Japan Society for the Promotion of Science.
• CDMSI: “Creation of new functional devices and high-performance materials to support next-generationindustries”, social and scientific priority issue to be tackled by using post-K computer from MEXT, Japan.

1Wireless LAN:
Eduroam and UTokyo-Guest can be used in the conference rooms. Detailed instruction for making registration
for UTokyo-Guest is available at the reception.
Tips for Oral Presentation:
Please be advised that each presentation is followed by 5min for discussion. Please leave enough time fordiscussion.
Tips for Poster Presentation:
Presentation time is from 17:00 to 18:40 on May 30. The poster session will be held in parallel in three sections:section A (P01-P27, lobby of Koshiba Hall), section B (P28-P56, room 231), section C (P57-P85, room 271).
Display time of the posters is from 14:00 on May 29 to 10:30 on May 31. Please put your poster on the poster
board between 12:30-14:00 on the first day, and remove it before 11:00 on the third day at the latest.

P67

P68

P69

P66

Corridor

Door

P65

P81 P80

P70

P64

P82 P79

P71
Corridor

P63 P83 P78

P72

P62 P84 P77 P73

Lecture room

P61 P85 P76 P75

Section C

P60

P59

P58

Door

P57

【Room.271】

Area E
Connecting
corridor【Room.231】
P36
P34

P35
P55 P56

P33

P54 P53P51 P52P50 P49

P32

P47 P48

P31

P46 P45
P43 P44

P37

P29

P06

P08

P05

Door

P04

P03

Door

P02

P38
P39

Corridor

P40

P25 P24 P21 P20 P17 P16 P13 P12 P09

P01

P26 P23 P22 P19 P18 P15 P14 P11 P10

P42 P41

Section B

P30

P07

P27

Door

Section A

P28

Door

Reception

Corridor

Door

Warehouse
Entrance
Connecting
corridor
Area W

2

KOSHIBA HALL

Oral Presentations - Wednesday, May 29
9:00 - 9:10

Opening

Chair : Masatoshi Imada
9:10 - 9:40

Gabriel Kotliar (Brookhaven National Labo. / Physics Dept., Rutgers Univ.)
Theory of Normal State and Superconductivity in Iron Pnictides and Chalcogenides.

9:45 - 10:15

Ivan Božović (Brookhaven National Labo. / Applied Physics Dept., Yale Univ.)
What Makes Cuprate Superconductors so Exceptional?

10:20 - 10:35

Takahiro Misawa (Institute for Solid State Physics, Univ. of Tokyo)
Development of many-variable variational Monte Carlo method (mVMC) and its applications
to high-Tc cuprates

10:40 - 11:00

Coffee Break

Chair : Fakher Assaad
11:00 - 11:20

Atsushi Fujimori (Dept. of Physics, Univ. of Tokyo)

11:25 - 11:45

Origin of the pseudogap in electron-doped and hole-doped cuprates revealed by ARPES
Marcello Civelli (Laboratoire de Physique des Solides, Univ. Paris-Sud)
A Cluster Dynamical Mean Field Theory Perspective on the pseudogap of cuprates: correlationand competition with the high Tc superconductivity

11:50 - 12:05

Shiro Sakai (Center for Emergent Matter Science, RIKEN)
Direct relation between electronic structures of Mott insulator and of high-temperature
superconductor

12:10 - 12:25

Youhei Yamaji (Dept. of Applied Physics, Univ. of Tokyo)
Hidden self-energy structures of high-temperature superconductivity

12:30 - 12:50

Group photo

12:50 - 13:55

Lunch

Chair : Roser Valenti
13:55 - 14:25

Ferdi Aryasetiawan (Mathematical Physics, Lund University)
Multi-Tier Self-Consistent GW+EDMFT Scheme

14:30 - 14:45

Kazuma Nakamura (Dept. of Basic Sciences, Kyushu Institute of Technology)
RESPACK: Ab initio software for many-body perturbation calculation and effective-model
derivation

14:50 - 15:05

Claude Ederer (Materials Theory, ETH Zurich)
Metal-insulator transitions in complex oxide thin films and heterostructures from DFT+DMFT

15:10 - 15:30

Shinji Tsuneyuki (Dept. of Physics / Institute for Solid State Physics, Univ. of Tokyo)
Transcorrelated method: the idea and its applications

15:35 - 15:55

Coffee Break3

Chair : Silke Biermann
15:55 - 16:25

Karsten Held (Institute for Solid State Physics, TU Wien)

16:30 - 16:50

p-tons - generic optical excitations of correlated systems
Yoshiteru Maeno (Dept. of Physics, Kyoto Univ.)
Non-Equilibrium Steady States (NESS) of a Mott-Anderson Insulator Candidate

16:55 - 17:15

Frank Lechermann (Institut für Theoretische Physik, Univ. Hamburg)
Doping effects in the strongly correlated vanadium oxides V2O3 and VO2

17:20 - 17:35

Ryotaro Arita (Dept. of Applied Physics, Univ. of Tokyo / RIKEN Center for Emergent Matter
Science)
Cluster multipole dynamics in non-collinear antiferromagnets

4

Oral Presentations – Thursday, May 30
Chair : Karsten Held
9:00 - 9:30

Antoine Georges (Center for Computational Quantum Physics, Collège de France / Flatiron
Institute)

9:35 - 9:55

Strong Correlations in Multi-Orbital Materials: Beyond Mottness.
Shik Shin (Institute for Solid State Physics, Univ. of Tokyo)
Topological superconductors and Multiple topological states in iron-based superconductors

10:00 - 10:20

Yuji Matsuda (Dept. of Physics, Kyoto Univ.)
Quantum oscillations and charge-neutral fermions in an insulator

10:25 - 10:40

Sergey Streltsov (Institute of Metal Physics, Russian Academy of Science)Spin-orbit-entangled jeff =1/2 state in 3d transition metal oxide: CuAl2O4

10:45 - 11:10

Coffee Break

Chair : Naoki Kawashima
11:10 - 11:40

Silke Biermann (Ecole Polytechnique)11:45 - 12:00

Spectral properties of Sr2IrO4Takashi Miyake (CD-FMat, AIST)
Magnetocrystalline anisotropy of rare-earth magnets

12:05 - 12:20

Arun Paramekanti (Univ. of Toronto)
Skyrmions: Classical spin crystals and chiral quantum liquids

12:25 - 12:40

Toru Sakai (Graduate School of Material Science, Univ. of Hyogo / QST SPring-8)
Spin Nematic Liquid of Low-Dimensional Quantum Antiferromagnets

12:45 - 14:05

Lunch

Chair : Gabriel Kotliar
14:05 - 14:35

Philipp Werner (Dept. of Physics, Univ. of Fribourg)

14:40 - 15:00

GW+DMFT simulation of lattice models in and out of equilibrium
Stefan Kaiser (Max-Planck Institute for Solid State Research / 4th Physics Institute, Univ. of
Stuttgart)
Higgs Spectroscopy in Unconventional Superconductors

15:05 - 15:25

Takami Tohyama (Dept. of Applied Physics, Tokyo Univ. of Science)
Characterization of photoexcited states in the half-filled one-dimensional extended Hubbard
model by machine learning

15:30 - 15:50

Coffee Break5

Chair : Philipp Werner
15:50 - 16:10

Ryo Shimano (Cryogenic Research Center and Dept. of Physics, Univ. of Tokyo)
Dynamical control of superconducting order parameter by light

16:15 - 16:30

Kenji Yonemitsu (Dept. of Physics, Chuo Univ.)
Photoinduced Charge Oscillations in Correlated Dimer Systems

16:35 - 16:55

Norio Kawakami (Dept. of Physics, Kyoto Univ.)
Non-Hermitian quantum phenomena in correlated systems

17:00 - 18:40

Poster session

6Oral Presentations – Friday, May 31
Chair : Takami Tohyama
9:00 - 9:30

Roser Valenti (Institut für Theoretische Physik, Goethe-Univ. Frankfurt)
Field- and Pressure-induced phases in generalized Kitaev models and materials

9:35 - 9:50

Yukitoshi Motome (Dept. of Applied Physics, Univ. of Tokyo)
Majorana signatures in proximate Kitaev spin liquids

9:55 - 10:10

Peter Prelovsek (J. Stefan Institute, Ljubljana, Slovenia / Faculty of Mathematics and Physics,
Univ. of Ljubljana)
Spin liquid in extended Heisenberg models on triangular lattice

10:15 - 10:30

Synge Todo (Dept. of Physics / Institute for Solid State Physics, Univ. of Tokyo)
Optical bistability in a quantum low photon-density regime

10:35 - 11:00

Coffee Break

Chair : Norio Kawakami
11:00 - 11:30

Fakher Assaad (Institut für Theoretische Physik und Astrophysik, Univ. Würzburg)

11:35 - 11:55

Intertwined orders in Dirac Fermions
Giorgio Sangiovanni (Institut für Theoretische Physik und Astrophysik, Univ. of Würzburg)
First-order topological phase transitions with correlated electrons

12:00 - 12:20

Yoshihiro Iwasa (Dept. of Physics and QPEC, Univ. of Tokyo / RIKEN Center for Emergent
Matter Science)Superconductivity in 2D Materials

12:25 - 12:40

Tomi Ohtsuki (Physics Division, Sophia Univ.)
Phase Diagrams and Scaling Behaviors of Disordered Weyl Semimetals

12:45 - 13:55

Lunch

Chair : Antoine Georges
13:55 - 14:25

Matthias Troyer (Microsoft Quantum)
The Quantum Computing Future of Strongly Correlated Electron Simulations

14:30 - 14:50

Naoki Kawashima (Institute for Solid State Physics, Univ. of Tokyo)
Tensor Network States for Lattice Systems

14:55 - 15:10

Yusuke Nomura (Dept. of Applied Physics, Univ. of Tokyo)
Machine learning for solving quantum many-body Hamiltonians

15:15 - 15:45

Masatoshi Imada (Waseda Research Institute for Science and Engineering, Waseda Univ. /Toyota Physical and Chemical Research Institute)
Perspectives on descriptions of correlated electron phenomena

15:50 - 16:00

Closing7

Poster Presentations – Section A (lobby of Koshiba Hall)
Presentation time: from 17:00 to 18:40 on May 30
Display time: from 14:00 on May 29 to 10:30 on May 31
P01

Yuji Aoki

P02

Nonmetallicity in self-doped BiS2-based Eu3Bi2S4F4: single crystal study
Hiromu Araki
Higher order topological phases of a disordered breathing Kagome model: A machine learning study

P03

Padmanabhan BalasubramanianInvestigations on double perovskites Pr2MnNiO6 and Nd2-xSrxMnNiO6: Spectroscopy and ab-initio
studies

P04

Maxime Charlebois
Density of states in variational Monte Carlo

P05

Naoya Chikano
Analysis of basic properties of compact representation of many-body Green’s function

P06

Hiroyuki Deguchi
Chiral-Glass Phase in a Ceramic YBa2Cu3O7 Superconductor at Low Magnetic Field

P07

Jean Baptiste Fankam Fankam
Electronic structure, chemical reactivity and optoelectronic properties of 4',5'-dibromo-2',7'-dinitro-3oxo-3h-spiro[2-benzofuran-1,9'- xanthene]-3',6'-diolate from first principles calculations

P08

Makoto Hagiwara
Singular I-V characteristics in the intermediate state of intergrain ordering of a ceramic superconductor
system found by a new measuring technique

P09

Imre Hagymasi
Dynamical topological quantum phase transitions in nonintegrable models

P10

Hiroki Hanate
Study on Harmonic Response and Phonon Dispersion with Inelastic X-Ray Scattering in theGeometrically Frustrated Iridate Ca5Ir3O12

P11

Kenji Harada

P12

Tensor network approach to non-equilibrium systems
Takumi Hasegawa
Raman scattering investigation on geometrically frustrated iridate Ca5Ir3O12

P13

Michihiro Hirata
Pressure-tuned velocity renormalization and excitonic fluctuations of 2D massless Dirac fermions in an
organic conductor

P14

Motoaki Hirayama
Effective Hamiltonian for cuprate superconductors derived from multi-scale ab initio scheme with level
renormalization

P15

Max Hirschberger
Skyrmion formation and enhanced emergent electrodynamics in centrosymmetric magnets

9

P16

Li Huang
Combining density functional theory with dynamical mean-field theory: Applications to the correlated
actinide materials

P17

Kota Ido
Laser-controlled superconductivity in correlated electron systems

P18

Shoma Iimura
Mechanism of unconventional full-gap superconductivity in Kondo lattice with semi-metallic
conduction bands

P19

Akihiko Ikeda
Search for excitonic condensation in LaCoO3 induced at ultrahigh magnetic fields

P20

Fumihiro Imoto
Development of Orbital-Free Density Functional Theory with Machine Learning

P21

Hiro Ishizuka
Optical rectification of spin current in magnetic insulators

P22

Ryuta Iwazaki
Nature of superconducting fluctuation in photo-excited systems

P23

Harald O. Jeschke

P24

Chromium breathing pyrochlores as a showcase for a variety of pyrochlore Hamiltonians
Ryosuke Kadono
Metallic Spin Liquid-like Behavior of LiV2O4

P25

Ryui Kaneko
Tensor network approach to the Kitaev spin liquid

P26

Naoyuki Katayama
Liquid dynamics of orbital molecules with long correlation length in layered LiVS2

P27

Takeo Kato
Quantum Critical Phenomena in Heat Transport via a Qubit

10

Poster Presentations – Section B (room 231)
Presentation time: from 17:00 to 18:40 on May 30
Display time: from 14:00 on May 29 to 10:30 on May 31
P28

Harukazu Kato
Microscopic Investigation of a phase transition between an insulator CaCu3Ti4O12 and a metal
CaCu3Ru4O12: Cu-NQR in CaCu3Ti4-xRuxO12

P29

Kohei Kawabata
Topological classification of non-Hermitian insulators and superconductors

P30

Aaram J. Kim
Spin and charge correlations across the extended pseudogap regime in the half-filled 2d Hubbard model

P31

Akito Kobayashi

P32

Electronic States in Organic Conductor a-(BETS)2I3
Akihiro Koda
Coupled Spin-Charge Fluctuation in the Semimetal Phase of All-In/All-Out Antiferromagnet Cd2Os2O7

P33

Koji Kudo
Correlated higher-order topological insulator on kagome lattice

P34

Daichi Manabe
Supercurrent generation by spin-twisting itinerant motion of electrons in a model for cuprate
superconductors

P35

Muhamad Nasruddin Manaf
Electron Correlation in the Anomalous Muonium in Silicon

P36

Yasuhiro H. Matsuda
Magnetic-field-induced insulator-metal transition in W-doped VO2

P37

Munehisa Matsumoto
Doping a rare-earth permanent magnet: ab initio studies on the impacts in multiple energy scales

P38

Tetsuya Minakawa
Low-energy effective models of spin-S anisotropic Kitaev models

P39

Masahito Mochizuki
Theoretical Study on the Magnetism-Driven Negative Thermal Expansion Phenomenon in the InversePerovskite Antiferromagnets

P40

Katsuhiro Morita
Ground-state phase diagram of the Kitaev-Heisenberg model on a kagome lattice

P41

Yuichi Motoyama
DSQSS – PIMC solver for quantum lattice model

P42

Hideaki Murase
Real-space observation of electronic crystallization

P43

Makoto Naka
Organic antiferromagnet as a spin current generator

P44

Joji Nasu
Non-equilibrium dynamics of fractional excitations in Kitaev spin liquids

11

P45

Takahiro Ohgoe
Ab initio studies on superconductivity and inhomogeneity in Hg-based cuprate superconductor

P46

Takahiro Ohgoe
Resummation of diagrammatic series with zero convergence radius for the unitary Fermi gas

P47

Daigo Ohki
Effect of Coulomb Interaction in Seebeck Coefficient of Organic Dirac Electron System

P48

a-(BEDT-TTF)2I3
Rikuto Oiwa
Time-reversal symmetry breaking superconductivity in hole-doped monolayer MoS2

P49

Shun Okumura
Spin-charge coupled phenomena in a chiral soliton lattice

P50

Shun Okumura
Numerical study of magnetic hedgehog crystals in itinerant chiral magnets

P51

Seishiro Ono
Symmetry Indicators for Topological Superconductors

P52

Shota Ono
Anomalous energy transfer between electrons and phonons in photoexcited metals

P53

Junya Otsuki
Strong-coupling formula of momentum-dependent susceptibilities in dynamical mean-field theory

P54

Robert Peters
Quantum oscillations in topological Kondo insulator

P55

Rico Pohle
Quantum spin liquids in the pyrochlore S=1/2 Heisenberg model with Dzyaloshinskii-Moriya
interactions

P56

Sarita Rajput
The role of exchange interactions on spin reorientation transition of Nd0.5Dy0.5FeO3: A first principle
study

12

Poster Presentations – Section C (room 271)
Presentation time: from 17:00 to 18:40 on May 30
Display time: from 14:00 on May 29 to 10:30 on May 31
P57

Panch Ram

P58

Inversion and Magnetic Quantum Oscillations in Kondo Insulators
Roman Rausch
Dynamics of the magnetic polaron

P59

Shiro Sakai
Superconductivity in quasiperiodic systems

P60

Tatsuki Sato
Magneto-optic study of thermally driven ferrimagnet-helimagnet transition in a chiral-polar magnet

P61

Thomas Schäfer
Quantum Criticality in the Two-Dimensional Periodic Anderson Model

P62

Makoto Shimizu
Two-dome superconductivity in FeS induced by a Lifshitz transition

P63

Hiroshi Shinaoka

P64

Dynamical susceptibility in DMFT: a sparse QMC sampling approach
Kazuya Shinjo
Machine learning study of photoexcited states in the half-filled one-dimensional extended Hubbard
model

P65

Hiroaki Shishido
Possible Enhancement of Coercivity in Rare-Earth Permanent Magnets on the Verge of a Valence
Transition

P66

Seiichiro Suga
Quantized Excitation Spectra by Confinement in (Quasi-)One-Dimensional S=1 Quantum Spin Systems

P67

Takuya Susa
Topological Transition in Hard-Core Bosonic Haldane Model

P68

Hidemaro Suwa

P69

Large-scale dynamical simulation of Hubbard model
Takafumi Suzuki

P70

Ground-state phase diagram of the extended Kitaev-G model on a honeycomb lattice
Kohei Suzuki
Superconducting fluctuations in S=1 one-dimensional Kondo lattice model under transverse magnetic
fields

P71

Terumasa Tadano
Ab initio derivation of effective Hamiltonian for La2CuO4/La1.55Sr0.45CuO4 heterostructures

P72

Takehiro Tani
Field-Angle Dependence of Interlayer Magnetoresistance in Organic Dirac Electron System

P73

Masaki Tezuka
Characterization of quantum chaos by two-point correlators

13

P74

Canceled

P75

Naoto Tsuji
Spin freezing crossover and SYK strange metal

P76

Masafumi Udagawa

P77

Dynamics of fractional excitations in quantum spin ice
Kentaro Ueda
Topological phase transitions induced by pressure, band-filling, and magnetic field in stronglycorrelated pyrochlore iridates

P78

Mizuki Urai
Disorder-enhanced quantum fluctuations in a quasi-two-dimensional organic Mott system

P79

Alena Vishina
High-throughput search for rare-earth-free permanent magnets

P80

Kodai Wakamatsu

P81

Thermoelectric effects in a doped spin liquid candidate k-(ET)4Hg2.89Br8
Hiroshi Watanabe

P82

Two Competing Superconducting Phases in Molecular Conductors k-(BEDT-TTF)2X
Masahiko G. Yamada
Theory of Proton-Driven Quantum Spin-Dipole Liquids

P83

Kazuki Yamamoto
Phase transitions in non-Hermitian fermionic superfluidity under complex-valued interactions

P84

Tsuneya Yoshida
Non-Hermitian perspective of strongly correlated systems

P85

Nobuyuki Yoshioka
Constructing Neural Stationary States for open quantum many-body systems

14

Oral Presentations – Wednesday, May 29

29-1

Theory of Normal State and Supercondctivity in Iron Pnictides andChalcogenides.
Gabriel Kotliar
Brookhaven National Laboratory, Upton NY 11973, USAandPhysics Department Rutgers University, Piscataway NJ 08540 USA

Hund metals are systems which derive their correlations from the Hund rule coupling rather
than the Hubbard U. Primary examples are the iron pnictides and chalcogenides and theruthenates.
In this talk I will describe recent progress in understanding this class of materials using
Dynamical Mean Field Theory and advanced quantum impurity solvers. We will stress inwhich way Hund correlations are different from those of materials near a Mott insulatingstate[1]. We will conclude on how the normal state characteristics of Hund metals impact
their superconducting states at lower temperatures[2].
[1] Deng et. al. arXiv:1708.05752[2] Lee et. al. Phys. Rev. Lett. 121, 187003 (2018)

17

29-2

What Makes Cuprate Superconductors so Exceptional?
Ivan Božović
Brookhaven National Laboratory, Upton NY 11973, USA
Applied Physics Department, Yale University, New Haven, CT 06520, USA
An account will be given of a very comprehensive experiment in which over 2,000 singlecrystal LSCO films were grown by molecular beam epitaxy and studied over the course of 12
years. The key parameters of the normal and superconducting states — r, RH,
magnetoresistance, Tc, l, x — have been measured precisely as a function of temperature T(down to 300 mK), magnetic field B (up to 90 T), doping, and in-plane azimuth angle f. [1]The key findings are as follows. (i) The superconducting phase stiffness is extremely low,comparable to Tc. (ii) The superfluid density Ns(T) decreases linearly with T, up to Tc. (iii) Tc
scales with Ns0 linearly but with an offset, except very close to the dome edges where it scalesas √Ns0. (iv) The superconducting state develops from an electronic nematic state that breaks
the C4 symmetry of the underlying crystal lattice. (v) The electron fluid behaves as if it werecomprised of two components, one Fermi-liquid (FL) like and the other showing r linear in T
and B; the later component diminishes and disappears with increased doping, together with thenematicity, Ns0, and Tc.
The related results of other groups show that the above appears to be typical of HTS cuprates
and independent on the details of the Fermi surface, the number of CuO2 planes in the unit cell,
the presence or absence of CuO chains, the density and the nature of dopants, thesuperconducting gap size, and even whether the cuprate is underdoped or overdoped.
We conclude that HTS in cuprates involves some new physics, beyond the standard FL-BCS
description. It seems to entail strong pairing, strong electron correlations, strong thermal phasefluctuations, and probably strong pair-breaking, intrinsic but T- and doping-dependent.
Impurities and disorder are probably not essential, at least in the first cut.
These are just some broad conceptual constraints; much tighter ones are imposed by theparticular surfaces in the (T,x,B,f) space depicted by r, RH, Ns, etc., which encode a wealth ofincisive information about the physics of HTS, and may provide a new impetus to the theory.
[1] Nature 547, 432 (2017); 536, 309 (2016); 472, 458 (2011); 455, 782 (2008); 422, 873 (2003). Science
361, 479 (2018); 326, 699 (2009); 316, 425 (2007); 297, 581 (2002). Nature Materials 12, 877 (2013);
12, 387 (2013); 12, 1019 (2013); 12, 47 (2013); 11, 850 (2012). Nature Physics 10, 256 (2014); 7, 298(2011). Nature Nanotechnology 9, 443 (2014); 5, 516 (2010). Nature Communications 2, 272 (2011).
Phys. Rev. Letters 106, 237003 (2011); 102, 107004 (2009); 101, 247004 (2008); 93, 157002 (2004);
89, 107001 (2002). Proc. Nat. Acad. Sci. 113, 4284 (2016); 107, 8103 (2010).

18

29-3

Development of many-variable variational Monte Carlo method(mVMC) and its applications to high-Tc cuprates
T. Misawa1, K. Yoshimi1, M. Kawamura1, S. Morita1, Y. Motoyama1,
T. Ohgoe2, K. Ido1, M. Imada2, and T. Kato1
1

Institute for Solid State Physics, The University of Tokyo, Japan
Department of Applied Physics, The University of Tokyo, Japan

2

High-accuracy analyses of theoretical models for quantum many-body systems are
expected to play important roles for clarifying the nature of novel quantum phases suchas high-Tc superconductivities and quantum spin liquids. Furthermore, recent theoreticalprogress enables us to obtain low-energy effective Hamiltonians for real materials in
non-empirical ways [1]. To clarify the electronic structures of real materials and designmaterials that have desired physical properties, it is a crucial step to perform
high-accuracy analyses on the low-energy effective Hamiltonians. Variational Monte
Carlo method [2] is one of promising ways to perform the high-accuracy calculations
for larger system sizes. Recent development of the theoretical method [3] andhigh-performance computers relaxes the limitations and biases imposed by the assumed
form of the variational wave functions and enable us to perform highly-accurate
calculations for the correlated electron systems [4,5].
We have developed mVMC (many-variable Variational Monte Carlo) [6,7], which issoftware for the highly-accurate variational Monte Carlo calculations with the simple
and flexible user interfaces. For widely studied models of the correlated electron
systems such as the Hubbard model, the Heisenberg model, and the Kondo-latticemodel, users can perform the calculation by preparing only one input file whose lengthis shorter than ten lines. By preparing the additional input files, users can also treat
general Hamiltonians such as the ab initio low-energy effective Hamiltonians. In thepresentation, we will explain the basic usage of mVMC and its recent applications to theab initio low-energy effective three-band (dp) Hamiltonians for the cuprates [8].
References.
[1] For a review, see M. Imada and T. Miyake, J. Phys. Soc. Jpn. 79, 112001 (2010).
[2] For a review, see C. Gros, Ann. Phys. 189, 53 (1989).
[3] S. Sorella, Phys. Rev. B 64, 024512 (2001).
[4] D. Tahara and M. Imada, J. Phys. Soc. Jpn. 77, 114701 (2008)[5] T. Misawa and M. Imada, Phys. Rev. B 90, 115137 (2014).
[6] T. Misawa et al., Comp. Phys. Commun. 236, 447 (2019).
[7] https://github.com/issp-center-dev/mVMC[8] M. Hirayama, T. Misawa et al., arXiv:1901.00763.

19

29-4

Origin of the pseudogap in electron-doped and hole-doped cuprates
revealed by ARPES
Atsushi Fujimori
Department of Physics, University of Tokyo
The origin of the pseudogap in underdoped cuprates continues to be a highly debatedissue until now. In this work, the s-symmetry pseudogap scenario proposed for thepseudogap of hole-doped cuprates [1] was tested for the “antiferromagnetic pseudogap”
of electron-doped cuprates, and was found to well explain the pseudogaps both in thehole- and electron-doped compounds [2]. Our ARPES study [4] also indicated asignature of the rotational-symmetry breaking (i.e., nematicity) in the pseudogap phase[3], but suggests that the nematicity is a result of the pseudogap opening and not theorigin of the pseudogap.
Collaboration with M. Horio, S. Nakata, T. Adachi, Y. Koike, H. Eisaki, D. Song, Y.
Yoshida, S. Ideta, K. Tanaka, M. Hashimoto, D. Lu, Z.-X. Shen, S. Sakai, and M. Imada
is gratefully acknowledged.
References:[1] S. Sakai et al., Phys. Rev. Lett. 111, 107001 (2013).
[2] M. Horio, S. Sakai, M. Imada, AF et al., arXiv:1801.04247.
[3] Y. Sato et al., Nat. Phys. 13, 1074 (2017).
[4] S. Nakata, AF et al., arXiv:1811.10028.

20

29-5

A Cluster Dynamical Mean Field Theory Perspective on the pseudogapof cuprates: correlation and competition with the high Tc
superconductivity
Marcello Civelli
Laboratoire de Physique des Solides, Université Paris-Sud et Paris-Saclay
A most striking feature of high-Tc cuprate superconductors is the appearance above thesuperconducting transition temperature of an unconventional metallic phase, known as
the pseudogap. The relation between the pseudogap and superconductivity has been
often at the front stage of the high-Tc debate. By exploiting cluster dynamical mean
field theory on the two dimensional Hubbard Model, we establish key facts on the role
played by the pseudogap on the cuprate phase diagram. We show that the pseudogap
competes with a more conventional metallic phase over a wide range of the doping,interaction and frustration. This competition leads to a pseudogap-to-conventional metaltransition in the high doping and strongly correlated region of the phase diagram, whichis relevant for the cuprate physics. This is in particular revealed by a correlation-driven
Lifshitz transition of the Fermi surface that explains the puzzling link between
pseudogap and the Fermi surface topology pointed out in recent experiments. We shallfocus in particular on the Raman response. In the superconducting phase, we show thatthe pseudogap and the superconductivity compete for the same electrons, producing an
unconventional form of the superconducting pairing that involves electrons in highenergy states. Also these findings produce a key hallmark, the so called “peak and dip”
feature, in the Raman response. These results reveal an unprecedented relationshipbetween the pseudogap and superconductivity, which eventually can be responsible forboosting up the Tc.

21

29-6

Direct relation between electronic structures of Mott insulator and ofhigh-temperature superconductor
Shiro Sakai
Center for Emergent Matter Science, RIKEN, Wako, Saitama 351-0198, JapanThe high-temperature superconductivity in copper oxides emerges when carriers are
doped into the parent Mott insulator. This well-established fact has, however, eluded a
microscopic explanation. Here we show that the missing link is a pole structure ofelectronic self-energy in the energy-momentum space. The continuous evolution of theself-energy pole with doping directly connects the Mott insulator and high-temperature
superconductivity. We show this by numerically studying an extremely small doping
region close to the Mott insulating phase in a standard model for cuprates, thetwo-dimensional Hubbard model, by means of a cluster extension of the dynamical
mean-field theory [1,2].
We first identify two relevant self-energy structures in the Mott insulator; the pole
generating the Mott gap and a relatively broad peak generating the so-called waterfall
structure, which is another consequence of strong correlations present in the Mott
insulator and has been discussed in the literature in connection with a spectral structure
observed in angle-resolved photoemission spectroscopy for cuprates. We then revealthat either the Mott-gap pole or the waterfall structure (the feature at the energy closer
to the Fermi level) directly transforms into another self-energy pole at the same energy
and momentum when the system is doped with carriers. The anomalous self-energy
yielding the superconductivity is simultaneously born exactly at this energy-momentum
point. Thus created self-energy pole, interpreted as arising from a hidden fermionicexcitation [3], continuously evolves upon further doping and considerably enhances thesuperconductivity. Above the critical temperature of superconductivity, the sameself-energy pole generates a pseudogap in the normal state [3]. We thus elucidate a
unified Mott-physics mechanism, where the self-energy structure inherent to the Mott
insulator directly gives birth to both the high critical superconducting temperature andthe pseudogap [4].
[1] G. Kotliar, S. Y. Savrasov, G. Pálsson, and G. Biroli, PRL 87, 186401 (2001).
[2] T. Maier, M. Jarrell, T. Pruschke, and M. H. Hettler, RMP 77, 1027 (2005).
[3] S. Sakai, M. Civelli, and M. Imada, PRL 116, 057003; PRB 94, 115130 (2016).
[4] S. Sakai, M. Civelli, and M. Imada, PRB 98, 195109 (2018).

22

29-7

Hidden self-energy structures of high-temperature superconductivity
Y. Yamajia, T. Yoshidab, A. Fujimoric, and M. ImadaaabDepartment of Applied Physics, the University of Tokyo
Graduate School of Human and Environmental Studies, Kyoto Universityc
Department of Physics, the University of Tokyo

Origin of high temperature superconductors has long been a major challenge in physics.
How electrons are mutually interacting is the key to identify the origin of
superconductivity. We are, however, able to observe motion of electrons only after
projection onto experimentally accessible degrees of freedom.
In this study, the Boltzmann machine learning [1, 2] is examined to extract physicalquantities hidden in experimental data, combined with rigorous and physically soundprior knowledge. The method is applied to a small set of the angle-resolved
photoemission spectroscopy (ARPES) spectra of copper oxide superconductors [3] toextract the normal and anomalous contributions in the self-energy separately, in which
scattering processes among electrons are encoded. We reveal that prominent peakstructures emerge both in the normal and anomalous self-energies, which are canceled in
the total self-energy and hence not directly visible in experiments including ARPES. The
revealed peaks make dominant contribution to the superconducting gap and hence
provide the decisive testimony for the origin of the superconductivity [4]. Furthermore,
the present achievement opens avenues for innovative machine-learning spectroscopy
method in general.
References:[1] D. H. Ackley, G. E. Hinton and T. J. Sejnowski, Cognitive Science 9, 147 (1985).
[2] P. Smolensky, Tech. Rep. (COLORADO UNIV AT BOULDER DEPT OF COMPUTERSCIENCE, 1986).
[3] T. Kondo, et al., Nature 457, 296 (2009).
[4] Y. Yamaji, T. Yoshida, A. Fujimori and M. Imada, unpublished.

23

29-8

Multi-Tier Self-Consistent GW+EDMFT Scheme
F. Nilsson1, L. Boehnke2, P. Werner2, and F. Aryasetiawan1
12Mathematical Physics, Lund University, Sweden
Department of Physics, University of Fribourg, Switzerland

A parameter-free scheme for computing the electronic structure of real materials withmoderate to strong electron correlations has been developed by merging the GW andextended DMFT methods [1,2]. The scheme sorts electrons according to their degree ofcorrelations into tiers, each treated with an appropriate level of approximation. Theinterplay between short- and long-range correlations arising from the strong onsite andlong-range Coulomb interaction is accounted for self-consistently.
The scheme is applied to study the spectral functions of the much investigated prototype
of correlated metals, SrVO3, as well as a closely related compound SrMoO3. The
long-range Coulomb interaction through self-consistency has the effects of enhancing
screening of the impurity U and reduces its value significantly. The self-consistent
impurity U becomes significantly smaller than the separation between the two satellitefeatures conventionally interpreted as Hubbard bands. Analysis of the results suggeststhat it is more appropriate to interpret the satellites as collective charge excitations(plasmons) arising from the long-range Coulomb interaction. Indeed, in the case of
SrMoO3 LDA+DMFT cannot account for the satellites with a reasonable value of U. We
have also studied sodium as a function of lattice constant to mimic correlations strength,
in order to evaluate the range of applicability of the scheme.

[1] L. Boehnke, F. Nilsson, F. Aryasetiawan, and P. Werner, Phys. Rev. B 93, 161104(R) (2016).
[2] F. Nilsson, L. Boehnke, P. Werner, and F. Aryasetiawan, Phys. Rev. Materials 1, 043803(2017).

24

29-9

RESPACK: Ab initio software for many-body perturbation
calculation and effective-model derivation
Kazuma Nakamura
Department of Basic Sciences, Kyushu Institute of Technology,
Kitakyushu 804-8550, Japan.
I present an ab initio program package RESPACK [1] for many-body perturbation
calculation and effective low-energy model derivation. RESPACK is possible tocalculate the maximally localized Wannier function, response function with random
phase approximation and related optical properties, frequency-dependent electronicinteraction parameters, and the GW spectral function, etc. It supports ab initio bandcalculation codes using norm conserving pseudopotentials plus plane wave basis set, andofficially supports xTAPP [2] and Quantum ESPRESSO [3] packages. An automatic
generation script from these band-calculation output to input files for RESPACK is
prepared. An input file for specifying calculation conditions is designed pursuing
simplicity and is given in a namelist format. RESPACK has a wide application including
simple metals, semiconductors, 3d/4d transition-metal compounds, organic and aromaticcompounds, etc. It supports OpenMP/MPI and intel/GNU compiler environments.
Interfaces to model-analysis solvers mVMC [4] and HF [5] are also provided; users can
automatically obtain these inputs via RESPACK. For code developments, I acknowledge
Yoshihide Yoshimoto, Yoshiro Nohara, Yusuke Nomura, Terumasa Tadano, Mitsuaki
Kawamura, and Maxime Charlebois. For making interface with mVMC and HF, I
thank to Takahiro Misawa, Kazuyoshi Yoshimi, and Yuichi Motoyama. I also thank to
Masatoshi Imada, Ryotaro Arita, Takashi Miyake for application of RESPACK tomaterial science.
[1] https://sites.google.com/view/kazuma7k6r[2] http://xtapp.cp.is.s.u-tokyo.ac.jp/[3] http://www.quantum-espresso.org/[4] https://github.com/issp-center-dev/mVMC
[5] http://issp-center-dev.github.io/HPhi/index.html

25

29-10

Metal-insulator transitions in complex oxide thin films andheterostructures from DFT+DMFT
Sophie Beck and Claude Ederer
Materials Theory, ETH Zurich, Switzerland
Emerging properties in complex oxide thin film and heterostructures can arise from a variety of different
factors such as epitaxial strain, defects, electronic and structural reconstruction at interfaces and surfaces,
charge transfer, or reduced dimensionality, which are extremely difficult to isolate experimentally. Here,
we use realistic density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations to
explore how these factors affect the metal insulator transition in several early transition metal oxides. We
demonstrate how the charge transfer across the interface gives rise to a quasi-two-dimensional metallic
electron gas at the interface between the two prototypical Mott insulators LaTiO3 and LaVO3, and how
the thickness of this emerging metallic layer can be tuned by epitaxial strain [1]. Furthermore, we analyze
the metal-to-insulator transition observed in CaVO3 thin films for decreasing film thickness [2,3]. Weshow that this transition is controlled by the interplay between epitaxial strain, dimensional confinement,
and an enhanced crystal-field splitting in the interfacial layers [4]. Our results also highlight the current
capabilities of the DFT+DMFT approach as a quantitative and predictive method for complex materials
systems.

References:[1] S. Beck, C. Ederer, in preparation (2019).
[2] M. Gu et al., J. Appl. Phys. 113, 133704 (2013).
[3] D. E. McNally et al., npj Quantum Mater. 4, 6 (2019).
[4] S. Beck et al., Phys. Rev. B 97, 075107 (2018).

26

29-11

Transcorrelated method: the idea and its applications
Shinji Tsuneyuki
Department of Physics, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Institute for Solid State Physics, The University of Tokyo
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, JapanThe transcorrelated (TC) method first developed by Boys and Handy [1, 2] is aunique approach to correlated electrons with Slater-Jastrow-type many-body wave
functions. In this method, by using similarity transformation of Hamiltonian with the
Jastrow factor, Hartree-Fock-like self-consistent-field equations are derived forone-electron states, with which we can calculate band structure of periodic systems
from first principles as well as the total energy. The method is also applicable to the
Hubbard model if we adopt Gutzwiller wave functions.
For these years we have shown that the TC method is a useful approach to condensed
matter [3-11]: it applies to metals in principle [3], computationally affordable [6,10],much improves the Hartree-Fock band structure of semiconductors [4,6,7], and iscompatible with post-HF methods [5,8,9]. In this talk, I review the progress in the TC
method for condensed matter including an application to a transition metal oxide [11].
[1] S.F. Boys and N.C. Handy, Proc. R. Soc. London, Ser. A 309, 209 (1969); ibid. 310,
63 (1969); ibid. 311, 309 (1969).
[2] N.C. Handy, Mol. Phys. 21, 817 (1971).
[3] N. Umezawa and S. Tsuneyuki, Phys. Rev. B 69, 165102 (2004).
[4] R. Sakuma and S. Tsuneyuki, J. Phys. Soc. Jpn. 75, 103705 (2006).
[5] S. Tsuneyuki, Prog. Theo. Phys. Suppl. 176, 134 (2008).
[6] M. Ochi, K. Sodeyama, R. Sakuma and S. Tsuneyuki, J. Chem. Phys. 136, 094108(2012).
[7] M. Ochi, K. Sodeyama and S. Tsuneyuki, J. Chem. Phys. 140, 074112 (2014).
[8] M. Ochi and S. Tsuneyuki, J. Chem. Theo. Comp. 10, 4098 (2014).
[9] M. Ochi and S. Tsuneyuki, Chem. Phys. Lett. 621, 177-183 (2015).
[10] M. Ochi, Y. Yamamoto, R. Arita and S. Tsuneyuki, J. Chem. Phys. 144, 4109
(2016).
[11] M. Ochi, R. Arita, and S. Tsuneyuki, Phys. Rev. Lett. 118, 026402 (2017).

27

29-12

p-tons --- generic optical excitations of correlated systems

Karsten Held
Institute for Solid State Physics, TU Wien, Austria
The interaction of light with solids gives rise to new bosonic quasiparticles, with theexciton being---undoubtedly---the most famous of these polaritons. While excitons are
the generic polaritons of semiconductors, we show that for strongly correlated systems
another polariton is prevalent---originating from the dominant antiferromagnetic or
charge density wave fluctuations in these systems. As these are usually associated witha wave vector k=(π,π,…) or close to it, we propose to call the derived polaritons π-tons.
These π-tons yield the leading vertex correction to the optical conductivity in all
correlated models studied: the Hubbard, the extended Hubbard model, theFalicov-Kimball, and the Pariser-Parr-Pople model, both in the insulating and in themetallic phase.
Acknowledgement: ERC Grant No. 306447; FWF SFB ViCoM F41 and P 30997.

28

29-13

Non-Equilibrium Steady States (NESS)of a Mott-Anderson Insulator Candidate
Y. Maeno
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
DC current can be a powerful tuning parameter of the electronic states of strongly
correlated electron systems in the vicinity of Mott transitions. We will preset on the novel
phenomena in ruthenium oxide systems under such non-equilibrium steady states(NESS).
Under DC current, Ca2RuO4 with a small Mott gap exhibits semimetallic transport
behavior with giant diamagnetism at low temperatures [1]. We attribute this behavior to
emergent “Mott semimetal” state, in which the upper and lower Hubbard bands becometiny electron and hole pockets with light quasiparticle masses responsible for the large
diamagnetism.
We then describe the NESS effects in the Mott-Anderson insulator candidate Ca3Ru2O7.
We will discuss similarities and differences of the NESS effects between these tworuthenate systems.
The results imply that in a non-equilibrium steady state (NESS) introduced by DC current,a variety of Mott insulators with small gaps may be driven into novel electronic states.
This work was done mainly in collaboration with C. Sow, R. Numasaki, S. Yonezawa, F.
Nakamura, T. Oka, S. Kitamura, K. Kuroki, J. Zhang, M. Liu, A. Millis, D. Shibata, T.
Yoshida, and N. Kikugawa. This work was supported by JSPS KAKENHI Nos.
JP26247060, JP15H05852, and JP15K21717.
Reference:[1] C. Sow, S. Yonezawa, S. Kitamura, T. Oka, K. Kuroki, F. Nakamura, Y. Maeno, Science 358,
1084 (2017).

2929-14

Doping effects in the strongly correlated vanadium oxides
V O and VO232Frank Lechermann
I. Institut für Theoretische Physik, Universität Hamburg, Germany
Describing the realistic effects of impurity doping in strongly correlated compounds is achallenging problem. Electronic and lattice degrees of freedom, both on a local andglobal level come into play, demanding for a sound description of the bonding andmany-body aspect on an equal footing.
We employ the combination of density functional theory (DFT) and dynamical
mean-field theory (DMFT) to address the impact of point defects in Mott-criticaltransition-metal oxides. The efficient but still highly accurate implementation of thecharge self-consistent DFT+DMFT method allows us to tackle the realistic correlation
problem on a close-to-standard materials-science level.
Focus is here on the intriguing physics of the hallmark vanadates V O and VO , bothshowing a metal-to-insulator transition (MIT) at stoichiometry with lowering
temperature. The intriguing phase diagram of V O with Cr- or Ti-doping will be
discussed, elucidating the rather different doping physics of the different chemical
impurities [1]. For rutile VO , we show the suppression of the MIT via the introductionof oxygen vacancies and clarify the underlying mechanism [2].
These studies on defect physics in strongly correlated materials are believed to be
relevant also for future technological applications in view of a possible control ofmaterial properties by correlated-defect engineering.
2

2

3

2

3

2

[1] F. Lechermann, N. Bernstein, I. I. Mazin and R. Valenti, PRL 121, 106401 (2018).

[2] P. Ganesh, Frank Lechermann, Ilkka Kylanpaa, Jaron Krogel, Paul R. C. Kent andOlle Heinonen: arXiv:1811.01145 (2018).

30

29-15Cluster multipole dynamics in non-collinear antiferromagnets
Takuya Nomoto1 and Ryotaro Arita1,2
1Department of Applied Physics, University of Tokyo
2
RIKEN Center for Emergent Matter Science

Recently, functional antiferromagnets are attracting an increasing amount of attention.
In contrast with ferromagnetic devices, those using antiferromagnets have several
advantages: They are robust against magnetic field perturbations, which is good for dataretention. Since they do not have strong stray fields, one can integrate high-density
memory. On top of those, the energy scale of antiferromagnets is usually higher than thatof ferromagnets, which is convenient for ultrafast data processing.
In this talk, we propose a systematic framework to investigate spin dynamics in
non-collinear antiferromagnets. Taking Mn3Sn as a representative example, we derive aneffective low energy model based on the multipole expansion of the magnetic structure,and investigate the uniform precession and the domain wall dynamics. We show that thesolution for the effective model accurately reproduces the numerical calculation of the
Landau-Lifshitz-Gilbert equations (Fig.1). Our results indicate that Mn3Sn has preferable
properties for applications to a racetrack memory and a spin torque oscillator, and thus, isa promising candidate for new devices by using the multipole degrees of freedom.

Fig. 1: Domain wall velocity as a function of time (t). Staggered magnetic field is applied
for t>1000. The open squares are numerical results for a ferromagnet (blue) and collinear
antiferromagnet (red). The open circles are those for Mn3Sn. The dashed lines L1, L2, L3 areanalytic solutions for FM, Mn3Sn, and collinear AFM, respectively.

31

Oral Presentations – Thursday, May 30

30-1

Strong Correlations in Multi-Orbital Materials: Beyond Mottness.
Antoine Georges
Center for Computational Quantum Physics
Collège de France, Parisand Flatiron Institute - Simons Foundation, New York,
Multi-band/multi-orbital materials such as transition-metal oxides, ironsuperconductors or twisted bilayer systems offer a fertile platform for exploring thephysics of strong electronic correlations. In this context, the interplay of the `Hubbard
U’ with Hund’s rule and spin-orbit coupling, as well as orbital differentiation, lead torich physics beyond the paradigmatic ‘Mottness’. In recent years, the concept of`Hund’s metals’ has emerged and has successfully explained the properties of ironsuperconductors and ruthenates. In this talk, I will consider mostly Sr2RuO4 – anamazing material which can serve as a precision laboratory for many-body physics. I
will report on very recent high-resolution ARPES experiments [A.Tamai et al.
arXiv:1812.06531] which allow to put the Dynamical Mean-Field Theory framework to
a direct test, review how Hund’s coupling is responsible for strong correlations in thismaterial and emphasize the importance of spin-orbit coupling.

Figure credit : A.Tamai et al. arXiv :1812.06531

35

30-2

Topological superconductors and Multiple topological states in iron-based
superconductors
Shik Shin
Institute for Solid State Physics, The University of Tokyo, Kashiwa Chiba 277-8581
Topological materials and unconventional iron-based superconductors are bothsignificant areas of study but, to date, relatively little overlap has been identified
between these two fields. However, the combination of topological bands andsuperconductivity promises the manifestation of exotic superconducting states including
Majorana fermions, the central component of topological quantum computation. Here,using laser-based, spin-resolved and angle-resolved photoemission spectroscopy anddensity functional theory calculations, we have identified both topological insulator andDirac semimetal states near the Fermi energy in different iron-based superconducting
compounds. Carrier doping can tune these topologically non-trivial bands to the Fermi
energy, potentially allowing access to several different superconducting topologicalstates in the same material. These results reveal the generic coexistence ofsuperconductivity and multiple topological states in iron-based superconductors,
indicating that this broad class of materials is a promising platform for high-temperature
topological superconductivity.
References[1] P. Zhang, S. Shin, et. al., Nature Physics (2018)[2] P. Zhang, S. Shin, et. al., Science 359, 6381(2018).

36

30-3

Quantum oscillations and charge-neutral fermions in an insulator
Y.Matsuda1, Y. Sato1, Z. Xiang2, Y. Kasahara1, T. Taniguchi1, S. Kasahara1,
L. Chen2, T. Asaba2, C. Tinksman2, O. Tanaka3, Y. Mizukami3, T.
Shibauchi3, F. Iga4, J. Singleton5, and L, Li2
1

Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
3
Department of Advanced Materials Science, University of Tokyo, Chiba, Japan
4
Graduate School of Science and Engineering, Ibaraki University, Mito, Japan5
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
2

Fermi surfaces in metallic systems can be detected by observing characteristic
quantum oscillations of magnetization and electrical resistivity in a strong magnetic
field--they are consequences of the quantization of the orbital motion of conduction
electrons. Such oscillations, in particular of the resistivity, are absent in insulators whichhave no Fermi surfaces.
Here we report a profoundly controversial behavior in a Kondo insulator YbB12. This
material is unmistakably an insulator with an electric resistivity magnitude far beyond
that of metals. However, its resistivity exhibits quantum oscillations. Analysis of theoscillations reveals the three-dimensional nature of the Fermi surface, demonstratingthat the signal arises from the bulk rather than the surface. Although the oscillations of
the resistivity and magnetization are observed in an insulator, their amplitude follow theconventional Fermi liquid theory of metals. The large effective masses determined by
the oscillations point to the presence of Fermi surfaces consisting of strongly correlated
electrons [1].
We also report the observation of gapless itinerant excitations of charge-neutral
fermions in the ground state of YbB12 using very-low-temperature heat-transportmeasurements. This demonstrates that YbB12 is an electrical insulator but a ''thermal
metal". Moreover, the more insulating crystal exhibits the more metallic thermal
properties, i.e., the opposite of conventional metals physics. In addition, despite thecharge neutrality, the neutral fermions can couple to a magnetic field [2].
[1] Z. Xiang et al. Science 362, 65 (2018).
[2]Y. Sato et al. a preprint

37

30-4

Spin-orbit-entangled jef f =1/2 state in 3d transition metaloxide: CuAl2O4
S.V. Streltsov
March 1, 2019

Spin-orbit (SO) Mott insulators are regarded as a new paradigm of magnetic materials, whose properties are largely influenced by the SO coupling and featured by highly
anisotropic bond-dependent exchange interactions between the spin-orbital entangled Kramers
doublets, as manifested in 5d iridates and 4d ruthenates. I will show that a very similar
situation can be realized in cuprates, when the Cu2+ ions reside in a tetrahedral environment, like in spinel compounds. A special attention will be paid to CuAl2 O4 , which was
experimentally found to retain cubic structure and does not show any long-range magnetic
ordering down to very temperatures (0.5 K). We argue that these are the strong Coulomb
correlations and the spin-orbit coupling, which conspire to suppress the Jahn-Teller distortions in CuAl2 O4 [1, ?]. The spin-orbit-entangled jef f =1/2 state is then naturally realizes inthe situation of t52g configuration and degenerate t2g subshell. This in turn explains unusual
magnetic properties of CuAl2 O4 . Using first-principles electronic structure calculations,
we construct a realistic model for the diamond lattice of the Cu2+ ions in CuAl2 O4 andshow that the magnetic properties of this compound are largely controlled by anisotropic
compass-type exchange interactions. This work was supported by Russian Science
Foundation (grant 17-12-01207).

References[1] S. V. Streltsov and D. I. Khomskii, Physics-Uspekhi 60, 1121 (2017).
[2] S. A. Nikolaev, I. V Solovyev, A. N. Ignatenko, V. Y. Irkhin, and S. V Streltsov, Phys.
Rev. B 98, 201106 (R) (2018).
[3] C.H. Kim, H. Cho, S. Baidya, V.V. Gapontsev, S.V. Streltsov, D.I. Khomskii, J.-G.
Park, A. Go, arXiv:1810.08594

1

38

30-5

Spectral properties of Sr2IrO4
S. Biermann, B. Lenz, C. Martins
Centre de Physique Théorique, Ecole Polytechnique, 91128 Palaiseau, France

The spin orbit compound Sr2IrO4 has
kept intriguing condensed matterscientists due to its apparent similarity
to copper oxides: it is isostructural to
La2CuO4, with a single hole in anotherwise filled t2g shell on the Ir site,
and the low-energy spectrum seems
well described by a single orbital
degree of freedom. The pure compoundis insulating at all temperatures, with
antiferromagnetism setting in below
240 K. Upon doping, a metallic state isreached, which has been found to
Fig. 1: Spectral function of electronexhibit exotic (potentially cuprate-like) doped Sr2IrO4 – experiment vs. theory [3].
features.
We will review recent advances in our theoretical modeling [1,2] of spectral propertiesof pure and doped Sr2IrO4 and related compounds, based on a novel cluster extension of
dynamical mean field theory [3,4].

References:[1] Cyril Martins, Markus Aichhorn, Loïg Vaugier, and Silke Biermann, Phys. Rev. Lett. 107,
266404 (2011); see also R. Arita, J. Kuneš, A. V. Kozhevnikov, A. G. Eguiluz, and M. Imada,
Phys. Rev. Lett. 108, 086403 (2012).
[2] C. Martins, M. Aichhorn, S. Biermann, J. Phys.: Condens. Matter, 29 263001 (2017).
[3] C. Martins, B. Lenz, L. Perfetti, V. Brouet, F. Bertran, and S. Biermann. Phys. Rev. Materials
2, 032001 (2018).
[4] B. Lenz, C. Martins, and S. Biermann, J. Phys.: Condens. Matter 31 293001 (2019).

39

30-6

Magnetocrystalline anisotropy of rare-earth magnets
Takashi Miyake
CD-FMat, AIST, Tsukuba, Japan

Modern high-performance permanent magnets mainly consist of 3d transition-metals
and rare-earth elements. High saturation magnetization and high Curie temperature
originate predominantly from interacting 3d electrons, whereas the 4f electrons play a
crucial role in strong magnetocrystalline anisotropy (MCA). There is no established wayof accurately calculating MCA of rare-earth magnets from first-principles, mainly
because localized 4f electrons are strongly correlated [1]. In the present study, we discuss
MCA of real rare-earth magnet compounds based on crystal-field theory. We will present
first-principles evaluation of crystal-field parameters, and discus how chemical doping
affects MCA through change in electronic states [2, 3]. We also present analysis offinite-temperature MCA using a classical spin model and constrained Monte Carlo
method [4].
References:[1] T. Miyake and H. Akai, J. Phys. Soc. Jpn. 87, 041009 (2018).
[2] T. Miyake et al., J. Phys. Soc. Jpn. 83, 043702 (2014).
[3] P. Delange et al., Phys. Rev. B 96, 155132 (2017).
[4] Y. Toga et al., Phys. Rev. B 94, 174433 (2016).

40

30-7

Skyrmions: Classical spin crystals and chiral quantum liquids
Arun Paramekanti
Affiliation: University of Toronto
Motivated by recent developments in studying skyrmions in thin film ferromagnets andfrustrated antiferromagnets, we discuss a triangular lattice spin model with anisotropic
spin exchange interactions which hosts such noncoplanar spin crystals. We consider both
ferromagnets with broken inversion symmetry which host Dzyaloshinskii-Moriya (DM)interactions, and antiferromagnets where symmetry forbids DM interactions but permits
so-called “compass exchange” terms which are relevant to recent experiments onmaterials such as Gd2PdSi3 [1]. We show that nonlinear mode-mode coupling in such
systems can lead to highly field tunable skyrmion crystals [2]. Finally, we present
numerical results on some spin-1/2 Heisenberg models, showing how quantum
fluctuations can melt skyrmion order, leading to chiral quantum spin liquids [3].

Fig. 1: Skyrmions in 2D Rashba ferromagnets and in centrosymmetric frustrated antiferromagnets[1] Takashi Kurumaji, Taro Nakajima, Max Hirschberger, Akiko Kikkawa, Yuichi Yamasaki,
Hajime Sagayama, Hironori Nakao, Yasujiro Taguchi, Taka-hisa Arima, and Yoshinori Tokura,
arXiv:1805.10719[2] S. Sorn, S. Divic, A. Paramekanti, arXiv:1810.08207[3] C. Hickey, L. Cincio, Z. Papic, A. Paramekanti, PRB 96, 115115 (2017)

41

30-8

Spin Nematic Liquid of Low-Dimensional Quantum Antiferromagnets
Toru Sakai
Graduate School of Material Science, University of Hyogo, Hyogo 678-1297, Japan
And QST SPring-8, Hyogo 679-5148, JapanThe spin nematic phase, which is a kind of multipole phases, has attracted a lot ofinterest in the field of the strongly correlated electron systems, as well as the quantum
spin liquid phase. Using the numerical exact diagonalization, the density matrix
renormalization group (DMRG) calculation, and the finite-size scaling analysis, it isfound that some spin nematic and spin liquid phases are induced by external magnetic
field in the anisotropic and/or frustrated quantum spin systems. In our previous work[1],it was found that a field-induced nematic phase appears at some critical field in theanisotropic spin ladder and the mixed spin chain. The nematic phase is characterized by
the power-law decay in the correlation function of the second-order spin moment. In
addition at some higher critical field a quantum phase transition can occur to theconventional field-induced Tomonaga-Luttinger liquid. Several typical magnetization
curves calculated by DMRG are presented.
Recently the field-induced nematic phase was observed on the frustrated spin laddersystem[2]. So we study on a frustrated spin ladder system[3], using the numerical
diagonalization and DMRG. As a result, it is found that several exotic quantum phases,
including the spin-nematic liquid phase. We also report some exact eigenstates of thepresent model and present several interesting phase diagrams[4].
Our recent numerical diagonalization study[5] on the S=1 spin ladder system with theeasy-axis single-ion anisotropy suggested that the field-induced nematicTomonag-Luttinger liquid (TLL) phase appears. In addition another spin nematic liquid
phases were predicted in the three-leg ladder system with the ring exchange interaction[6].
Some realistic candidates which possibly exhibit the spin nematic phase will be proposed.
[1] T. Sakai, T. Tonegawa and K. Okamoto, Physica Status Solidi B 247, 583 (2010) .
[2] N. Buttgen et al., Phys. Rev B 90, 134401 (2014) .
[3] T. Hikihara, T. Tonegawa, K. Okamoto and T. Sakai, J. Phys. Soc. Jpn. 86, 054709 (2017).
[4] T. Tonegawa et al., J. Phys. So. Jpn. 87, 104002 (2018).
[5] R. Yamanokuchi, K. Okamoto and T. Sakai, to appear in Proceedings of International
Conference on Megagauss Magnetic Field Generation and Related Topics (2018).
[6] Y. Tachibana, K. Okamoto and T. Sakai, in preparation.

42

30-9

GW+DMFT simulation of lattice models in and out of equilibrium
Denis Golez (1), Lewin Boehnke (1), Martin Eckstein (2),Philipp Werner (1)(1) Department of Physics, University of Frioburg, 1700 Fribourg, Switzerland(2) Department of Physics, University of Erlangen-Nuernberg, 91058 Erlangen,Germany
The GW+DMFT method provides a consistent scheme for the description of correlationand screening effects in lattice systems and a powerful tool for the study of
nonequilibrium states [1,2]. We will discuss the formalism and its implementation forsimple lattice models (single-orbital Hubbard and three-orbital "d-p” model) andinvestigate the effect of dynamical screening on the local interaction and spectral function.
For the d-p model of charge transfer insulators, we demonstrate that the GW contribution
to the self-energy is not relevant in equilibrium, but crucially important for thedescription of photo-excited states [3]. These model-level studies are a first step in thedevelopment of parameter-free ab-initio simulation approaches for correlated materials.

[1] S. Biermann, F. Aryasetiawan, and A. Georges, Phys. Rev. Lett. 90, 086402 (2003).
[2] D. Golez, L. Boehnke, M. Eckstein, and P. Werner, Phys. Rev. Lett. 118, 246402 (2017).
[3] D. Golez, L. Boehnke, M. Eckstein, and P. Werner, arxiv:1808.02264 (2018).

43

30-10

Higgs Spectroscopy in Unconventional Superconductors
Stefan Kaiser
Max-Planck Institute for Solid State Research, Stuttgart Germany
4th Physics Institute, University of Stuttgart, Germany
The complex phase diagram of cuprate high-Tc superconductor is due to an intriguinginterplay of different orders such as the pseudogap, or charge order. Despite intensive
spectroscopic measurements of these ordered states, yet the microscopic mechanism
behind high-Tc superconductivity is still lacking.
In particular the collective dynamics of the superconducting order parameter, the Higgs
mode, was not accessible so far. Here we report on “Higgs spectroscopy” in cuprate
superconductors that we accomplished by using a high-field multicycle THz pulse, which
nonlinearly couples to the superconducting condensate and leads to characteristic third
harmonic generation [1]. We identify the driven Higgs amplitude response of thesuperconducting order parameter in four archetypal families of cuprate thin films and wereport on a novel collective mode universally exhibited by optimally doped samples. In
addition, we find a finite Higgs-like response above Tc that might be interpreted as finite
pairing amplitude even above Tc.
This remarkable example illustrates the power of Higgs Spectroscopy as novel tool toinvestigate the eigenmodes of coherent condensates – also beyond superconducting
condensates [2] - with respect to their symmetries and possible couplings to external
modes or interplay with competing or intertwined orders.
[1] H. Chu et al. “New collective mode in superconducting cuprates uncovered by
Higgs spectroscopy.” arxiv1901.06675 (2019).
[2] D. Werdehausen “Coherent order parameter oscillations in the ground state of theexcitonic insulator Ta2NiSe5.” Science Advances 4, eaap8652 (2018).

44

30-11

Characterization of photoexcited states in the half-filled
one-dimensional extended Hubbard model by machine learning
Takami Tohyama1), Kazuya Shinjo1), Shigetoshi Sota2), and Seiji Yukoki2)
1)

Department of Applied Physics, Tokyo University of Science, Tokyo, JAPAN2)
RIKEN Center for Computational Science, Kobe, Hyogo, JAPANNonequilibrium processes in strongly correlated electron systems can provide new
insights into the dynamical properties of these systems, which can be qualitatively
different from their weakly interacting counterparts. One such example is nonequilibrium
induced phase transition. As the system is driven away from the equilibrium, undercertain conditions, a ‘‘crossover’’ from one state to another metastable state may occur.
Such an indication has been reported theoretically in the half-field one-dimensional
extended Hubbard model (1DEHM) [1], where by tuning the laser frequency and strengtha sustainable charge order enhancement is found when the system is originally in the
spin-density-wave (SDW) phase but close to the transition to charge-density wave
(CDW). However, the detailed nature of the photoexcited state over full parameter spaceis still unclear.
To characterize the photoexcited states in the 1DEHM, we apply supervised machinelearning [2]. We use the following procedure. Firstly, using density-matrix
renormalization group (DMRG), we calculate the ground state of 1DEHM withcorrelated hopping terms that are suggested from the Floquet time-independent effectivemodel of a driven Hubbard model. We next prepare entanglement spectrum (ES) for the
possible ground states with SDW, CDW, bond-charge-density wave (BCDW), andbond-spin-density wave (BSDW). Using these ES, we construct neural network withfour layers. Finally we use the neural network to judge the ES of photoexcitaed wave
functions of the 1DEHM. Predicted phases are checked by time-dependent local andnon-local correlation functions obtained by time-dependent DMRG. Using thisprocedure, we predict a possible BSDW for the photo-driven 1DEHM in a region of the
CDW ground state and confirm this by calculating the various time-dependent
correlation functions.
[1] H. Lu, S. Sota, H. Matsueda, J. Bonca, and T. Tohyama, Phys. Rev. Lett. 109, 197401(2012).
[2] K. Shinjo, S. Sota, and T. Tohyama, in preparation.

45

30-12

Dynamical control of superconducting order parameter by light
Ryo Shimano
Cryogenic Research Center and Department of Physics, The University of Tokyo

Optical manipulation of quantum system has gained a growing interest over decades.
With the recent development of ultrafast advanced light sources, the concept is now
being applicable to complex condensed matter systems with avoiding a thermalization
effect at least in a transient manner. In particular in superconductors, the light-control ofsuperconducting order parameter is a fascinating subject as it potentially elucidates thehidden phases which are inaccessible by static perturbation, and also provides deep
insights into the ground sate properties in equilibrium. Toward this aim we haveadopted two approaches:1) dynamical control of superconducting order parameter bylight, in particular in terahertz frequency range, and 2) quenching of those orders thatcompetes with superconductivity. Along with the first scheme, after a brief review on
our recent observation of the Higgs mode, namely the amplitude oscillation of orderparameter in conventional and unconventional superconductors [1-3], we report ourrecent investigation on its extension to a two-band superconductor FeTexSe1-x. A
transient enhancement of order parameter as well as the relation between the multipleorder parameters will be discussed. In the second scheme, we investigated thephoto-quench dynamics in a nematic phase of FeSe and in a charge density wave phaseof transition-metal dichalcogenides. Nonequilibirum dynamics of those symmetry
broken ground states will be discussed.
[1] R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, and R.

Shimano, Phys. Rev. Lett. 111, 057002 (2013).
[2] R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa, H. Terai, Z.
Wang, H. Aoki, and R. Shimano, Science 345, 1145 (2014).
[3] K. Katsumi, N. Tsuji, Y. I. Hamada, R. Matsunaga, J. Schneeloch, R. D. Zhong, G. D.
Gu, H. Aoki, Y. Gallais, and R. Shimano, Phys. Rev. Lett. 120, 117001 (2018).

46

30-13

Photoinduced Charge Oscillations in Correlated Dimer Systems
Kenji Yonemitsu
Department of Physics, Chuo University, Bunkyo, Tokyo 112-8551, Japan
Photoinduced phenomena in correlated electron systems are not limited tomelting/ordering phase transitions (including those to hidden states) or ultrafast control
of conduction/magnetism. Here we report characteristic electron dynamics that emerge
only after a strong pulse of an oscillating electric field is applied.
In an organic dimer-Mott insulator/superconductor, N-(BEDT-TTF)2Cu[N(CN)2]Br,
which is known for intriguing critical phenomena, stimulated emission (i.e., increased
reflectivity and transmittance at a specific energy) has been observed after a strong lightpulse is applied [1]. Its energy is on the high-Z side of the linear absorption spectrum.
This is theoretically shown by exact diagonalization as due to a nonlinear charge
oscillation, which is related to a discrete time crystal in many-body localized driven
systems [2]. The oscillation is regarded as an electronic breathing mode [Fig. 1] andwidely observed in dimerized systems. The initially uniform charge density isdisproportionated inside a dimer and coherently oscillates after photoexcitation. Its
frequency is expressed by all of the (i.e., both intra- and inter-dimer) transfer integralsthat connect charge-rich to -poor sites. The result is consistent with the experimental
results for N-(BEDT-TTF)2Cu[N(CN)2]Br and N-(d-BEDT-TTF)2Cu[N(CN)2]Br.
This nonlinear charge oscillation has a finite life time (for interacting systems), but its
life time is longer than any charge oscillation responsible for the linear opticalconductivity. In fact, the nonlinear charge oscillation becomes dominant in the Fourier
spectrum as the Coulomb interaction is increased. Thus, it is regarded asinteraction-induced synchronization. Because it is triggered by a pulse excitation,
thermalization is avoidable, which is in contrast to a discrete time crystal.
In the experiment, the amplitude of the nonlinear charge oscillation is enhanced near
the superconducting transition temperature. The effect of superconductivity is discussed
using a mean field approximation. Depending on the interaction strength, different typesof nonlinear charge oscillations are shown to appear [3]. For the electronic breathingmode, the result is consistent with the experiment.
References:[1] Y. Kawakami et. al., Nat. Photonics 12, 474 (2018).
[2] KY, J. Phys. Soc. Jpn. 87, 044708 (2018).
[3] KY, J. Phys. Soc. Jpn. 87, 124703 (2018).
Fig. 1: Electronic breathing mode47

30-14

Non-Hermitian quantum phenomena in correlated systems
Norio Kawakami
Department of Physics, Kyoto University
Recently, non-Hermitian quantum phenomena have attracted a great deal of attention. We
here address the following two interesting issues in this context.
1. Non-Hermitian Kondo effect [1]: We first investigate the Kondo effect in an open
quantum system motivated by recent experiments with ultracold alkaline-earth(-like)atoms. Due to inelastic collisions
and the associated atom losses, thissystem is described by a nonHermitian extension of the Kondo
problem. We show that the nonHermiticity induces anomalous
reversion of renormalization-group,
leading to a unique quantum phaseFig. 1: RG flow of real- imaginary Kondo couplings
transition (Fig.1). Furthermore, byexactly solving the non-Hermitian Kondo Hamiltonian, we obtain the critical line
consistent with the renormalization-group flow.
2. Non-Hermitian perspective on Kondo lattice systems [2,3]: We then address a slightly
different topic: a two-dimensional Kondo lattice model in equilibrium with special
emphasis on its non-Hermitian properties [2], following a recent proposal by Kozii andFu [4]. We confirm that the single-particle spectral weight shows the exceptional points(EPs). Correspondingly, the spectral weight exhibits the band touching, resulting in a
structure similar to the Fermi arc. Furthermore, we find that the paramagnetic phase in the
low temperature region shows a significant difference from non-interacting fermions: theimaginary part of the self-energy yields the Fermi loop without any defective points. We
extend the notion to symmetry-protected exceptional rings [3].
This work is done in collaboration with M. Nakagawa, M. Ueda, K. Takasan, T.
Yoshida, R. Peters and Y. Hatsugai.
References :[1] M. Nakagawa, N. Kawakami and M. Ueda, Phys. Rev. Lett. in press[2] T. Yoshida, R. Peters and N. Kawakami, Phys. Rev. B98, 035141 (2018)[3] T. Yoshida, R. Peters, N. Kawakami and Y. Hatsugai, arXiv:1810.06297[4] V. Kozii and L. Fu, arXiv:1708.05841

48

Oral Presentations – Friday, May 31

31-1

Field- and Pressure-induced phases in generalized Kitaev
models and materials
Roser Valentí
Institut für Theoretische Physik, Goethe-Universität Frankfurt,
Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany
In the search for novel materials’ properties, the generation and manipulation of highly
entangled quantum states is a grand challenge of solid state research. Amongst the mostentangled proposed states are quantum spin liquids. In this context, the exactly solvable
Kitaev Z2 spin-liquid model, for which finely tuned anisotropic interactions exactly
fractionalize spins into fermionic Majorana spinons and gauge fluxes has activated anenormous amount of interest. Most specially since possible realizations may beachieved in octahedral coordinated spin-orbit-coupled 4d5 and 5d5 insulators. However,the low symmetry environment of the known Kitaev materials also allows interactions
beyond the Kitaev model that open possible new routes for further exotic excitations.
Based on ab initio and many-body simulations and comparison to experimental
observations, we will discuss in this talk, the challenges that one faces in designing
such materials and in identifying the origin of their excitations. We will further present
recent results [1-6 ] on possible field- and pressure-induced phases obtained by adding
to the Kitaev model further interaction terms and will discuss the relevance of theresults in relation to honeycomb iridates and α-RuCl3 .
Work done in collaboration with Steve M. Winter, Kira Riedl, David Kaib, Ying Li,
Andreas Honecker, Pavel A. Maksimov, Alexander L. Chernyshev and Radu Coldea[1] Riedl et al. arXiv:1809.03943[2] Li et al. arXiv:1807.02124 (PRL in press)[3] Biesner et al. PRB 97, 220401(R) (2018)[4] Winter et al. PRL 120, 077203 (2018)[5] Hermann et al. PRB 97, 020104(R ) (2018)[6] Winter et al. Nature Comm. 8, 115 (2017)

51

31-2

Majorana signatures in proximate Kitaev spin liquids
Yukitoshi Motome
Department of Applied Physics, The University of TokyoA Majorana fermion is a fermionic particle that is its own antiparticle. The exotic
particle has long been pursued since the proposal by Ettore Majorana in 1937. Recently, it
has attracted renewed interest in condensed matter physics, where it can be realized as
quasi-particles in quantum states of matter [1]. In this talk, we discuss a candidate forsuch realization of Majorana fermions, quantum spin liquids, where strong quantum
fluctuations suppress any conventional symmetry breaking and leave the electron spins
disordered but quantum entangled. In particular, we focus on the so-called Kitaev spin
liquids, where the electron spins are fractionalized into Majorana fermions and Z2 fluxes[2]. We will give an overview of our recent findings, especially focusing on the peculiar
properties associated with the fractional excitations, such as thermal fractionalization [3],signature of fermionic excitations [4], dichotomy of static and dynamical spincorrelations [5], and Majorana topological states in an applied magnetic field [6]. We will
also discuss our recent proposals for materials exploration [7].
References (If necessary):[1] F. Wilczek, Nat. Phys. 5, 614 (2009).
[2] A. Kitaev, Ann. Phys. (NY) 321, 2 (2006).
[3] J. Nasu, M. Udagawa, and Y. Motome, Phys. Rev. Lett. 113, 197205 (2014); Phys. Rev. B 92,
115122 (2015).
[4] J. Nasu, J. Knolle, D. L. Kovrizhin, Y. Motome, and R. Moessner, Nat. Phys. 12, 912 (2016).
[5] J. Yoshitake, J. Nasu, and Y. Motome, Phys. Rev. Lett. 117, 157203 (2016); Phys. Rev. B 96,
024438 (2017); ibid., 96, 064433 (2017); S.-H. Do et al., Nat. Phys. 13, 1079 (2017).
[6] J. Nasu, J. Yoshitake, and Y. Motome, Phys. Rev. Lett. 119, 127204 (2017); Y. Kasahara et al.,Phys. Rev. Lett. 120, 217205 (2018); Y. Kasahara et al., Nature 559, 227 (2018); J. Nasu, Y.
Kato, Y. Kamiya, and Y. Motome, Phys. Rev. B 98, 060416(R) (2018); Y. Nagai, T. Jinno, Y.
Yoshitake, J. Nasu, Y. Motome, M. Itoh, and Y. Shimizu, preprint (arXiv:1810.05379).
[7] R. Sano, Y. Kato, and Y. Motome, Phys. Rev. B 97, 014408 (2018); S.-H. Jang, R. Sano, Y.
Kato, and Y. Motome, preprint (arXiv:1807.01443); Y. Sugita, Y. Kato, and Y. Motome, in
preparation.

52

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Spin liquid in extended Heisenberg models on triangular latticeP. Prelovsek
J. Stefan Institute, Ljubljana, Slovenia
Faculty of Mathematics and Physics, University of Ljubljana, Slovenia
Motivated by recent experiments, revealing a spin liquid phase in the layered 1T-TaS[1], we study the finite-temperature static and dynamical properties of the extended J -J
Heisenberg model [2]. Here, frustration enables to reach reliable results to considerably
lower T relative to the unfrustrated lattices. Indications for the onset of spin liquid phaseare visible in thermodynamic quantities, as the specific heat, but also in dynamical
structure factor, being related to experimentally relevant NMR relaxation rate 1/T ,
2

1

2

1

which can be used as the hallmark for the spin liquid. Further I will present the
reduced-basis approach using the triangle as the basic unit which allows the unifying
treatment of the Heisenberg models on triangular and kagome lattices. The approachenables the study of larger systems, but reveals also similarities and differences between
both lattices, in particular in relation to the existence of spin-liquid regimes.
[1] M. Klanjsek et al., Nat. Phys. 13, 1130 (2017).
[2] P. Prelovsek and J. Kokalj, Phys. Rev. B 98, 035107 (2018).

53

31-4

Optical bistability in a quantum low photon-density regime
Synge Todo
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan

Sz

Spins coupled with photon degrees of freedom in the cavity (Fig.1) exhibit various
responses to the incident laser field depending on the average number of photons in thecavity. One of the most characteristic phenomena is the dynamical first-order phasetransition, in which the number of photons in the cavity shows bistability with respect tothe laser intensity, and jumps discontinuously between the states. Until now, the analysesof this phenomenon have been carried out based on the mean-field treatment that becomesexact in the limit of infinite number of spins, or based on numerical simulations for small
spin number in the limit of infinite number of photons. We realized a large-scale
simulation utilizing the parallelization with respect to the photon number and thesymmetry of the spin state, and examined thesize dependence of the photon number
distribution and the relaxation time in the low
photon-density regime from the eigenvalues
and eigenstates of the time evolution operator.
We show that in the low photon density regime Fig. 1: Spins in the cavity.
the laser frequency dependence of the-0.1
metastable state is qualitatively different from
-0.2
that in previous studies, and discuss the opticalbistability from the effective free-energy-0.3
picture of the dynamical first-order phase-0.4transition. Furthermore, we calculate thehysteresis loop (Fig.2), which appears when the
-0.5
0.01
0.02
0.03
0.04
0.05
laser intensity is periodically modulated, from
Driving amplitude ξ
the viewpoint of the Floquet operator, andFig. 2: Hysteresis loop and limit cycle underclarify that a dynamical phase transition
periodic modulation of laser intensity.
phenomenon appears with respect to the periodof modulation.
[1] T. Shirai, S. Todo, H. de Raedt, S. Miyashita, Phys. Rev. A 98, 043802 (2018).
[2] T. Shirai, S. Todo, H. de Raedt, S. Miyashita, in preparation.

54

31-5

Intertwined orders in Dirac Fermions

F. F. Assaad
Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland,
97074 Würzburg, Germany

There has recently been a flurry of negative sign free fermionic models thatexhibit exotic phases and phase transitions. After reviewing these advances, I
will place the emphasis on models of Dirac fermions in 2+1 dimensions withdynamically generated anti-commuting mass terms. For eight component Diracfermions as realized in Graphene, one can find 56 quintuplets of anticommuting
mass terms. For example the three antiferromagnetic masses and two valencebond solid ones form such a quintuplet. Another quintuplet combines thequantum spin Hall mass terms with the two s-wave superconducting ones. Here,I will present a model free of the negative sign problem that dynamicallygenerates the quantum spin Hall and s-wave superconducting phases and show
results supporting a direct and continuous phase transition between these twostates [1]. More generally, we will argue that models of Dirac fermions
with dynamically generated anti-commuting mass terms are a golden route torealize and investigate exotic quantum phase transitions beyond theGinzburg-Landau-Wilson paradigm.
[1] Y. Liu, Z. Wang, T. Sato, M. Hohenadler, C. Wang, W. Guo, and F. F.
Assaad, Superconductivity from the Condensation of Topological Defects in a
Quantum Spin-Hall Insulator, ArXiv:1811.02583 (2018).

55

31-6

First-order topological phase transitions with correlated electrons
Giorgio Sangiovanni
Institut für Theoretische Physik und Astrophysik
University of Würzburg, Am Hubland, 97074 Würzburg, Germany

Ha
ll
I

Sp

in

[HgTe/CdTe quantum wells]

0.25

References[1] A. Amaricci, J. C. Budich, et int., G. Sangiovanni,
Phys. Rev. Lett. 114, 185701 (2015)[2] A. Amaricci, J. C. Budich, et int., G. Sangiovanni,
Phys. Rev. B 93, 235112 (2016)[3] A. Amaricci, L. Privitera, et int., B. Trauzettel,
Phys. Rev. B 95, 205120 (2017)[4] A. Amaricci, A. Valli, et int., M. Capone,
Phys. Rev. B 98, 045133 (2018)

56

0.2

Ha
ll
I

Sp
in

[HgTe/CdTe quantum wells]

Qu
an
tu
m

Mott

Insul
ator

ns

ul
ato
r

y

an
tu
m

Qu

Mott

y

Insul
ator

ns

ul
ato
r

y (finite dimension)

When the bandstructure of a topologically trivial insulator evolves into that of aquantum spin Hall, the gap closes and inverts gradually. The situation can drasticallychange if the electrons experience a strong Coulomb repulsion. The second-order phasetransition turns into a discontinuous one and the system eschews to be fine-tuned to agapless semimetal [1].
periodic boundary conditions
10
0.5 that two-orbital microscopic models
In this talk I am going 25
to showfortwo25
25 and
(a)
(b)
0.40
-5.9
three-dimensional topological 0.45
insulators display a first-orderphase transition. This
Uncorrelated
topological transition
20
20
20
0.38
originates from the asymmetry0.4resulting from the rapid growth of-6.0the many-body
U=5.5
character on the non-trivial15side, while the band insulator
nature15 persists in1its Hartree
15 [1,
2.76 2.78 2.8 2.82 2.84
0.36
0.35
Quantum
CriticalPoint2]. This "blending" between non-trivial topology and local orbital physics
is the key
-6.64
(c)
10
10
10
0.3
0.34
physical ingredient which allows us to define local Bandobservables
divergingatthequantum
Insulator
5 0.255
5metallic AF
critical point.
0.1 -6.65
Triple point
0.32
channel
U=7.0
Mott
edge
The boundary modes in the 0.2
topologically
non-trivial
phase0.4are influenced
by the0.2
0.3
1.6
3.0825 3.0850 3.0875 3.0900
x0.1
x
x
1/U
(a)
(b)
(c)
presence of the high-spin Mott insulatingphase at even larger valuesof U. We indeed
observe a boundary reconstruction in 2D: the edge
10
0.5
(a)
-5.9
states get gapped out in the outmost part of the slab
Uncorrelated
0.45
topological transitionand settle in sub-peripheral layers [3]. Upon
-6.0
0.4
U=5.5
allowing for time-reversal symmetry breaking this12.76 2.78
0.35
turns into an interesting coexistence of correlated
Quantum Critical Point
-6.64
0.3
metallic modes and antiferromagnetism [4].
Band Insulator

0.1 -6.65

Triple point

0.1

U=7.0
0.2

1/U

0.3

0.4

Fig. 1: Phase diagram of theBernevig-Hughes-Zhang-Hubbard
model. Taken from Ref. [1]3.0825 3.08

31-7

Superconductivity in 2D Materials
Yoshihiro Iwasa
Department of Physics and QPEC, University of Toyo, Tokyo 113-8656, JapanRIKEN Center for Emergent Matter Science, Wako 351-0198, Japan
2D materials are a rich materials platform of designing novel properties and functions.
One examples is superconductiviy in 2D materials, obtained by mechanically exfoliatedmono or a few layers, heterointerfaces, electric-double-layer transistors, andmolecular-beam-epitaxy grown or chemical vapor deposited atomically thin layers [1].
All these 2D superconductors have very high crystallinity in marked contrast with theconventional 2D superconductors with amorphous or granular structures, and thus
provide opportunities for investigating the intrinsic nature of 2D superconductors,
reflecting the high crystallinity of 2D materials. Nowadays, people are excited about the
new opportunity of flatband physics and superconductivity in twisted bilayer graphene[2].
Here we review the novel aspects of highly crystalline 2D superconductors withparticular focus on the gate-induced 2D superconductivity using electric double layertransistor devices. Topics include quantum metallic states (Bose metal) [3], quantum
Griffiths phases [4], dramatically enhanced Chandrasekhar-Clogston-Pauli limit due tothe spin-orbit interactions [5], and nonreciprocal superconducting transport [6]. Also,
EDLTs was found to offer a new opportunity to control electrochemical reactions, whichallows us to access superconductivity in monolayer FeSe [7] and very lightly doped
superconductor in ZrNCl toward BCS-BEC crossover [8].
References[1] Y. Saito, T. Nojima, and Y. Iwasa, Nat. Rev. Mater. 2, 16094 (2016).
[2] Y. Cao et al., Nature, 556, 43 (2018), Nature, 556 80 (2018).
[3] Y. Saito et al., Science 350, 409 (2015)[4] Y. Saito et al., Nat. Comm. 9, 778 (2018)[5] Y. Saito et al., Nat. Phys. 12, 144 (2016)[6] R. Wakatsuki et al., Sci. Adv. 3, e1602390 (2017).
[7] S. Shimizu et al., Nat Comm. 10, 825 (2019).
[8] Y. Nakagawa et al., Phys. Rev. B98, 064512 (2018).

57

31-8

Phase Diagrams and Scaling Behaviors of Disordered Weyl Semimetals

Tomi Ohtsuki1, Xunlong Luo2, Ryuichi Shindou2
1) Physics Division, Sophia Univ., Kioicho 7-1, Tokyo, Japan
2) Intl. Center for Quantum Materials, Peking Univ., Beijing, China
Recent experimental discovery of Weyl semimetals stimulated extensive
studies of this novel topological material. In the presence of disorder, Weyl
semimetal systems show rich phase diagram: Weyl semimetal (WSM), band
/Chern insulator (BI/CI), as well as diffusive metal phases. By changing themass parameter, the system undergoes CI to WSM transition, while
increasing the strength of disorder induces diffusive metal (DM) phase.
We demonstrate that the machine learning method to detect surface Fermi
arc states gives reasonable phase diagram1,2) (see Figure 1). We further
show that rich unconventional scaling behaviors are expected across and on
the phase boundaries.3,4)
3

2.4

W

DM

’phaseDiagramWSM.txt’

2.5

2.2
2

x

1.8

2

1.5

CI

while the vertical axis W indicates the strength

0.5

0

0.35

0.4

0.45

0.5

0.55

β

The

horizontal axis b indicates the mass parameter,

1

WSM

1.2
1

learning the surface Fermi arc states.

of disorder.

1.6
1.4

Fig. 1: Phase diagram obtained by machine

0.6

References[1] T. Ohtsuki and T. Ohtsuki: J. Phys. Soc. Jpn, 85, 123706 (2016), 86, 044708 (2017).
[2] S. Liu, T. Ohtsuki, R. Shindou: Physical Review Letters 116, 066401 (2016).
[3] X. Luo, B. Xu, T. Ohtsuki, R. Shindou: Physical Review B 97, 045129 (2018).
[4] X. Luo, T. Ohtsuki, R. Shindou: Physical Review B 98, 020201 (2018).

58

31-9

The Quantum Computing Future ofStrongly Correlated Electron Simulations
Matthias Troyer
Microsoft Quantum, Redmond, WA, USA
The exponential complexity of simulating correlated quantum systems on computers has
motivated Richard Feynman to propose that we should use computers employing quantum
principles to simulate Nature. As we make progress towards scalable implementations of such
quantum computers it is timely to start thinking about how we would use them to solve important
problems in correlated electron science. Correlated quantum systems will be one of the mostimportant applications of quantum computers.
In my talk I will present an overview of how quantum computers will revolutionize this field in
the future. I will discuss quantum algorithms for the simulation of ground states, excited states
and non-equilibrium dynamics. I will present ideas of solving the infamous negative sign problemof quantum Monte Carlo simulations on quantum computers.
I will also go into specific runtime estimates for the above simulations. We found that, despite an
exponential improvement in scaling compared to classical computers the brute-force ab-initio
simulation of materials and molecules is still challenging even on quantum computers. Effectivemodels, like variants of the Hubbard model, will thus be the initial targets for quantum computers.
Hybrid quantum/classical methods and embedding approaches like dynamical mean field theory
will also become more powerful when accelerated with quantum computers.
I will end with a vision of how our field of research may look like in the age of quantum
computing.

5931-10Tensor Network States for Lattice Systems
Naoki KAWASHIMAThe Institute for Solid State Physics, The University of Tokyo
While the tensor network representation has been used in variational calculations oflattice many-body problems since 1960s, it is relatively recently that the concept drewattention it deserves. In the last few decades, various efficient scheme for practicalcomputation were devised while the connection to the analytical theories became clearer.
One of the most well-known examples for the latter may be the AKLT state for thesymmetry protected topological order. While it is not the ground state of the original
problem, i.e., the S=1 antiferromagnetic Heisenberg chain, it captures its essence in termsof the 1D tensor network state (i.e., a matrix-product state) with only a binary degree of
freedom for each virtual index. I present some tensor-network calculations [1] of theKitaev honeycomb model, which is the exactly solvable model for topological spin
liquids. Whereas no compact tensor network representation is known for the ground stateof the Kitaev honeycomb model, we propose one with only binary degree of freedom as agood initial state for the variational calculation. This state satisfies most of the desirable
properties that the exact ground state is known to possess. By introducing a variationalparameter in the form of the tensor network operator (sometimes referred to as PEPO), we
can further improve the wave function, though it increases the necessary bond dimension
to 4 or larger. With the bond dimension of 8 we reach relative error of 10-4 (in energy)
with only two tunable parameters. While these results are specialized for the Kitaev
honeycomb model, they may serve as a prototype for tensor-network application to thespin liquid states.
[1] Hyun-Yong Lee, Ryui Kaneko, Tsuyoshi Okubo and Naoki Kawashima, unpublished.

60

31-11

Machine learning for solving quantum many-body Hamiltonians
Yusuke Nomura
Department of Applied Physics, University of Tokyo
Machine learning techniques can be used to extract essential feature of quantum
many-body wave functions (=vectors with exponentially large dimensions), and to obtaincompact representations of many-body states. Indeed, recently, variational wave function
written in terms of restricted Boltzmann machine (RBM) has been introduced to represent
ground states of quantum spin Hamiltonians [1]. The RBM wave function has beenshown to be able to represent ground states of spin Hamiltonians in high accuracy [1].
In the present study, we introduce efforts to improve the accuracy of RBM wave
functions by i) combining concepts from machine learning and physics [2] and by ii)
introducing deep Boltzmann machine (DBM) having additional hidden layer on top of the
RBM structure [3].
In i), we introduce a new form of variational wave function, called RBM+PP, which
combines RBM with pair-product (PP) wave functions [2]. The combined wave function
can be applied not only to bosonic models but also to fermionic models. The combined
method improves the accuracy beyond that ever achieved by RBM and conventional
wave-function method separately, thus proving its power as an accurate solver.
In ii), we show a deterministic approach to generate deep-Boltzmann-machine (DBM)
network to represent ground states of many-body Hamiltonians [3]. Thanks to more
flexible representational power of DBM compared to RBM, the DBM state can reproduce
the exact imaginary-time Hamiltonian evolution with the number of hidden units
proportional to the system size and imaginary time, respectively. Once the network is
constructed, physical quantities can be measured by sampling both the visible and hidden
variables. The present construction of classical DBM network provides a novel
framework of quantum-to-classical mappings (In special cases, it becomes equivalent topath-integral formalism).
The present works have been done in collaboration with Andrew S. Darmawan, Youhei
Yamaji, Masatoshi Imada, and Giuseppe Carleo.
[1] G. Carleo and M. Troyer, Science 355, 602 (2017).
[2] Y. Nomura, A. S. Darmawan, Y. Yamaji, and M. Imada, Phys. Rev. B 96, 205152 (2017).
[3] G. Carleo, Y. Nomura, and M. Imada, Nat. Commun. 9, 5322 (2018)61

31-12

Perspectives on descriptions of correlated electron phenomena
Masatoshi Imada
Waseda Research Institute for Science and Engineering, Waseda University,
Shinjuku-ku, Tokyo, 169-8555, Japanand
Toyota Physical and Chemical Research Institute, Nagakute, Aichi, Japan
Strongly correlated electron systems have been a challenging and central subject inphysics for decades. Emergent phenomena including exotic magnetism, unconventionaltransport, and superconductivity have extensively been studied as playgrounds ofemergent and universal concepts of nature and as seedbeds of potential functionality for
future innovative applications.
After the discovery of cuprate high-Tc superconductors, it has been widely recognizedthat deeper understanding of fermionic many-body systems relevant to realistic
materials is required beyond conventional theoretical frameworks. Necessity ofdescribing realistic materials representable originally by exponentially large Hilbert
space has urged us to develop multi-scale ab initio method with downfolding, allowing
correct descriptions of materials by using only low-energy degrees of freedom, which
can be solved within the available computational cost by accurate low-energy solvers in
the next step. High accuracy is required because of the severe competitions among
various quantum phases and fluctuations, hallmark of strongly correlated systems.
Theoretical and computational methodologies have been updated and improved formore accurate and realistic descriptions for both of better downfolding procedure suchas the constrained GW and more accurate solvers such as tensor network and machinelearning tools. The improved scheme now allows quantitative and predictable
descriptions of real correlated materials even for challenging issues including
unconventional superconductivity of the cuprate and iron-based superconductors. It also
inspires us to novel concepts such as spin liquids, fractionalization with emergent or
dark fermions and Planckian fluids with unparticle concept. The research front is now
extended also to fields such as non-equilibrium phenomena, non-periodic systems
including interfaces, surfaces and random systems, and topological materials withstrong correlation effects. After summarizing some of recent achievements andinteresting proposals presented in this workshop, I discuss my personal view for futurechallenges to be tackled.

62

Poster Presentations – Section A

P01

Nonmetallicity in self-doped BiS2-based Eu3Bi2S4F4:
single crystal study
Yuji Aoki, Ryuji Higashinaka, Hideaki Endo, Joe Kajitani,
Ryotaro Sakatani, and Tatsuma D. Matsuda
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397
BiS2-based layered materials [1] show rich variety of correlated electron physics,
which are partly attributed to locally-noncentrosymmetric BiS2 layers with strong
spin-orbit coupling [2]. We have succeeded in growing single crystals and have revealed
a quantum critical behaviors (lnT divergence in specific heat) in CeOBiS2[3],anisotropic-gap superconductivity and the absence of a Pauli paramagnetic limit in
LaO0.5F0.5BiS2[4], and lattice instabilities La(O,F)BiS2 [5]. In this presentation, we
focus on Eu3Bi2S4F4 (see Fig. 1 for the crystal structure).
Eu3Bi2S4F4 has been reported to exhibit superconductivity with a transition
temperature of 1.5 K using polycrystalline samples [6]. Studies using our single crystals
have revealed anomalous thermodynamic and transport properties. Mixed valent Eu
ions with the average valence of +2.19 indicates the self-doped electron density in BiS2
layers to be 0.29 electrons per Bi site, which is consistent with our ARPES
measurements [7]. It has been revealed, however, that the resistivity shows ananomalous nonmetallic behavior with 1/T3 dependence without any sign ofsuperconductivity, in marked contrast to a metallic behavior in the polycrystals. Thisbehavior
suggests
anunconventional
electron-localization mechanism working in the single
crystals, possibly caused by Eu2+/Eu3+ distribution (orfluctuations) and/or lattice instabilities inherent to BiS2
layers.
References[1] Y. Mizuguchi et al., Phys.Rev. B 86, 220510 (2012).
[2] X. Zhang et al., Nat.Phys. 10, 387 (2014).
[3] R. Higashinaka et al., J.Phys.Soc.Jpn. 84, 023702 (2015).
[4] Y. C. Chan et al., Phys. Rev. B 97, 104509 (2018).
[5] R. Sagayama et al., J. Phys. Soc. Jpn., 84, 123703 (2015).
[6] H. F. Zhai et al., J. Am. Chem. Soc., 136, 15386 (2014).
[7] E. Paris et al., Phys Rev. B, 95, 035152 (2017).

65

Fig. 1: Crystal structure of
Eu3F4Bi2S4 (I4/mmm, #139).

P02

Higher order topological phases of a disordered breathing Kagomemodel: A machine learning study
Hiromu Araki, Tomonari Mizoguchi, Yasuhiro Hatsugai
University of Tsukuba
A higher order topological insulator (HOTI) is a new concept of topological states ofmatter, characterized by the emergent boundary states whose co-dimension is higher thanone. So far, several tight-binding models for HOTI are proposed [1,2], which triggered
many theoretical and experimental studies. However, its robustness against disorders
has not been well understood.
In this presentation, we show the phase diagram of HOTI phases in a breathing Kagomemodel in the presence of disorders, by using a machine learning technique [3]. We findthat the corner states survive against the finite strength of disorder potential as long as theenergy gap is not closed, indicating the stability of the HOTI phases against the disorders.
We also discuss the relation between the HOTI phase and the Z3 Berry phase [4], which isa bulk topological invariant.

Fig. 1: Phase diagram of the breathing kagome model. Horizontal axis is thebreathing parameter and the vertical axis is the disorder parameter.

References:[1] F. Schindler, A. M. Cook, et. al., Science Advances 4 (2018)[2] M. Ezawa, Phys. Rev. Lett. 120, 026801[3] H. Araki, T. Mizoguchi, Y. Hatsugai Phys. Rev. B 99, 085406 (2019)[4] Y. Hatsugai and I. Maruyama, Europhys. Lett. 95, 20003 (2011)

66

P03

Investigations on double perovskites Pr2MnNiO6 and Nd2-xSrxMnNiO6:
Spectroscopy and ab-initio studies
Padmanabhan Balasubramanian
Department of Allied Sciences, Graphic Era University, Dehra Dun, Uttarakhand 248002,
India.
The electronic structure of double perovskite Pr2MnNiO6 was studied using core x-ray
photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS) and ab-initio
calculations1. Based on charge transfer multiplet analysis of the Ni/Mn 2p XPS and XAS
spectra, we find charge transfer energies of 3.5 and 2.5 eV for Ni and Mn respectively. The
ground state of Ni2+ and Mn4+ ions reveal a higher d electron count of 8.21 and 3.38
respectively as compared to the ionic values. The partial density of states show a chargetransfer character of the system for U- J ³ 2 eV wherein an increase in U does not change theband gap. With increase in U − J, the spectral weight of O 2p states increase below EF, while
the spectral weights of Ni and Mn 3d states, become smaller in comparision. The O 1s edge
absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from
XPS and XAS. Our studies reveal that the material is a p − d type charge transfer insulator
with an intermediate covalent character as per the to the Zannen–Sawatzy–Allen phase
diagram.
In second part of my talk, I shall discuss effect of Sr doping on Nd2MnNiO6. The compound
Nd2-xSrxMnNiO6(x=0, 0.2, 0.4, 0.5 and 1) is investigated experimentally from bulk
magnetization, x-ray spectroscopy and ab-initio methods2. There occurs a structuraltransformation from monoclinic (P21/n, for x = 0 to 0.5) to cubic (!"3", for x = 1). The
systematic reduction in magnetic moment at 5 K suggests an increase in antisite disorders
with doping. Our GGA-based calculations for the ordered supercell, predict half metallic
character for doping (x > 0) samples due to delocalization of Ni eg orbitals. The theoreticalmagnetic moment of Mn remains constant irrespective of doping, x. However, the moment of
Ni shows a drastic reduction with doping, from 1.4 µB to 0.7 µB suggesting an increase incovalent character of the system. Temperature dependent resistivity measurements exhibit aclear metallic region for x = 0.2 sample, while for x > 0.2, the metallicity gets suppressed due
to increase in anti-site disorders. Calculations on supercell with anti-site disorder was
systematically performed with flip of 1 and 2 Mn/Ni sites which yield drastic reduction in Ni
moments. With doping, the disordered anti-ferromagnetic phase yields lowest energy,
especially for x=1.
References[1] Padmanabhan Balasubramanian, Shalik Ram Joshi, Ruchika Yadav, Frank M F de Groot, Amit
Kumar Singh, Avijeet Ray, Mukul Gupta, Ankita Singh, Satyendra Maurya, Suja Elizabeth, Shikha
Varma, Tulika Maitra and Vivek Malik J. Phys.: Condens. Matter 30, 435603 (2018).
[2] Amit Kumar Singh, Padmanabhan Balasubramanian , Ankita Singh, M K Gupta and Ramesh
Chandra J. Phys.: Condens. Matter 30, 355401 (2018).

67

P04

Density of states in variational Monte Carlo
Maxime Charlebois1, Kota Ido2, Takafumi Suzuki1, Masatoshi Imada1
1

Department of Applied Physics, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
2
Institute for Solid State Physics, The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Variational Monte Carlo is a powerful tool to study the strongly interacting electronmaterials with a large cluster. It is a technique that is optimized to find the ground state|𝜑⟩ and to estimate average values of operator A from this ground state:〈A〉 =⟨𝜑 |A|𝜑 ⟩⟨𝜑 |𝜑⟩= ∑x 𝜌(x)⟨𝜑 |A|x⟩⟨𝜑 |x⟩

where2|⟨𝜑 |x⟩|𝜌(x) = ⟨𝜑|𝜑⟩ ,and |x⟩ is a specific state of the system. The sum over specific states is estimated using
Monte Carlo sampling. In this poster, I want to show how it is possible to estimate theGreen function, thus the density of state using this approach.
The Green function require the inverse of the Hamiltonian. However, for a large system,
it is impossible to have a complete representation of the Hamiltonian. A solution is toapproximate the Hamiltonian with a subspace composed of many different excitations
and work with this new matrix in this subspace.

Fig. 1: Denstity of state for a Hubbard chain for different interactions levels U.

The density of state for a one-dimensional Hubbard chain is shown in Fig. 1. We can showthat it correctly reproduces the Mott gap opening with interaction.
This project has been conducted with the open source package mVMC.
Reference:[1] Misawa et al. Computer Physics Communications 235 (2019) 447-462

68

P05

Analysis of basic properties of compact representation of many-body
Green’s function
Naoya Chikano, Kazuyoshi YoshimiA, Junya OtsukiB, Hiroshi Shinaoka
Saitama univ., ISSPA, Okayama univ.B
The Matsubara Green’s function is a building block of a variety of quantum many-bodytheories ranging from the dynamical mean-field theory to quantum Monte Carlo methods.
As the data size of the Green’s function grows at low temperature, practical calculations
using these methods often suffer from large computation time and massive memory
consumption. Recently, some of the authors and co-workers proposed a compact
representation for the imaginary-time dependence of the Green’s functions [1,2,3]. The
basis functions of this “intermediate representation (IR)” are a solution of an integral
equation based on a physical ground. The absence of analytic and accurate numericalsolutions however prevents the application of the IR to practical calculations as well as
detailed analysis of its properties.
We propose an efficient numerical method for computing an accurate numerical solutionof the basis functions [4]. Based on the obtained numerical solution, we study theproperties of the IR basis in detail. In particular, we show that for any system the numberof the required expansion coefficients grows only logarithmically with inverse
temperature ß (see the figure). To promote theapplication to the IR basis to practical
calculations, we published the numericalsolutions and Python/C++ libraries as “irbasis”[5]. In this presentation, we will show the resultof analysis of the basic properties of the IR basis
and an outline of “irbasis”.
[1] H. Shinaoka, J. Otsuki, M. Ohzeki, K. Yoshimi,
PRB 96, 035147 (2017).
[2] J. Otsuki, M. Ohzeki, H. Shinaoka, K. Yoshimi,

Fig.1: The number of expansion coefficientsrequired to represent the Green’s function.

PRE 95, 061302(R) (2017).
[3] H. Shinaoka, J. Otsuki, K. Haule, M. Wallerberger, E. Gull, K. Yoshimi, M. Ohzeki, PRB 97,
205111 (2018).
[4] N. Chikano, J. Otsuki, H. Shinaoka, PRB 98, 035104 (2018).
[5] N. Chikano, K. Yoshimi, J. Otsuki, H. Shinaoka, arXiv:1807.05237.

69

P06

Chiral-Glass Phase in a Ceramic YBa2Cu3O7 Superconductor at Low
Magnetic Field
Hiroyuki Deguchi1, Yuki Kato1, Takanori Hujii1, Masaki Mito1,
Akihiko Hisada2, Kuniyuki Koyama2, and Makoto Hagiwara3
1

Faculty of Engineering, Kyusyu Institute of Technology
Faculty of Science and Technology, Tokushima University
3
Faculty of Engineering and Design, Kyoto Institute of Technology
2

Ceramic cuprate superconductors composed of sub-micron size grains are considered
to be made up of random Josephson-coupled networks of 0 and junctions, and show
successive phase transitions. The first transition is the intragrain superconducting one at
Tc1 and the second is the glass transition among the grains at Tc2 (< Tc1) at which a sharp
peak of nonlinear susceptibility is observed. This critical phenomenon at Tc2 suggests
the onset of the chiral-glass phase predicted by Kawamura [1]. The third transition at Tc3(< Tc2) is the intergrain superconducting one, where the resistivity of the ceramics
becomes zero. Recently, we studied the field dependence of the intergrain transition
temperatures Tc2 and Tc3 in the ceramic YBa2Cu4O8, and obtained the field-temperaturephase diagram of the intergrain ordering [2]. The chiral-glass phase exists in the narrow
field-temperature region between the intergrain superconducting transition and theintragrain superconducting one at low magnetic field.
In this work we investigated the field dependence of Tc2 and Tc3 in the ceramic
YBa2Cu3O7. The field-temperature phase diagram is obtained as shown in Fig.1. The
values and field dependence of Tc3
are different from those of Tc2
below 100 Oe. However, thetransition line for Tc2 coincides thatfor Tc3 in the magnetic field above
100 Oe. The chiral-glass phase
exists in the narrow region below
100 Oe. The results suggest that thechiral-glass ordering takes place at
finite fields in ceramic cuprate
superconductors.

Fig. 1: Phase diagram in the ceramic YBa2Cu3O7[1] H.Kawamura, J.Phys. Soc. Jpn. 79 (2010) 011007.

[2] H.Deguchi et al., Journal. Phys. Conf. Series 871 (2017) 012011.

70

P07

Electronic structure, chemical reactivity and optoelectronic propertiesof 4',5'-dibromo-2',7'-dinitro-3-oxo-3h-spiro[2-benzofuran-1,9'xanthene]-3',6'-diolate from first principles calculations
J. B. F. Fankama,*, G. W. Ejuhb, c, d, J. M. B. NdjakaaaUniversity of Yaounde I, Faculty of Science, Department of Physics, P.O.Box 812
Yaounde, Cameroon.
b
Department of General and Scientific Studies, IUT-FV Bandjoun, University of
Dschang, P.M.B 134, Bandjoun, Cameroonc
CETIC (Centre d’Excellence Africain en Technologies de l’Information et de laCommunication), Université de Yaoundé I, B.P. 8390 Yaoundé, Cameroon.
dDepartment of Electrical and Electronic Engineering, National Higher Polytechnic
Institute, University of Bamenda, P. O. Box 39, Bambili, Cameroon
Abstract
In this article, the molecular structure, electric parameters and optoelectronic
properties of 4',5'-dibromo-2',7'-dinitro-3-oxo-3H-spiro[2-benzofuran-1,9'-xanthene]3',6'-diolate have been theoretically studied. We used the RHF and DFT (PBE1PBE,
MPW1PW91, B3PW91 and B3LYP) approach to calculate the optimized parameters,
molecular structure, electric parameters and optoelectronic properties of the tilted
compound with the cc-pVDZ basis set. In addition, we calculated the chemical
reactivity to show the stability of the molecule. As for the titled compound, the LUMO–
HOMO energy gap value of RHF is the largest, B3LYP is the smallest, indicating theformer which is slightly stable than the later. The larger the HOMO-LUMO energy gap,
the harder and more stable (less reactive) the compounds. The lowest value (3.78eV) is
shown in B3LYP is most stable and the highest value (8.81eV) in RHF is least stable.
The energies of frontier orbitals are used to determine several chemical reactivity
parameters as a measure of relative stability and reactivity. The effect of correlation
decreased the value of HOMO-LUMO energy, chemical hardness and increased the71electronic chemical potential and electrophilicity. Our results suggest that this molecule
have potential applications as linear and nonlinear optical materials. Due to the large
hyperpolarizability of this molecule, we think that these molecules have potential
applications in the ﬁeld of optoelectronic and can be a promising material for opticallimiting applications.
Keywords: electronic structure; chemical reactivity; optoelectronic properties;
RHF and DFT; and optical limiting applications.

72

P08

Singular I-V characteristics in the intermediate state of intergrainordering of a ceramic superconductor system found by a newmeasuring technique
Makoto Hagiwara1, Yôsuke Mutobe1, Yuusuke Fujii1, Shinzi Morishita1,
Kuniyuki Koyama2 and Hiroyuki Deguchi3
1

2

Kyoto Institute of Technology
Faculty of Science and Technology, Tokushima University
3
Faculty of Engineering, Kyusyu Institute of Technology

Superconductive ceramics in which fine grains are bonded through each weak junction
can show intergrain ordering as for phase angle difference between the adjacent grains.
Thus, nature of the intermediate state of such weak junction network systems attracts
physical interests, for it is considered to be neither normal-conductive nor
super-conductive. Actually, anomaly of nonlinear resistivity has been found in theintermediate region of ceramic YBaCuO (Y124) in our preceding studies. However,origin of the anomaly had not been well discussed, since the intrinsic I-V feature was
hard to be clarified. This problem is chiefly due to temperature modulation effects
caused by sweeping applied current for the measurements.
Then in our work, several measurement techniques have been devised to solve thisregion of Josephson tunneling characteristics with

Responce Voltage V / Vexperimental problem. The basis (named SSPD) is that the pulsed applied current isused and its height is swept along sinusoidal curve across zero. Further ingenuity is thatthe pulse sequence is arranged alternatingly trace sine and cosine waves (named CPmode), which keeps the average electric power by the applied current constant. Utilizingthese methods, intrinsic I-V characteristic of Y124 ceramics has been searched at thetemperature region of the intermediate state of the
0.6
intergrain ordering.
39 K
0.4
CP modeFigure 1 shows the I-V curve at 39 K revealed by
0.2
SSPD and CP mode observation. We find the0.0
nonlinear nature is confined to the vicinity of theorigin. The curve appears to trace dissipative -0.2
-0.4

threshold voltage 70 nV, though supercurrent
region does not appear in the sweep cycle. Thus -0.6-5 -4 -3 -2 -1 0 1 2 3 4 5Applied Current I /mA
found nature is considered to be essential for the
Fig. 1: Voltage (V ) vs. current (I )fluctuated intermediate state.
curve for ceramic YBaCuO at 39 K.

73

P09

Dynamical topological quantum phase transitions in nonintegrable
modelsImre Hagymási1*, Claudius Hubig2 and Ulrich Schollwöck1
1. Department of Physics, Arnold Sommerfeld Center for Theoretical Physics (ASC),
Fakultät für Physik, Ludwig-Maximilians-Universität München, D-80333 München,
Germany
2. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748
Garching, Germany

We consider sudden quenches across quantum phase transitions in the spin-1 XXZ
model with single-ion-type anisotropy, where the initial state is a symmetry-protected
topological phase and the ground state of the final Hamiltonian has no topological order.
We demonstrate that dynamical phase transitions may occur during these quenches thatare identified by nonanalyticities in the rate function for the return probability. In
addition, we show that the temporal behavior of the string order parameter is intimately
related to the subsequent dynamical phase transitions. Using the tools of quantum
information we reveal the intrinsic entanglement structure of the time-evolved statesand point out that in certain cases the dynamical quantum phase transitions areaccompanied by enhanced two-site entanglement.

74

P10

Study on Harmonic Response and Phonon Dispersion with Inelastic
X-Ray Scattering in the Geometrically Frustrated Iridate Ca5Ir3O12
H. Hanate1, K. Matsuhira1, K. Nakamura1, S. Haneta1, T. Hasegawa2,and S. Tsutsui3
Kyushu Inst. of Tech.1, GSIAS Hiroshima Univ.2, JASRI/SPring-83
Ca5Ir3O12 has a hexagonal structure with noncentrosymmetric space group of 𝑃6̅2𝑚(No. 189). In the structure, 1D chains of the edge-sharing IrO6 octahedra form triangularlattices in the c-plane, as shown in Fig. 1. The average valence of Ir ions in Ca5Ir3O12 is+4.67, so Ir4+ and Ir5+ exist in a ratio of 1 : 2. This situation can lead to the geometricalfrustration of charge on both the triangular lattice in c-plane and 1D chains along the caxis [1]. It is reported that Ca5Ir3O12 exhibits an antiferromagnetic below 7.8 K and asecond-order phase transition at 105 K [1]. The origin of this phase transition at 105 K is
not clear at present; this is "hidden order" [1]. Recently, nonlinear electrical conductivity
along the c-axis in a single crystal of Ca5Ir3O12 is discovered even above 105 K [2].
We performed the harmonic voltage response experiments by application of sine-wave
current in order to investigate the non-linear electrical conductivity in detail. In addition,
we measured the phonon dispersion by using inelastic X-ray scattering experiments at
SPring-8 to reveal the origin of hidden order. We found that the phonon dispersion
observed at room temperature is in good agreement with the result by GGA calculations
with spin orbit interaction. We will report these results in this presentation.

Fig. 1 Crystal structure of Ca5Ir3O12 [2][1] M. Wakeshima, et. al., Solid State Commun., 125, 311 (2003).
[2] K. Matsuhira, et. al., J. Phys. Soc. Jpn. 87, 013703 (2018).

75

P11

Tensor network approach to non-equilibrium systems
Kenji Harada
Graduate School of Informatics, Kyoto University, Kyoto 606-8501, Japan
Recently, new numerical tools based on a tensor network representation is extensively
developed for strongly correlated systems. Using them, we get many new results forquantum and classical phases of equilibrium systems. A tensor network represents
informational correlations in a system. Therefore, it gives us a new informational point ofview for a macroscopic phase of a system. However, the application of tensor network fora non-equilibrium system is not so much, and we have little understanding of theinformational aspect of a macroscopic non-equilibrium system.
Here, we show new application of tensor networks for some non-equilibrium models.
For example, we focus on a (1+1)-dimensional directed percolation (DP), a canonicalmodel of a non-equilibrium continuous transition to a phase dominated by a single special
state called the "absorbing" state. Using a tensor network scheme, we numericallycalculate the time evolution of state probability distribution of DP. We find a universal
relaxation of Renyi entropy at the absorbing phase transition point and a new singularity
in the active phase where the second-order Renyi entropy has a cusp and the dynamical
behavior of entanglement entropy changes from asymptotically-complete
disentanglement to finite entanglement. We confirm that the absorbing state, though its
occurrence is exponentially rare in the active phase, is responsible for these phenomena.
This interpretation provides us with a unified understanding of time-evolution of the
Renyi entropy at the critical point as well as in the active phase.
[1] Kenji Harada and Naoki Kawashima, “Entropy governed by the absorbing state of directed

Fig. 1: The (1+1)-dimensional directed percolation at a non-equilibrium critical point.
percolation”, axXiv:1902.10479.

76

P12

Raman scattering investigation on geometrically frustrated iridate
Ca5Ir3O12
Takumi Hasegawa1, Wataru Yoshida2, Kazuma Nakamura2, Norio Ogita1,and Kazuyuki Matsuhira2
1

Graduate School of Integrated Arts and Sciences, Hiroshima University,
Higashi-Hiroshima 739-8521, Japan.
2
Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan.
Iridium oxides have attracted much attention as a strongly correlated electron systemwith strong spin-orbit interaction (SOI). The d-orbitals of Ir are divided into Jeff=3/2 and
Jeff=1/2 states under strong SOI. Ca5Ir3O12 has the intermediate valence Ir14/3+ ions, if
we assume Ca2+ and O2- ions. The 3 Jeff=1/2 bands will be filled by one 5d electron.
This will lead to metallic state, which is also obtained from the ab initio DFT
calculation [1]. However, the conductivity is insulative even at room temperature [2]. In
addition, Ca5Ir3O12 indicates a second order transition at Ts = 105 K; Ca5Ir3O12 showsmagnetic ordering at 7.8 K [2]. The transition mechanism at Ts is remains unresolved;
powder X-ray and neutron diffraction experiments have not detected any structuralchange at Ts until now [2]. To clarify the transition mechanism at Ts, we performed
Raman scattering measurements on Ca5Ir3O12 [3].
Ca5Ir3O12 has a hexagonal structure with noncentrosymmetric space group P-62m. The
Ir-O chains along c-axis arrange on triangular lattice separated by Ca ions [1]. Raman
active modes are 6A1’ + 13E’ + 6E’’. Among them, we have observed 6A1’ + 9E’ + 5E’’at room temperature. Below Ts, new 23 peaks have been observed. This is a clear
evidence of the structural change at Ts. This large number of new peaks indicates anappearance of a superlattice structure. The polarization dependence below Ts can beanalyzed under -6 point group. Therefore, the -6 symmetry will conserve at thetransition. Moreover, we have observed at least one new mode as a broad and weak
peak from higher temperature than Ts. This suggests a possible local distortion around Irions above Ts, relevant to a mixed valence states of the Ir ions.
[1] K. Matsuhira, K. Nakamura, Y. Yasukuni, Y. Yoshimoto, D. Hirai, and Z. Hiroi, J. Phys. Soc.
Jpn. 87, 013703 (2018).
[2] M. Wakeshima, N. Taira, Y. Hinatsu, and Y. Ishii, Solid State Commun., 125, 311 (2003).
[3] T. Hasegawa, W. Yoshida, K. Nakamura, N. Ogita, and K. Matsuhira, submitted to J. Phys.
Soc. Jpn.

77

P13

Pressure-tuned velocity renormalization and excitonic fluctuations of
2D massless Dirac fermions in an organic conductor
M. Hirata1, K. Ishikawa2, D. Liu2, D. Ohoki3, G. Matsuno3,
A. Kobayashi3, K. Miyagawa2, M. Tamura4, C. Berthier5, K. Kanoda2
1

Institue for Materials Research, Tohoku University, 2University of Tokyo,
3
Nagoya University, 4Tokyo University of Science, 5CNRS-LNCMI, UGA

The discovery of graphene and topological materials constitutes a novel
paradigm in the research of condensed matter not only for their relativistic or topologicalproperties, but also for their potential link to the collective wisdom of strongly correlated
electron systems. The 2D massless Dirac fermions in the pressurized organic conductor

-(BEDT-TTF)2I3 (being charge neutral due to stoichiometry) [1] provide an excellent
testing ground in this regard, bringing the physics of short-range and long-range parts of
the Coulomb interaction to the table; at low pressure (P) a metal-insulator transition withstripe-type charge ordering takes place in which the nearest-neighbour repulsive
interaction plays a decisive role [2]. At high P, by contrast, the transition ceases above a
critical value PC ≃ 1.0 GPa, but instead an anomalous logarithmic increase of the Fermi
velocity and excitonic fluctuations show up due to the long-rang part of the interaction [35] that is unscreened at the Dirac points, where the carrier density is vanishingly small.
In this talk we will report on our NMR results at a range of P in -(BEDTTTF)2I3 coupled with supportive theoretical analyses by means of renormalizationgroup (RG) techniques and a gap equation based on a ladder-type diagram. Focusing on
the spin-lattice relaxation rate 1/T1 that is sensitive to the thermal excitations at the
Dirac points, we show that a remarkable suppression of spin excitations develops withineach Dirac cone as P is reduced towards PC. At low temperature the additional upturn of
1/T1, previously reported at high P [5], shifts to a higher temperature upon decreasing P.
These data are consistent with our numerical analyses signalling amplification of the log
velocity renormalization and the excitonic fluctuations by the reduction of P. Probablescenarios and relevant calculations will be discussed and compared with the results.
[1] S. Katayama, A. Kobayashi and Y. Suzumura, J. Phys. Soc. Jpn. 75, 054705 (2006)[2] H. Seo J. Phys. Soc. Jpn. 69, 805 (2000)[3] H. Isobe and N. Nagaosa, J. Phys. Soc. Jpn. 81, 113704 (2012)[4] M. Hirata et al, Nat. Commun. 7, 12666 (2016)[5] M. Hirata et al, Science 358, 1403 (2017)

78

P14

Effective Hamiltonian for cuprate superconductors derived frommulti-scale ab initio scheme with level renormalization
Motoaki Hirayama1, Takahiro Misawa2, Takahiro Ohgoe3, Youhei Yamaji3,and Masatoshi Imada3
1) RIKEN Center for Emergent Matter Science, 2) Institute for Solid State Physics,
University of Tokyo, 3) Department of Applied Physics, University of Tokyo
Mechanism of high temperature superconductivity in copper oxide superconductors discovered
more than thirty years ago is still under active debates. One of the reasons of the controversies issevere competitions of completely different orders, particularly, d-wave superconductivity,
antiferromagnetism and charge inhomogeneous states, while they are still controversial.
Therefore, more quantitative ab initio studies are needed based on the realistic parametrization of
the cuprate superconductors to reach conclusive, and quantitative understanding of themechanism.
In this study, we propose a more accurate and realistic description of the low-energy effective
Hamiltonian based on the multi-scale ab initio scheme for correlated electrons (MACE). We
derive three-types (three-band, two-band and one-band) of effective Hamiltonians for
HgBa2CuO4 andthree-band effective Hamiltonian for La2CuO4 by improving theconstrained-GW approximation combined with the self-interaction correction (cGW-SIC)[1].
The improvement for the treatment of the interband Hartree energy turns out to be crucially
important, because the solution of the present improved Hamiltonian shows excellent agreementwith the experimental results, for instance, for the charge gap (2 eV) and antiferromagnetic
ordered moment (0.6 µB) of the mother compound of La2CuO4, in sharp contrast to the estimatesby the previous Hamiltonian, 4.5 eV and 0.8 µB, respectively. To our knowledge, this is the firstsimultaneous and quantitative reproduction of these quantities by ab initio methods without
introducing adjustable parameters. We also predict that the Mott gap and the magnetic orderedmoment for HgBa2CuO4 is roughly 0.7 eV and 0.4 µB, respectively, if the mother compound
becomes available, indicating weaker electron correlations than La2CuO4. The obtained
Hamiltonians will serve to further clarify the electronic structures of these copper oxide
superconductors and to elucidate the superconducting mechanism in an ab initio fashion.
[1] M. Hirayama, Y. Yamaji, T. Misawa, and M. Imada, Phys. Rev. B 98, 134501 (2018).
[2] M. Hirayama, T. Misawa, T. Ohgoe, Y. Yamaji, and M. Imada, arxiv:1901.00763

79

P15

Skyrmion formation and enhanced emergent electrodynamics incentrosymmetric magnets
Max Hirschberger1*, Taro Nakajima1*, Shang Gao1, Licong Peng1, Akiko
Kikkawa1, Takashi Kurumaji1‡, Markus Kriener1, Yuichi Yamasaki2,3, Hajime
Sagayama4, Hironori Nakao4, Kazuki Ohishi5, Kazuhisa Kakurai5, Yasujiro
Taguchi1, Xiuzhen Yu1, Taka-hisa Arima1,6, and Yoshinori Tokura1,7
1

RIKEN Center for Emergent Matter Science (CEMS),Wako 351-0198, Japan.
Research and Services Division of Materials Data and Integrated System (MaDIS), National
Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan
3
PRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan
4
Institute of Materials Structure Science, High Energy Accelerator Research Organization,
Tsukuba, Ibaraki 305-0801, Japan
5
Neutron Science and Technology Center, Comprehensive Research Organization for Scienceand Society (CROSS), Tokai, Ibaraki 319-1106, Japan
6
Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561,
Japan
7
Department of Applied Physics, University of Tokyo, Bunkyo-ku 113-8656, Japan*: equal contribution
2

Research on magnetic skyrmions, which are nanometric whirls in non-collinear magnets, has long
been focused on non-centrosymmetric compounds (such as the B20 family) or interfaces, where
Dzyaloshinskii-Moriya interactions favor twisted spins structures with fixed helicity.
We report on our recent experimental efforts to study skyrmion formation and associated transport
responses in centrosymmetric magnets, where DM interactions are absent or cancel out globally, and where
left- and right-handed spirals are nearly degenerate. In the Heisenberg systems Gd2PdSi3 and Gd3Ru4Al12,
spiral spin structures are stabilized by RKKY interactions. In applied magnetic field, a skyrmion state canbe realized in these systems, as confirmed by resonant elastic x-ray scattering and real-space imaging in atransmission electron microscope. The size of a single skyrmion is typically smaller than 3 nm, resulting ina giant emergent magnetic field detectable by means of the topological Hall and Nernst effects, which haverecently been observed in our laboratory.

80

P16

Combining density functional theory with dynamical mean-field
theory: Applications to the correlated actinide materials
Li Huang
Institute of Materials, China Academy of Engineering Physics
The combination of density functional theory and dynamical mean-field theory (dubbed
as DFT + DMFT) is one of the most powerful and successful method that everestablished to study the physical properties of strongly correlated materials. In theframework of the DFT + DMFT method, the band picture is provided by the classic
density functional theory, while the electronic correlation effect is captured by anon-perturbative many-body approach, namely the dynamical mean-field theory. In thistalk, I will briefly introduce the history, basic ideas, pros and cons of the DFT + DMFT
method. And then I will present some selected DFT + DMFT results that my group has
made in recent years about the unusual properties of some correlated actinide materials,
including pressure-driven insulator-metal transition in cubic UO2, magnetic moment
puzzle in CmO2, and valence state transition in Cf.
References[1] Li Huang and Haiyan Lu, Phys. Rev. B 99, 045122 (2019)[2] Li Huang and Haiyan Lu, Phys. Rev. B 99, 045109 (2019)[3] Haiyan Lu and Li Huang, J. Phys.: Condensed Matter 30, 395601 (2018)[4] Li Huang, Yilin Wang, and Philipp Werner, EPL 119, 57007 (2017)

81

P17

Laser-controlled superconductivity in correlated electron systems
K. Ido1, T. Ohgoe2, and M. Imada2
Institute for Solid State Physics, The University of Tokyo, Japan
Department of Applied Physics, The University of Tokyo, Japan
Recently, laser-controlled superconductivity (SC) has been studied extensively to go
beyond the limit of superconducting critical temperature Tc in equilibrium. An approachis a selective excitation of phonon modes of correlated electron materials by strong laser
irradiation. Surprisingly, indication of superconductivity far above Tc has been observedin cuprate superconductors[1]. However, this approach is a specific one which depends
on the detail of phonon modes of the material.
In this study, we propose an alternative and more general way to enhance SC in acorrelated electron system without lattice degrees of freedoms. To achieve our goal, we
simulated nonequilibrium dynamics in a hole-doped Hubbard model with intermediate
couplings under laser irradiation by using a many-variable variational Monte Carlo
method in and out of equilibrium[2,3]. For the ground states, it was shown that SC grows
with increasing the on-site interaction under the charge uniform condition, but it
severely competes with charge inhomogeneity in the strong coupling region [4,5].
When a charge uniform ground state is used as the initial state, we found that the SC
can be enhanced by irradiating strong laser with high frequency [6]. This enhancement iscaused owing to the following mechanisms. First, the effective interaction between
carriers is enhanced by the dynamical localization mechanism, which drives the system
into strong coupling regions. Second, the irradiation allows uniform strong SC
dynamically stabilized without deteriorating into equilibrium inhomogeneities thatsuppress SC. We also show the results when lasers are irradiated to an inhomogeneousstate. We found that the inhomogeneity dynamically melts by the same laser irradiation asthat for the uniform state, and thus it causes the resultant enhancement of the SC.
References[1] D. Fausti et al., Science 331, 189 (2011), S. Kaiser et al., Phys. Rev. B 89, 184516 (2014).
[2] D. Tahara and M. Imada, J. Phys. Soc. Jpn. 77, 114701(2008).
[3] K. Ido, T. Ohgoe, and M. Imada, Phys. Rev. B 92, 245106 (2015).
[4] T. Misawa and M. Imada, Phys. Rev. B 90, 115137 (2014).
[5] K. Ido, T. Ohgoe, and M. Imada, Phys. Rev. B 97, 045139 (2018).
[6] K. Ido, T. Ohgoe, and M. Imada, Sci. Adv. 3, e1700718(2017).

82

P18

Mechanism of unconventional full-gap superconductivity
in Kondo lattice with semi-metallic conduction bands
Shoma Iimura1, Motoaki Hirayama2, and Shintaro Hoshino1
Department of Physics, Saitama University, JapanRIKEN Center for Emergent Matter Science (CEMS), Japan1

2

Non-local and anisotropic Cooper pairings are usually favored in heavy-electronmaterials with localized f-electron. On the other hand, the full-gap nature of thesuperconducting states in CeCu2Si2 [1] and UBe13 [2] have been revealed by the specificheat measurements in a rotating magnetic-field. Therefore, it is desirable to propose a
new mechanism of full-gap superconductivity that is specific to heavy-electron materials.
Here we theoretically study the Kondo lattice with semi-metallic conduction bands,
where conduction electrons and holes compete in screening localized f-moment. Basedon the mean-field approach, we reveal that the quantum-mechanical superposition
between conduction electron and hole arises from the formation of Kondo-singlet states(Fig.1). The resultant ground state spontaneously breaks gauge symmetry to make thesystem superconducting. The superconducting state is characterized by the composite pair
amplitude, which describes theformation of the three-body boundstate composed of the conducting
electron, hole and the localized spin
moment [3]. We have demonstrated
here that the semi-metallic conduction
bands with electron and hole Fermi
surfaces are closely related to thecomposite pairing. In addition, we
discuss the possible application to real Figure 1. Conceptual picture of the mechanism ofheavy-electron materials.
superconductivity in Kondo lattice with semi-metallic
conduction bands.

References[1] S. Kittaka, Y. Aoki, Y. Shimura, T. Sakakibara, S. Seiro, C. Geibel, F. Steglich, H. Ikeda, andK. Machida, Phys. Rev. Lett. 112, 067002 (2014).
[2] Y. Shimizu, S. Kittaka, T. Sakakibara, Y. Haga, E. Yamamoto, H. Amitsuka, Y. Tsutsumi, andK. Machida, Phys. Rev. Lett. 114, 147002 (2015).
[3] P. Coleman, A. M. Tsvelik, N. Andrei, and H. Y. Kee, Phys. Rev. B 60, 3608 (1999).

83

P19

Search for excitonic condensation in LaCoO3 induced at
ultrahigh magnetic fields
A. Ikeda, Y. H. Matsuda, D. Nakamura, K. Sato2

2

Institute for Solid State Physics, Univ. of Tokyo, Kashiwaoha, Japan
Ibaraki College, National Institute of Technology, Hitachinaka, Japan

In a cobaltite, LaCoO3, the spin-state degree of freedom of Co+3 (d6) varies from high(HS), intermediate and low spin states (LS) with S = 0, 1, 2, respectively, due to thebalance of Hund's coupling and crystal field splitting. A first order magnetic transition
induced by magnetic field at 60 T is reported [1] from a non-magnetic ground state to amagnetized state, whose origin is still unclear. By constructing B-T phase diagram, werecently clarified that the magnetic phase is divided into two phases separated by thetemperature of ~30 K [2].
Theories argue that the field induced magnetic phase may be originated in the excitonic
condensation. The exotic phase appears in cobaltites as a result of the spontaneous
hybridization of LS and HS states [3-5]. In excitonic condensation, the multipole order ofspin and orbitals are realized the interference between different spin states, whichsometimes involves nematicity of spin and orbitals.
In the present study, we have investigated the high magnetic field phase diagram of
LaCoO3 with magnetostriction measurements. We have developed a high speed
magnetostriction measurement system for detecting the spin-state evolution of LaCoO3[6]. So far, we have confirmed that the B-T phase diagram of LaCoO3 constructed by themagnetization measurement is complemented by the magnetostriction measurement.
[1] K. Sato, A. Matsuo, K. Kindo, Y. Kobayashi, and K. Asai, J. Phys. Soc. Jpn. 78, 093702 (2009).
[2] A. Ikeda, et al., Phys. Rev. B 93, 220401(R) (2016).
[3] T. Tatsuno, E. Mizoguchi, J. Nasu, M. Naka, and S. Ishihara, J. Phys. Soc. Jpn. 85, 083706 (2016).
[4] A. Sotnikov and J. Kuneˇs, Scientific Reports 6, 30510 (2016).
[5] T. Kaneko and Y. Ohta, Phys. Rev. B 90, 245144 (2014).
[6] A. Ikeda, et al., Rev. Sci. Instrum. 88, 083906 (2017).

84

P20

Development of Orbital-Free Density Functional Theory with Machine
Learning
Fumihiro Imoto
Department of Applied Physics, The University of Tokyo, Japan
Orbital-free density functional theory (OFDFT) circumvents the use of N
single-particle orbitals for the expression of the non-interacting kinetic energyfunctional, where N is the number of electrons in the system. The explicit formulation of
the kinetic energy term makes it possible to simulate systems that consists of more than
105 atoms with the computational cost of O(N). However, such an explicit formula withdesired accuracy, especially in non-metallic systems, has been a long-standing question.
In this work, we construct a semi-local kinetic energy density functional (KEDF)
for semiconductors via the multilayer neural network (NN), which is widely used in thecontext of supervised machine learning. We adopt the electron density and its gradients
up to the second order as the descriptor for the NN, whereas the kinetic energy density
is taken as the response variable. The training data has been adopted from diamond Si
and diamond C. After the training, we have found that the NN KEDF that depends on
up to the second-order density gradient can approximate the kinetic energy density
better than any conventional KEDFs in diamond Si, diamond C, and zincblende SiC.
We also perform OFDFT electron density optimization using the NN KEDF and
its functional derivative. We have developed a real-space OFDFT (RS-OFDFT) code,
which is an MPI- and OpenMP-paralleled real-space finite-difference code. The
structural properties obtained by the NN KEDF have shown fairly good agreement withthe experimental values in the case of diamond Si, diamond C, and zincblende SiC.
However, the convergence of OFDFT density optimization is extremely slow, especiallywhen the NN KEDF is employed. We will present several algorithms to solve such a
convergence problem along with numerical results obtained by implementing the new
optimization algorithms in the RS-OFDFT code.

85

P21

Optical rectification of spin current in magnetic insulators
Hiroaki Ishizuka1 and Masahiro Sato2
1

Department of Applied Physics, University of Tokyo
2
Department of Physics, Ibaraki University

Insulator spintronics has considerable advantages over the metallic counterpart. Spincurrent in the insulators are carried by chargeless magnetic excitations; it is decoupledfrom charge current and free from Ohmic loss. In addition, the relaxation time of angularmomentum is typically longer in the magnetic insulators. Owing to these potential
advantages, insulator spintronics has become an active field in the spintronics.
Despite the increasing interest, the method of generating spin current, especially, withelectromagnetic waves has been less explored. Unlike the electronic excitations, thechargeless magnetic excitations do not couple to scalar potential. Therefore, the control ofspin current by the electromagnetic fields is generally difficult. In this talk, we propose anovel method for generating spin current using nonlinear optical effects. Using anonlinear response theory, we study the generation of spin current in antiferromagnets.
We show that the three types of spin-light couplings induce a dc spin current in a spin
chain with a spin-liquid ground state: inverse Dzyaloshinskii-Moriya, Zeeman, andmagneto-striction couplings [1]. We find that the dc spin current appears in a model inwhich the total spin angular momentum is conserved. This is in sharp contrast to theestablished methods such as spin pumping in insulators and optical spin-current
generation in metals. Our results also imply that the spin-orbit interaction is unnecessary
for the generation of spin current [2]. This is a potential advantage for application because
the spin-orbit interaction tends to shorten the relaxation time of spin angular momentum.
[1] H. Ishizuka and M. Sato,
preprint (arXiv:1811.11515)[2] H. Ishizuka and M. Sato, in preparation.

Fig.

1:

Schematic

noncentrosymmetricfigureofthemagnets

weconsidered. A linearly-polarized THz
light is applied to the magnetic insulator
to generate spin current.

86

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By using pump-probe spectroscopy, the light-induced superconductivity (SC) havebeen proposed in 2D systems such as La1.675Eu0.2Sr0.125CuO4 [1] or YBa2Cu3O6.45 [2]above the transition temperature. Also in the 3D material, K3C60, whose SC transition
temperature is Tc=19.8K, it is suggested that light-induced SC can arise at around 100K
[3]. These experiments have attracted much attention recent years. As for the theoreticalresearches, it is discussed that the induced photons cause the change in the effective
interaction among electrons through the change in phonons by lights, and also the direct
effect to the electronic systems and so on [4-7].
In this study, we study the superconducting fluctuation effects in photo-excited systems
above the transition temperature. We have analyzed the pump-probe spectroscopy
theoretically by using time dependent Ginzburg-Landau theory with a simple setup shownin Fig.1. The SC fluctuation and current, whichis induced
frompump-lights, show
Singlepump-shot
model
Double pu(a) a short-time behavior,(b)
exponential decay in long-time limit. For
by contrast, we have (c)
A(t)
A(t)
revealed that they behave in a non-trivial
manner: 2⇡/⌦
power-law E(t)
pump
pump
probe
A0
and logarithmic decays are found dependingonthespatialA0prE1
dimensions. The non-equilibrium component Aof the current,
t1
0
1
t
probe
which derives from probe-lights has always negative
2⇡/⌦
2⇡/⌦
contribution after injecting the lights in each spatial
pump
t
t1
0 t0
t
0
1
dimension. In this poster presentation, we will discuss theA1
Fig 1: A sketch for the modelchange of the SC fluctuation, current and paraconductivity asof the electric field considered
functions of evolving time and pump-light amplitude.
<

P22

Nature of superconducting fluctuation in photo-excited systems
Ryuta Iwazaki1, Naoto Tsuji2 and Shintaro Hoshino1
1Department of Physics, Saitama University, Japan
2
RIKEN Center for Emergent Matter Science (CEMS), Japan[1] D. Fausti, et al., Science 331(6014), 189-191 (2011).

[2] S. Kaiser, et al., Phys. Rev. B 89, 184516 (2014).

[6] D. M. Kennes and A. J. Millis, Phys. Rev. B 96, 064507 (2017).

[7] Y. Lemonik and A. Mitra, Phys. Rev. B 98, 214514 (2018).

87

in our theory 𝐸" and Ωmeans the intensity of theelectric field and its frequencyof the probe-lights[3] M. Mitrano, et al., Nature 530, 461-464 (2016).

[4] D. M. Kennes, E. Y. Willner, D. R. Reichman and A. J. Millis, Nat. Phys. 13, 479 (2017).

[5] Y. Murakami, N. Tsuji, M. Ecktein and P. Werner, Phys. Rev. B 96, 045125 (2017).

P23

Chromium breathing pyrochlores as a showcase for a variety of
pyrochlore Hamiltonians
H. O. Jeschke1, P. Ghosh2, Y. Iqbal2, T. Müller3, R. Thomale3, J. Reuther4,
M. J. P. Gingras5
1

Research Institute for Interdisciplinary Science, Okayama University
2
Department of Physics, Indian Institute of Technology Madras
3
Institute for Theoretical Physics and Astrophysics, Julius-Maximilians-Universität
Würzburg
4
Fachbereich Physik, Freie Universität Berlin
5Department of Physics and Astronomy, University of Waterloo
We investigate all six structurally characterized chromium-based breathing pyrochlores
using a combination of density functional theory and pseudofermion functional
renormalization group calculations. We show that the distinct magnetic behavior of thematerials LiInCr4O8, LiGaCr4O8, LiInCr4S8, LiGaCr4S8, CuInCr4S8 and CuInCr4Se8 canbe matched one-to-one to their chemical composition, namely, oxides, sulfides and theselenide, with each realizing very different pyrochlore Hamiltonians. While the oxides
form breathing pyrochlore antiferromagnets displaying correlations reminiscent of a
perturbed Coulomb spin ice phase, in the sulfides, the ground state degeneracy exhibits
a dimensional reduction leading to line-like degeneracies in momentum space
originating from an effective face centered cubic lattice mapping in the low temperature
regime. In the selenide we reveal the presence of an approximate spiral spin liquidcharacterized by a sphere-like degenerate manifold of states in reciprocal space. Thisrich variety of effects is rooted in the subtle interplay of longer-range exchange
couplings, up to third nearest-neighbor on the pyrochlore lattice, and also holds the key
to successfully describing the available experimentally observed magnetic response of
the sulfides and the selenide.

88

P24

Metallic Spin Liquid-like Behavior of LiV2O4
H. Okabe1,2, M. Hiraishi1,2, A. Koda1,2, K. M. Kojima1,2, S. Takeshita1,2,
I. Yamauchi3, Y. Matsushita4, Y. Kuramoto2,5, and R. Kadono1,2
1Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan
2
SOKENDAI, Tsukuba, Ibaraki 305-0801, Japan
3
Graduate School of Science and Engineering, Saga University, Saga 840- 8502, Japan
4
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan
5
Department of Physics, Kobe University, Kobe 657-8501, Japan
Quantum phase transition and associated critical behavior of electronic states has been
a central focus of condensed matter physics in the past decades. In particular, the“metallic spin-liquid” state is attracting much interest as a novel non-Fermi liquid state,
where it is predicted to emerge near the quantum critical point (QCP) next to thespin-glass state. Furthermore, the metallic spin liquid is intensively discussed as a stage
for application of the so-called anti-de-Sitter/conformal field theory (AdS/CFT)
correspondence developed in the quantum field theory of gravity and entropy of black
holes.
In this contribution, we suggest that LiV2O4 spinel would serve as a promising stage
for the understanding the metallic spin liquid [1]. LiV2O4 is known to exhibit heavy
fermion-like behavior below a characteristic temperature TK~20 K, while it preserves aparamagnetic state down to ~10-2 K due to geometrical frustration. We show that the
dynamical spin susceptibility c(q,ω) in LiV2O4 exhibits anomalous duality which ismodeled as a sum of itinerant (cF) and local (cL) components, and that the behavior ofcL(q,ω) inferred from muon experiment is in line with that theoretically predicted forthe metallic spin liquid [2]. The anomaly coexists with the marginal Fermi-liquidbehavior inferred from the -lnT dependence of the electronic specific heat over a wide
temperature range below TK. We argue that such unusual properties of LiV2O4 can be
attributed to the putative metallic spin liquid state emerging near QCP between spinglass and Fermi liquid states.

References[1] H. Okabe, M. Hiraishi, A. Koda, K. M. Kojima, S. Takeshita, I. Yamauchi, Y. Matsushita, Y.
Kuramoto, and R. Kadono, Phys. Rev. B. 99, 041113(R) (2019).
[2] S. Burdin, D. R. Grempel, and A. Georges, Phys. Rev. B 66, 045111 (2002).

89

P25

Tensor network approach to the Kitaev spin liquid
R. Kaneko1, T. Okubo2, H.-Y. Lee1, Y. Yamaji3, N. Kawashima1
1

Institute for Solid State Physics, The University of Tokyo, Japan2
Department of Physics, The University of Tokyo, Japan3
Department of Applied Physics, The University of Tokyo, JapanThe Kitaev spin liquid has attracted much attention because of its exact solvability [1]and possible realization in Iridium and ruthenium compounds [2]. However, theeffective models may include additional spin exchange interactions that could
destabilize the spin liquid [3]. General-purpose numerical methods that do not resort to
the Majorana fermion representation are needed for solving such generalized models.
To this end, we investigate the applicability of the two-dimensional tensor-network
method in the Kitaev honeycomb systems. We first demonstrate the usefulness of thetensor product state (TPS) or the projected entangled-pair state (PEPS), which is
described in the spin degrees of freedom, in the simplest isotropic ferromagnetic Kitaev
model. We construct an optimal TPS in a small bond dimension (D=2) according to therequired physical symmetries as well as the vortex-free configuration of the Z2 gauge
field. This ansatz state already exhibits the criticality of the exact Kitaev spin liquid [4].
In contrast to the previous study [5], the lower-cost imaginary time evolution (simple
update) is found to be enough to obtain accurate energy for D=4. The energy is further
improved as D is increased.
We then make use of this ansatz for more generalized Kitaev models that contain theinteraction anisotropy, the Heisenberg exchange interaction, and the magnetic field. For
all cases, we find the presence of the Kitaev spin liquid phase.
References:[1] A. Kitaev, Ann. Phys. (NY) 321, 2 (2006).
[2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010); J. Chaloupka,
G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 110, 097204 (2013).
[3] X. Liu, T. Berlijn, W.-G. Yin, W. Ku, A. Tsvelik, Y.-J. Kim, H. Gretarsson, Y. Singh, P.
Gegenwart, and J. P. Hill, Phys. Rev. B 83, 220403(R) (2011); Y. Singh and P. Gegenwart, Phys.
Rev. B 82, 064412 (2010); Y. Singh, S. Manni, J. Reuther, T. Berlijn, R. Thomale, W. Ku, S.
Trebst, and P. Gegenwart, Phys. Rev. Lett. 108, 127203 (2012).
[4] H. Y. Lee, R. Kaneko, T. Okubo, and N. Kawashima, arXiv:1901.05786.
[5] J. O. Iregui, P. Corboz, and M. Troyer, Phys. Rev. B 90, 195102 (2014).

90

P26

Liquid dynamics of orbital molecules with long correlation length
in layered LiVS2
N. Katayama1, K. Kojima1, S. Tamura1, S. Hattori1, K. Saitoh2, H. Sawa1
1

2

Department of Physical Science and Engineering, Nagoya University
Institute of Materials and Systems for Sustainability, Nagoya University

Electronic instabilities in transition metal compounds often lead to the complex
arrangement of orbital molecules in a spin singlet state in low temperature. Examples
include dimer in Li2RuO3 [1] and pairs of trimer and tetramer in AlV2O4 [2]. Recent
local structural studies utilizing the pair distribution function (PDF) revealed that thedisordered orbital molecules appear ubiquitously even in high temperature paramagnetic
phases [3]. The central issue to be solved is whether such disordered orbital molecules
exhibit liquid-like dynamics or glass-like statics.
To address this issue, we performed the structural studies of LiVO2 and LiVS2 withtriangular lattices. Although high temperature metallic nature in LiVS2 is significantly
different from the insulating behavior in LiVO2, both compounds enter the nonmagnetic
insulator state in low temperatures [4]. In LiVO2, the low temperature orbital molecule
patterns were conjectured to be trimer by J.B. Goodenough on 1963 [5], although it has
never been crystallographically clarified. Meanwhile, low temperature orbital molecule
patterns remain unclear in LiVS2.
Here, we present that vanadium trimers are crystallographically clarified both in
LiVO2 and LiVS2 in low temperature phases. Remarkably, upon heating higher than thetrimer transition temperature of 314 K in LiVS2, the trimer molecules completely
disappear and, instead, disordered zigzag chain molecules with long correlation lengthof several hundred angstrom appear with fluctuating in the order of msec~sec.
[1] Y. Miura et al., J. Phys. Soc. Jpn. 76 (2007) 033705.
[2] Y. Moribe et al., Phys. Rev. Lett. 96 (2006) 086406.,
A.J. Browne et al., Phys. Rev. Meter. 1 (2017) 052003(R).
[3] S.A.J. Kimber et al., Phys. Rev. B 89 (2014) 081408(R).
A.J. Browne et al., Phys. Rev. Meter. 1 (2017) 052003(R).
K.R. Knox et al., Phys. Rev. B 88 (2013) 174114.
[4] N. Katayama et al., Phys. Rev. Lett. 103 (2009) 146405.
[5] J.B. Goodenough “Magnetism and the Chemical Bond” Interscience and John Wiley, N.Y.
1963 p270.

91

P27

Quantum Critical Phenomena in Heat Transport via a Qubit
Takeo Kato
Institute for Solid State Physics, The University of Tokyo
Quantum critical phenomena (QCP) near zero-temperature second-order phasetransition have been studied for long time in condensed matter physics. It is stillchallenging to realize them in controlled experimental systems, and has recently beenstudied, e.g., via transport properties in multi-channel Kondo quantum dots [1]. In thispresentation, we consider heat transport carried by photons via a quantum two-statesystem (a qubit) [2]. For an appropriate environment (the sub-ohmic reservoir), it is
known that the system undergoes quantum phase transition at a critical system-reservoir
coupling. We discuss QCP in heat transport through temperature dependence of a thermal
conductance [3]. We also propose a superconducting circuit coupled to a flux qubit as afeasible experimental system realizing the sub-ohmic reservoir. We expect that recent

Heat current

Qubit
Reservoir L

Temperaturedevelopment in calorimetry will enable us to observe QCP proposed here in near future.
This work is the collaborated research with Tsuyoshi Yamamoto.

Incoherent tunneling
Quantum criticalCo-tunnelingReservoir R
System-reservoir coupling

Fig. 1: (Left) A schematic of heat transport via a qubit. (Right) The phase diagram of thequbit coupled to the sub-ohmic system.
(Times, 11pt)

References[1] Z. Iftikhar et al., Science 360, 1315 (2018).
[2] T. Yamamoto, M. Kato, T. Kato, and K. Saito, New J. Phys. 20, 093014 (2018).
[3] T. Yamamoto and T. Kato, Phys. Rev. B 20, 245412 (2018).

92

Poster Presentations – Section B

P28

Microscopic Investigation of a phase transition between an insulator
CaCu3Ti4O12 and a metal CaCu3Ru4O12: Cu-NQR in CaCu3Ti4-xRuxO12
H. Kato1, D. Kuno1, H. Sakurai2, T-H. Kao2,3, H-D. Yang3
1Fac. of Science and Technology, Kochi University, Kochi 780-8520, Japan
National Institute for Material Science (NIMS), Tsukuba 305-0044, Japan
3
Dept. of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
2

A-site-ordered cubic perovskite systems, CaCu3B4O12, are regarded as perovskite
systems ABO3 involving periodically-ordered nonmagnetic Ca and Jahn-Teller Cu ionsat A site. For the B = Ti case, CaCu3Ti4O12 is an antiferromagnetic insulator showing an
enormously large dielectric constant [1], although a Ru member, CaCu3Ru4O12, is aparamagnetic metal [2,3,4]. The 3d electron at Cu changes its nature from an insulatingstate to an itinerant. In fact, the complete solid solution system, CaCu3Ti4-xRuxO12(CCTRO), which was first synthesized using a high-pressure technique and its
macroscopic properties were measured by our group, shows an insulator-to-metaltransition with the Ru content x [5]. A Mott transition picture was safely applied, wherea critical line of the two phases is considered to appear around xc ~ 2-3 [5].
We perform a Cu-NQR study on CCTRO, in order to investigate microscopic changeof the Cu-3d electron through the transition. The 63Cu spectrum in CCTRO consists ofseven peaks with their central frequencies unchanged along x. This could be understoodin terms of Cu-site-splitting arising from local coordination of the neighboring B siteions (Ti/Ru), which also implies the used samples are chemically homogeneous. For asample with x > 2.0, 1/T1T of each peak are similar in value at high temperatures andshows a temperature independent behavior, indicating existence of a metallic phase in
the sample. The 1/T1T value at high temperatures is larger as x decreases. This
corresponds to an increase of the density of the state at Fermi surface toward thetransition to an insulating phase, which might coincide with the Mott transition picture.
Moreover, around xc, significant temperature dependence of 1/T1T is seen below ~ 100K,
where the magnitude varies with the measured peak. The origin is discussed in terms of
microscopic coexistence of the metallic and insulating phases.
[1] A. Ramirez, et al., Sol. Sta. Comm., 115 (2000) 217[2] A. Collomb, et al., Phys. Sta. Sol. (a), 41 (1997) 459[3] W. Kobayashi, et al., J. Phys. Soc. Jpn., 73 (2004) 235[4] H. Kato, et al., J. Phys. Soc. Jpn., 78 (2009) 0547078-1-8[5] T. H. Kao, et al., Phys. Rev. B 95 (2017) 1951411-1-9

95

P29

Topological classification of
non-Hermitian insulators and superconductors
Kohei Kawabata1, Ken Shiozaki2, Masahito Ueda1,3, and Masatoshi Sato2
Department of Physics, University of Tokyo
Yukawa Institute for Theoretical Physics, Kyoto University
3
RIKEN Center for Emergent Matter Science (CEMS)
1

2

Non-Hermiticity [1,2] enriches topological phases beyond the existing framework forHermitian topological phases. Whereas unusual features with no Hermitian counterparts
were extensively explored [3-5], a full understanding about the role of symmetry andtopology in non-Hermitian physics has still been elusive. In particular, complete
topological classification of non-Hermitian insulators and superconductors has yet to beestablished, which serves as a non-Hermitian generalization of the 10-fold classificationof Hermitian ones [6,7] according to the Altland-Zirnbauer internal symmetry [8].
Here we develop a general classification theory of non-Hermitian topological phases[9]. We demonstrate that non-Hermiticity ramifies the celebrated Altland-Zirnbauer
symmetry classification for insulators and superconductor, leading to 38-fold symmetry
class instead of the 10-fold one. Moreover, we reveal that two types of energy gaps are
relevant for non-Hermitian systems due to the complex nature of energy spectra. Basedon the 38-fold symmetry and the two types of complex-energy gaps, we complete
classification of non-Hermitian topological phases in arbitrary dimensions andsymmetry classes. Our work establishes a theoretical framework for the fundamental
and comprehensive understanding of non-Hermitian topological phases and paves theway towards uncovering unique phenomena and functionalities that emerge from theinterplay of non-Hermiticity and topology.
[1] C. M. Bender & S. Boettcher, Phys. Rev. Lett. 80, 5243 (1998).
[2] R. El-Ganainy et al., Nat. Phys. 14, 11 (2018).
[3] K. Esaki, M. Sato, K. Hasebe & M. Kohmoto, Phys. Rev. B 84, 205128 (2011).
[4] H. Shen, B. Zhen & L. Fu, Phys. Rev. Lett. 120, 146402 (2018).
[5] KK, S. Higashikawa, Z. Gong, Y. Ashida & M. Ueda, Nat. Commun. 10, 297 (2019).
[6] A. P. Schynder, S. Ryu, A. Furusaki & A. W. W. Ludwig, Phys. Rev. B 78, 195125 (2008).
[7] A. Kitaev, AIP Conf. Proc. 1134, 22 (2009).
[8] A. Altland & M. R. Zirnbauer, Phys. Rev. B 55, 1142 (1997).
[9] KK, K. Shiozaki, M. Ueda & M. Sato, arXiv: 1812.09133.

96

P30

Spin and charge correlations across the extended pseudogap regime in
the half-filled 2d Hubbard model
Author(s): Aaram J. Kim, Fedor Simkovic IV.1, and Evgeny Kozik
Affiliation(s): Department of Physics, King’s College London, Strand, London WC2R
2LS, UK

The Hubbard model at half-filling on the square lattice is known to feature a crossoverfrom metal to antiferromagnetic (AF) insulator via the extended pseudogap regime. We
study spin and charge correlation functions across the pseudogap regime with controlled
accuracy using the diagrammatic Monte Carlo approach and provide reliable benchmarks
for experimental verification with quantum emulators on optical lattices in the currently
accessible temperature range. The charge and spin correlation functions change theircharacter as the system crosses over from metal to AF insulator. Near the AF insulator
regime, we observe the strongly enhanced AF peak in spin correlation and overall
suppressed charge correlations. The charge correlation function, however, changes its
temperature dependence at sizably weaker interaction strength compared to the spincorrelation function. In terms of the energetics, we show that there exists the subregion
within the extended pseudogap regime, where the system gains both the kinetic andpotential energy simultaneously as the temperature decreases.
[1] current address: Centre de Physique Theorique, Ecole Polytechnique, CNRS, Universite
Paris-Saclay, 91128 Palaiseau, France and College de France, 11 place Marcelin Berthelot, 75005
Paris, France.

97

P31

Electronic States in Organic Conductor -(BETS)2I3
Akito Kobayashi1, Daigo Ohki1, Takehiro Tani1, and Kazuyoshi Yoshimi21Dept. of Phys. Nagoya Univ., 2ISSP The Univ. of Tokyo

The two dimensional electron system in the organic conductor -(BEDT-TTF)2I3 has
attracted much attention, since it exhibits a unique metal-insulator transition at T=135K.
The metal and insulator phases have been attributed to the massless Dirac electron
phase with the inversion symmetry and charge order phase due to the inversion
symmetry breaking, respectively [1-3].
The organic conductor -(BETS)2I3, being a related material of -(BEDT-TTF)2I3,
exhibits the characteristic transport and NMR properties similar to the Dirac electron
phase in -(BEDT-TTF)2I3 at T>50K [4,5]. The low temperature phase at T<50K is an
insulator, while the phase transition is unclear. In contrast to -(BEDT-TTF)2I3,
however, the inversion symmetry is not broken in the low temperature phase.
The numerical results of the extended Hückel method [6] and the 1st principle
calculation [7] indicates that -(BETS)2I3 is a semimetal with both electron and hole
pockets. As a candidate of the low temperature phase, a band insulator induced by themean field of the nearest neighbor Coulomb interactions has been proposed [8]. It isalso proposed that the spin-orbit interaction play an important role [9]. Thus theelectronic state of -(BETS)2I3 is still controversial.
In the present study, an extended Hubbard model describing -(BETS)2I3 is derived
using the 1st principle calculation. The electronic state is examined using the mean field
theory and the exact diagonalization.
[1] K. Kajita et al. J. Phys. Soc. Jpn. 83, 072002 (2014).
[2] N. Tajima, M. Tamura, Y. Nishio, K. Kajita, and Y. Iye, J. Phys. Soc. Jpn. 69, 543 (2000).
[3] S. Katayama, A. Kobayashi, and Y. Suzumura, J. Phys. Soc. Jpn. 75, 054705 (2006).
[4] N. Tajima, private communication.
[5] K. Hiraki et al., J. Phys. Soc. Jpn. 80, 014715 (2011).
[6] R. Kondo et al., J. Phys. Soc. Jpn. 78, 114714 (2009).
[7] P. Alemany et al., Phys. Rev. B 85, 195118 (2012).
[8] T. Morinari and Y. Suzumura, J. Phys. Soc. Jpn. 83, 094701 (2014).
[9] S. M. Winter, K. Riedl, and R. Valentí, Phys. Rev. B 95, 060404(R) (2017).

98

P32

Coupled Spin-Charge Fluctuation in the Semimetal Phase of
All-In/All-Out Antiferromagnet Cd2Os2O7
A. Koda1,2, H. Hirose3*, M. Miyazaki1†, H. Okabe1, M. Hiraishi1,
I. Yamauchi1‡, K. M. Kojima1,2§, J. Yamaura,3**, Z. Hiroi3, and R. Kadono1,2
1Muon Science Laboratory and Condensed Matter Research Center, Institute ofMaterials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan2
Department of Materials Structure Science, SOKENDAI, Ibaraki 305-0801, Japan
3
Institute for Solid State Physics, University of Tokyo, Chiba 277- 8581, Japan
We report on a novel electromagnetic f uctuation in Cd2Os2O7 pyrochlore over the temperature
range where the all-in/all-out (AIAO) antiferromagnetic order below TN ≃ 227 K coexists withthe semimetal phase above T∗ ≃ 150 K. Muon depolarization observed for T∗ < T < TN indicatesthat the magnetic f uctuation is induced by the metallic carriers conducting on the AIAO spin
texture. Remarkably, the relevant spin-charge dynamics is hidden to other magnetic probes,
suggesting local character of excitation associated with strong spin-orbit coupling and a peculiar
frequency range of f uctuation (∼107–1010 s−1) uniquely accessible to muon. The onset of such a
coupled spin-charge excitation is presumed to enhance electric resistivity to mimic
metal-insulator (MI) transition at TN, while the true MI transition occurs around T∗ upon openingof the indirect charge gap. We also discuss possibility for the formation of a magnetic polaron-likeband in the semimetal phase.

*

Present address: NIMS, Tsukuba, Ibaraki 305-0003, Japan
Present address: Muroran Institute of Tech., Muroran, Hokkaido 050-8585, Japan‡
Present address: Saga University, Saga 840-8502, Japan§
Present address: TRIUMF, Vancouver, BC V6T2A3, Canada**
Present address: MCES, Tokyo Institute of Technology/IMSS, KEK, Japan†99

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P33

Correlated higher-order topological insulator on kagome lattice
Koji Kudo, Tsuneya Yoshida, and Yasuhiro Hatsugai

Graduate School of Pure and Applied Sciences, University of Tsukuba

The electron-electron correlation gives birth to a variety of nontrivial topologicalphenomena. One of the typical examples is the topological Mott insulator [1,2] hosting
gapless edge spin excitations. Its difference from the conventional topological insulators(TIs) is the absence of the gapless charge excitations at the edge, which is driven by thestrong correlation effects. Two-dimensional higher-order TIs [3], which support gaplesstopological corner states although edge states are gapped, are a new class of topologicalphases. While it is expected that the electron-electron interactions also bring further
diversity to higher-order TIs, the theoretical studies for correlated higher-order TIs aresparse.
In this poster, the correlation effects on the higher-order TIs are investigated by solving
the kagome Hubbard model exhibiting the corner states in the noninteracting case [4] [seeFig. 1 (a)]. We numerically demonstrate that the electron-electron correlation results in
the gapless corner spin excitations. Figure 1 (b) shows the energy spectrum given by theeffective spin Hamiltonian for the open system as a function of tan 𝜙 = 𝑡' /𝑡) , where𝑡' (𝑡) ) is the nearest-neighbor hopping parameter associated with the upward(downward) facing triangles. The result indicates that the ground state is accompanied by
the 8-fold degeneracy for 0 ≤ 𝜙 ≲ 𝜋/4, which suggests the presence of the higher-ordercorner states where the spin excitation is gapless. We also confirm that the spincounterpart of the Z3 Berry phase [5], the topological invariant detecting the upward or
downward trimers pattern, characterizes
theπ correlated
corner-Mott states.
π
π
π
5π
π
0 12 6 4 3 12 2(a)(b) -0.30
0.3 Sz=-14. Sz=-4. Sz=6.
Fig.1 (a) Sketch of kagome

Sz=-11.
Sz=-5.
Sz=-10.
Sz=-9.
Sz=-8.
Sz=-7.
Sz=-6.
Sz=-5.

Sz=-3.
-0.35 Sz=-13.
Sz=-12.
0.4 Sz=-11. Sz=-2.
-0.40
Sz=-1.
-0.45
Sz=-14. Sz=-10.
Sz=-4. Sz=0.
Sz=6.
Sz=-13. Sz=-9.
Sz=-3. Sz=1.
Sz=7.
8-fold0.5-0.50
degeneracy
Sz=-12. Sz=-8.
Sz=-2. Sz=2.
Sz=8.
-0.55
Sz=-11. Sz=-7.
Sz=-1. Sz=3.
Sz=9.
Sz=4.
Sz=-10.
Sz=0.
Sz=10.
0.6 Sz=-6.
-0.60
Sz=5.
Sz=-9. Sz=-5.
Sz=1.
Sz=11.
-0.65
Sz=-14.
Sz=-4.
Sz=6.
Sz=-8.
Sz=2.
Sz=12.
Sz=-13.
Sz=7.
Sz=-7.
Sz=3.
Sz=13.
0.70 Sz=-3.
-0.70
π
π ⇡/4
π
π
Sz=-12.
Sz=-2.
Sz=8.
Sz=-6.
Sz=4.612
4Sz=14.
3[1] D. Pesin and L. Balents, Nature Physics 6, 376 EP (2010). [2] T. Yoshida, R. Peters, S.

Fujimoto, and N. Kawakami, Phys. Rev. Lett. 112, 196404 (2014). [3] W. A. Benalcazar, B. A.

Bernevig, and T. L. Hughes, Science 357, 61 (2017). [4] M. Ezawa, Phys. Rev. Lett. 120, 026801

Sz=-1.
Sz=5.
Sz=0.
Sz=1.
Sz=2.
Sz=3.
Sz=4.
Sz=5.

Sz=9.
ϕ
Sz=10.
Sz=11.
Sz=12.
Sz=13.
Sz=14.

100

Sz=7.
Sz=8.
Sz=9.
Sz=10.
Sz=11.
Sz=12.
Sz=13.
Sz=14.

5 π ⇡/2π
122

latticeandthe(2018). [5] Y. Hatsugai and I. Maruyama, EPL (Europhysics Letters) 95, 20003 (2011).
boundary

condition. (b) Energy spectrum

E/U as function of 𝜙. U is thestrength of the interaction. The

total Sz is expressed by the colorof plots.

P34

Supercurrent generation by spin-twisting itinerant motion of electrons
in a model for cuprate superconductors
Daichi Manabe and Hiroyasu Koizumi
Center for Computational Sciences, University of Tsukuba, Tsukuba, Japan
Superconductivity is a phenomenon in which externally supplied electric current flows
through a system without voltage drop. This current flowing state is dissipationless, thus,
thermodynamically stable. At zero temperature, this will lead to the conclusion that thiscurrent flowing state is energetically stable one, thus, must be one of local energy
minimum states.
In the BCS theory based understanding, the mechanism for the supercurrent generation
is explained using the dc Josephson effect. The Josephson effect occurs in asuperconductor-insulator-superconductor junction (Josephson junction), where theinsulator part is so thin that electrons can tunnel through it. Josephson considered the
Cooper pair tunneling through it; each superconductor in the junction is assumed to be
characterized by the phase conjugate to the Cooper pair number and predicted that thedissipationless current is generated by the difference of the phases of the two
superconductors. Then, regarding superconductor as a collection of Josephson junctions,the supercurrent in a superconductor is explained as due to the gradient of the phase.
However, this explanation does not guarantee the existence of an energy minimum
under the external current feeding condition.
Through the study of high temperature superconductivity in cuprates, a newsupercurrent generation mechanism has been proposed by one of the present authors.
The spin-twisting circular motion of electrons is shown to generate supercurrent. In thepresent work, we further consider this mechanism. We will show that if the Rashba
spin-orbit interaction exists, energy minima occur at nonzero feeding current [1]. We
have found such minima in the system exhibiting spin twisting itinerant motion. The
supercurrent is shown to have the London equation form that exhibits the flux quantum
h/2e.

Reference[1] J. Supercond. Nov. Magn. https://doi.org/10.1007/s10948-018-4977-0

101

P35

Electron Correlation in the Anomalous Muonium in Silicon
Muhamad Nasruddin Manaf 1, Susumu Minami 1, Fumiyuki Ishii 2 andMineo Saito 3
1

Graduate School of Natural Sciences and Technology, Kanazawa University,
Kanazawa 920-1192, Japan
2
Nanomaterials Research Institute, Kanazawa University, Kanazawa 920-1192, Japan
3
Faculty of Mathematics and Physics, Kanazawa University, Kanazawa 920-1192,
Japan
The muon spin resonance (µSR) is an effective tool to determine the dynamics ofhydrogen in materials [1,2]. The most stable atomic configuration of muonium in silicon
is located at the bond center and is commonly recognized as anomalous muonium [3,4].
Several first-principles calculations have already been done but the origin of the negative
Fermi contact interaction constant (FCIC) has not been clarified [5-7]. In this study, we
carry out reliable first-principles calculations and quantitatively reproduce theexperimental FCIC [4]. We find that a large size of supercell is necessary to obtainreliable results. Furthermore, by analyzing calculation results, we clarify that the negative
FCIC is due to the electron correlation effect, i.e., we find that the negative value is not
explained based on a simple one-electron approximation which leads to the zero value
and the electron correlation effect induces the negative value.
References :[1] R.L. Lichti, Protons and Muons in Materials Science, edited by E.A. Davies and S.F.J. Cox(Taylor and Francis, London, 1996).
[2] Hydrogen in Semiconductors, edited by J. Pankove and N.M. Johnson (Academic, New York,
1991).
[3] B.D. Patterson et.al., Phys. Rev. Lett. 40, 1347 (1978).
[4] R.F. Kiefl, et.al., Phys. Rev. Lett. 60, 224 (1988).
[5] C.G. Van de Walle and P.E. Blӧchl, Phys. Rev. B 47, 4244 (1993).
[6] R.H. Luchsinger, Y. Zhou and P. Meier, Phys. Rev. B 55, 6927 (1997).
[7] A.R. Porter, M.D. Towler and R.J. Needs, Phys. Rev. B 60, 13534 (1999).

102

P36

Magnetic-field-induced insulator-metal transition in W-doped VO2
Yasuhiro H. Matsuda1, Akihiko Ikeda1, Daisuke Nakamura1,
Shojiro Takeyama1, Yuki Suga2, and Yuji Muraoka2
Institute for Solid State Physics, University of Tokyo
Research Institute for Interdisciplinary Science, Okayama University
12

VO2 undergoes a first-order metal-insulator (MI) transition at 340 K [1] along withdimerization of vanadium atoms. Although the MI transition has been studied for morethan 60 years, the mechanism of the transition is still highly controversial [2-4]. It has
been claimed that the electron-electron correlation is fundamentally important, and thus
VO2 is a Mott-Hubbard insulator [2] or a Peierls-Mott insulator [3]. On the other hand,
there is an another aspect that the structural instability of VO2 is essential and formationof a molecular orbital of the V-V dimer causes the localization of the d electrons,
resulting in the MI transition [4].
In the present work, we have investigated the near-infrared optical-transmission in a
V1-xWxO2 (x = 0.06) thin film [5] at ultrahigh magnetic fields of up to 520 T and 14 K.
The MI transition temperature of the x = 0.06 film is around 86 K which is lower thanthat of pure VO2 [6]. The magnetic field is generated by the electromagnetic flux
compression technique [7]. The wavelength of a laser line used is 1.977 µm.
A distinct field-induced decrease of the transmission was observed at fields greaterthan 120 T and the transmission becomes as weak as that in the room-temperature
metallic phase at 490 T, which indicates that the magnetic-field-induced insulator tometal transition occurs. Since origins of the MI transitions in VO2 and V1-xWxO2 (x =0.06) are essentially the same [6], the observed field-induced metallization suggests thecollapse of the V-V dimers; the molecular orbital of the pair of V4+ (d1, s = 1/2) ionswould be suppressed by magnetic field when the spin singlet state becomes unstable.
References[1] F. J. Morin, Phys. Rev. Lett. 3, 34 (1959).
[2] T. J. Huffman et al., Phys. Rev. B 95, 075125 (2017).
[3] C. Weber et al., Phys. Rev. Lett. 108, 256402 (2012).
[4] Z. Hiroi, Progress in Solid State Chemistry 43, 47 (2015).
[5] Y. Muraoka and Z. Hiroi, Appl. Phys. Lett. 80, 583 (2002).
[6] K. Shibuya et al., Appl. Phys. Lett. 96, 022102 (2010)[7] D. Nakamura et al., , Rev. Sci. Instrum. 89, 095106 (2018).

103

P37

Doping a rare-earth permanent magnet:
ab initio studies on the impacts in multiple energy scales
Munehisa Matsumoto
Institute for Solid State Physics, University of Tokyo
Rare-earth permanent magnet (REPM) poses us important problems bothtechnologically and fundamentally. A practical question concerns the prospect for
exploiting a bunch of good magnetic properties in the typical operation temperature
range spanning from TL=300K to TH=450K on the basis of 4f-3d intermetallic
ferromagnets without too much of rare metals [1]. Prerequisites include a) strong
magnetization, b) accordingly strong Ising-type anisotropy, c) Curie temperature that ishigh enough compared to TH, all of which backed-up by d) robust crystal structure.
Gaining all of these is rare: Nd-Fe-B magnet made of the particular champion magnet
compound Nd2Fe14B [2] has been excellent except for its relatively low Curie
temperature at 585K. Here a trade-off goes on between the strong magnetization and theCurie temperature. A possibility to go beyond Nd2Fe14B was suggested by ab initio
prediction for NdFe12N and subsequent fabrication of a thick-film sample on a special
substrate [3]. Here the structure stability is traded off by the other three properties.
These competing trends span multiple characteristic energy scales. That is, a) comesfrom the energy scale of O(1) eV in the exchange splitting of 3d-electron bands inFe-based part, b) amounts to 1meV - 10meV at most c) is the order of 0.1 eV, andformation energy associated with d) comes slightly below 100 meV per atom for
Nd2Fe14B. While celebrated Slater-Pauling curve can in principle be a guideline to
maximize the magnetization with an optimal Fe-Co mixing ratio, the above competing
energies in multiple-scale hierarchy and a particular lattice expansion in REPM
complicates the problem. We discuss the possible improvement of REPM compounds byinspecting the effects of various elements doped into Nd2Fe14B from first principles bycombining structure optimization and coherent potential approximation [4]. A marginal
space between Nd2Fe14B and a possible maximum in the intrinsic magnetic properties is
identified, in which NdFe12N seems to be located.
References[1] H. Shishido, MM, M. Takamura, T. Ueno, K. Saito, in this conference.
[2] M, Sagawa et al., JAP 55, 2083 (1984); for a review, see J. F. Herbst, RMP 63, 819 (1991).
[3] T. Miyake et al., JPSJ 83, 043702 (2014); Y. Hirayama et al., Scr. Mater. 95, 70 (2015).
[4] MM, preprint [arXiv:1812.10945].

104

P38

Low-energy effective models of spin-S anisotropic Kitaev models
Tetsuya Minakawa,1 Joji Nasu,2 and Akihisa Koga1
12

Department of Physics, Tokyo Institute of Technology
Department of Physics, Yokohama National University

It is known that the S=1/2 Kitaev model has a quantum-spin-liquid ground state with
topological order as an exact solution [1]. In an anisotropic limit, it is mapped to the toriccode [2], which is a promising application to the topological quantum computation.
Recently, the extension to arbitrary spin for the Kitaev model has been done to understandthe Kitaev model from the theoretical viewpoint [3-7].
In this presentation, we clarify low-energy properties of spin-S Kitaev models in an
anisotropic limit. First, we derive the low-energy effective form of the local conserved
quantity [3], which is found out to be explicitly different between the half-integer spin
and integer spin cases. Next, we show the low-energy effective Hamiltonian, which is
obtained by the simple perturbation theory. As expected by the effective form of thelocal conserved quantity, the form of the effective Hamiltonian is also different between
the half-integer spin and integer spin cases. In the former case, the effective model ismapped to the toric code as well as the case of S=1/2, whose ground-state degeneracy
depends on the topology of the surface. On the other hand, in the latter case, theeffective model is equivalent to a free spin model under a uniform magnetic field,
whose ground state is nondegenerate [8].

[1] A. Kitaev, Ann. Phys. 321, 2 (2006).
[2] A. Kitaev, Ann. Phys. 303, 2 (2003).
[3] G. Baskaran, D. Sen, and R. Shankar, Phys. Rev. B 78, 115116 (2008).
[4] J. Oitmaa, A. Koga, and R. R. P. Singh, Phys. Rev. B 98, 214404 (2018).
[5] T. Suzuki and Y. Yamaji, Physica B: Condensed Matter 536, 637 (2018).
[6] I. Rousochatzakis, Y. Sizyuk, and N. B. Perkins, Nat. Commun. 9, 1575 (2018).
[7] A. Koga, H. Tomishige, and J. Nasu, J. Phys. Soc. Jpn. 87, 063703 (2018).
[8] T. Minakawa, J. Nasu, A. Koga: arXiv:1811.05668, accepted for publication in Phys. Rev. B.

105

P39

Theoretical Study on the Magnetism-Driven Negative Thermal
Expansion Phenomenon in the Inverse-Perovskite Antiferromagnets
Masahito Mochizuki
Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan
We will discuss our recent theoretical studies [1,2] on the magnetism-driven negative
thermal expansion (NTE) phenomenon in the inverse perovskite antiferromagnets
Mn3AN (A=Zn, Ga, etc.). It was experimentally discovered almost 50 years ago that theNTE occurs in Mn3AN triggered by a magnetic phase transition, where the crystal
volume expands abruptly as the system enters a specific non-coplanar antiferromagnetic
phase called G5g-type upon cooling. Because composites of usual and NTE materials
enable us to realize the zero-thermal-expansion materials, social demands on NTE
materials rapidly increase along with the development of nanotechnology and precision
optical and mechanical devices, in which slight changes in length and volume become
fatal.
In this study, we theoretically investigate a mechanism and properties of
the NTE phenomenon in Mn3AN based on a classical spin model withcompetingbond-length-dependent
spin-exchangeinteractions.
We
numerically reproduced the experimentally observed volume expansion uponcooling triggered by the G5g-type antiferromagnetic order and revealed thatthe expansion occurs so as to maximize an energy gain of thenearest-neighbor antiferromagnetic interactions. The mechanism revealed isfound to be not specific to the inverse perovskite magnets but might be
expected also in magnets with other crystal structures.

(a) Inverse-perovskite type structure of Mn3AN, (b) G5g-type antiferromagnetic order.
[1] K. Takenaka and H. Takagi, Appl. Phys. Lett. 87, 261902 (2005).
[2] M. Mochizuki et al., Phys. Rev. B 97, 060401(R) (2018).
[3] M. Kobayashi and M. Mochizuki, Phys. Rev. Materials 3, 024407 (2019).

106

P40

Ground-state phase diagram of the Kitaev-Heisenberg model on a
kagome lattice
Katsuhiro Morita, Masanori Kishimoto, and Takami Tohyama
Department of Applied Physics, Tokyo University of Science, Tokyo 125-8585, JapanThe Kitaev-Heisenberg model on the honeycomb lattice has been studied for the
purpose of finding exotic states such as quantum spin liquid and topological orders. On
the kagome lattice, in spite of a spin-liquid ground state in the Heisenberg model, thestability of the spin-liquid state has hardly been studied in the presence of the Kitaev
interaction. Therefore, we investigate the ground state of the classical and quantum spin
systems of the kagome Kitaev-Heisenberg model. In the classical system, we obtain anexact phase diagram that has an eight-fold degenerated canted ferromagnetic phase and asubextensive degenerated Kitaev antiferromagnetic phase. In the quantum system, using
the Lanczos-type exact diagnalization and cluster mean-field methods, we obtain two
quantum spin-liquid phases, an eight-fold degenerated canted ferromagnetic phasesimilar to the classical spin system, and an eight-fold degenerated q=0, 120o orderedphase induced by quantum fluctuation.

Reference:
Katsuhiro Morita, Masanori Kishimoto, and Takami Tohyama, Phys. Rev. B 98, 134437 (2018).

107

P41

DSQSS – PIMC solver for quantum lattice model
Y. Motoyama1, K.Yoshimi1, A. Masaki-Kato2, N. Kawashima1, and T. Kato1
1

ISSP, Univ. of Tokyo,

2

Hitachi Ltd.

The path-integral Monte Carlo method (PIMC) is one of the most powerful numericalmethods for studying quantum many-body problems [1]. By using PIMC we cancalculate the expectation value of an observable within a statistical error even for verylarge systems in arbitrary dimension at finite temperature as long as the system is freefrom the sign problem. The Monte Carlo method generally suffers from the slow
convergence near the critical point (critical slowing down). For discrete space problems,
several update methods, e.g. the loop algorithm and the directed-loop algorithm, havebeen developed to overcome the critical slowing down. These methods have been
extended for massive parallelization for modern supercomputers [2,3].
We are developing a PIMC program package for quantum lattice problems, which wecall DSQSS (Discrete Space Quantum Systems Solver) [4]. Since DSQSS adopts the
directed-loop algorithm for update scheme, it can update configurations effectively evenunder a symmetry-breaking field, such as magnetic field and chemical potential. By
editing two XML files, a lattice XML file describing how bonds connect sites and analgorithm XML file describing how worm heads should be scattered at vertices, users
can perform calculation for any model on any lattice (graph). Since these XML files arecomplicated, DSQSS comes with utility tools to generate these files forcommonly-studied models and lattices such as XXZ spin models and hypercubic lattices.
DSQSS also offers the parallelized multi worm algorithm for massively parallel
calculation[3]. DSQSS v2, the latest stable version, came in March 2019 with several
new features such as more user-friendly interfaces. In the presentation, we will
introduce DSQSS, explain the basic usage, and show some calculation results.
References :[1] For a review, J. Gubernatis, N. Kawashima, and P. Werner, “Quantum Monte Carlo Methods,”
Cambridge University Press (2016).
[2] S. Todo, H. Matsuo, and H. Sitara, appears in CPC (2019), DOI:10.1016/j.cpc.2019.01.004.
[3] A. Masaki-Kato et al., PRL 112, 140603 (2014).
[4] https://github.com/issp-center-dev/dsqss

108

P42

Real-space observation of electronic crystallization
Hideaki Murase, Shunto Arai, Yuya Hirakawa, Tatsuo Hasegawa,Kazuya Miyagawa, Kazushi Kanoda
Department of Applied Physics, University of Tokyo
Electron-electron Coulomb interactions are frustrated in the quasi-2D organic
conductors, -(BEDT-TTF)2RbZn(SCN)4, in which holes in a quarter-filled band areinteracting on an anisotropic triangular lattice. This material exhibits a charge ordering at
200 K whereas rapid cooling (faster than  5 K/min) suppresses the charge ordering,which gives way to supercooled liquid or glassy freezing [1,2]. The previous resistanceand NMR measurements suggested that the supercooled charge liquid and glass evolves
into the charge order (electronic crystallization) through nucleation and subsequent
growth, which have been widely observed in atomic and molecular systems [3,4].
We have examined the spatial profile of the electronic crystal growth in thissystem by Raman imaging, exploiting the distinct Raman spectra in the charge-ordered
and charge-glass (or supercooled) states (Fig.1). Just below the charge-order temperature,
the supercooled liquid evolves rapidly and inhomogeneously into the charge order once a
nucleation occurs after a long incubation time, whereas the crystallization from the glass
at low temperatures evolves slowly in the form of finely distributed microcrystals (Fig.1).
This behavior is consistent with the well-known nucleation and growth mechanism.

Fig. 1:(left) Raman spectrum of the MM’ = RbZn salt. (right) The crystallization at hightemperature (up) and at low temperature (down).
[1] F.Kagawa, et al, Nat. Phys. 9, 2642 (2013). [2] T.Sato, et al, Phys. Rev. B 89, 121102(R)(2014). [3] T.Sato, et al, Science 357, 1378–1381 (2017). [4] S.Sasaki, et al, Science 357,
1381-1385(2017).

109

P43

Organic antiferromagnet as a spin current generator
Makoto Naka1, Satoru Hayami2, Hiroaki Kusunose3, Yuki Yanagi4,
Yukitoshi Motome5, and Hitoshi Seo6,7
1

Waseda Institute for Advanced Study, Waseda University, Tokyo 169-8050, Japan2
Department of Physics, Hokkaido University, Hokkaido 060-0810, Japan3
Department of Physics, Meiji University, Kanagawa 214-8571, Japan
4
Institute for Materials Research, Tohoku University, Miyagi, 980-8577, Japan
5
Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
6
Condensed Matter Theory Laboratory, RIKEN, Saitama 351-0198, Japan
7
Center for Emergent Matter Science (CEMS), RIKEN, Saitama 351-0198, Japan

Spin current–a flow of electron spins without a charge current–is an ideal information
carrier free from Joule heating for electronic devices. The spin Hall effect [1,2], whicharises from the relativistic spin-orbit coupling, enables us to generate and detect spincurrents in inorganic materials, taking advantage of their constituent heavy atoms. In
contrast, organic materials consisting of molecules with light elements have been
believed to be unsuited for spin current generation. Here we show that a class of organic
antiferromagnets with checker-plate type molecular arrangements, such as a typicalstrongly correlated system κ-(BEDT-TTF)2X [3], can serve as a spin current generator.
The mechanism relies on a peculiar spin splitting of the energy bands and a real spaceanisotropy of the electron transfers, owing to the pattern of molecular orientation andantiferromagnetic ordering. Reanalyzing the well-studied Hubbard-type model [4], weshow that a spin current can be generated by applying a temperature gradient in theantiferromagnetic insulating state or an electric field in the doped metallic state [5]. We
find that the spin-current conductivity is given by a symmetric tensor, in stark contrast
to the spin Hall or spin Nernst effect described by an antisymmetric tensor. Our findings
provide another route to create a spin current and open a new field of spintronics basedon organic magnets having advantages of small spin scattering and long lifetime.
References:[1] S. Murakami, N. Nagaosa, and S. C. Zhang, Science 301, 1348 (2003).
[2] J. Sinova et al., Phys. Rev. Lett. 92, 126603 (2004).
[3] K. Kanoda, J. Phys. Soc. Jpn. 75, 051007 (2006).
[4] H. Kino and H. Fukuyama, J. Phys. Soc. Jpn. 65, 2158 (1996).
[5] M. Naka, S. Hayami, H. Kusunose, Y. Yanagi, Y. Motome, and H. Seo, arXiv:1902.02506(2019).

110

P44

Non-equilibrium dynamics of fractional excitations
in Kitaev spin liquids
Joji Nasu1 and Yukitoshi Motome2
1

Department of Physics, Yokohama National University, Japan
Department of Applied Physics, University of Tokyo, Japan

2

Insulating magnets provide a good playground for quantum many-body effects on thespin degree of freedom of electrons in solids. Among them, quantum spin liquids (QSLs),
where any magnetic orders are absent even at zero temperature, have attracted greatinterest for a number of decades as it could exhibit topological orders and possess
fractional excitations with unconventional statistics. Particularly, the Kitaev model has
been intensively studied as its ground state is exactly shown to be a QSL with Majorana
quasiparticles fractionalized from quantum spins [1]. Although a lot of attempts havebeen devoted to the observations of the fractional quasiparticles, the experimental
identification remains elusive. This is due to the difficulty on observing eachquasiparticle separately using experimental probes.
In this study, to observe the fractional excitations, we focus on the time evolution of their
non-equilibrium dynamics. In our previous work, we examined the magnetic-field effecton the ferromagnetic and antiferromagnetic Kitaev models using the Hartree-Fockapproximation in the Majorana fermion representation [2]. Here, we examine thetransient magnetic dynamics yielded by the magnetic-field quench by extending theprevious work. In the Kitaev model, a quantum spin is fractionalized into low-energy
localized Majorana fermions and high-energy itinerant ones. We find that two kinds ofMajorana fermions are observed separately in distinct time-scales in the time evolution of
the magnetization; the low-energy localized Majorana fermions survive as long-lived
excitations whereas the high-energy itinerant ones are damped in the early stage.
References:[1] A. Kitaev, Ann. Phys. (N. Y.) 321, 2 (2006).
[2] J. Nasu, Y. Kato, Y. Kamiya, and Y. Motome, Phys. Rev. B 98, 060416(R) (2018).

111

P45

Ab initio studies on superconductivity and inhomogeneityin Hg-based cuprate superconductor
T. Ohgoe1, M. Hirayama2, T. Misawa3, K. Ido3, Y. Yamaji1, and M. Imada1
1

Department of Applied Physics, The University of Tokyo,
2
RIKEN Center for Emergent Matter Science,3
Institute for Solid State Physics, The University of Tokyo
Despite many experimental and theoretical studies on high-Tc cuprates, understanding
its rich physics remains one of the most important outstanding problems in materials
science. In particular, charge order found in underdoped regions has been recently
established as a ubiquitous feature and their competitions with superconductivity are
attracting great attentions, where the simple Hubbard model on a square lattice has often
been studied and recent progress on the numerical accuracy has allowed to reach an
overall consensus. However, neither the period of the spin/charge order nor its
dominance over homogeneous superconductivity is consistent with experimental
observations [1-3]. Therefore, realistic first-principles studies are highly desirable.
In this study [4], we solved an ab initio low-energy effective Hamiltonian of
HgBa2CuO4+δ which has been recently derived without any adjustable parameters[5].
To solve the Hamiltonian accurately, we employed a many-variable variational Monte
Carlo method [6] combined with a tensor-network and/or the power Lanczos method
together with the energy-variance extrapolation for a controlled calculation. Our results
quantitatively reproduce experiments including a wide hole-density region ofsuperconducting phase while competing with a period-4 charge order near the holedensity δ~0.1 consistently with recent X-ray scattering measurements. To understandthe superconducting amplitude and stability, crucial role of realistic off-site interactions
is revealed. Furthermore, we find that the enhancement of superconductivity is wellcorrelated with charge fluctuations rather than spin fluctuations.
References[1] B.-X. Zheng et al., Science 358, 1155 (2017).
[2] K. Ido, T. Ohgoe, and M. Imada, Phys. Rev. B 97, 045139 (2018).
[3] A. S. Darmawan, Y. Nomura, Y. Yamaji, and M. Imada; Phys. Rev. B 98, 205132 (2018).
[4] T. Ohgoe, M. Hirayama, T. Misawa, Y. Yamaji, and M. Imada, arXiv:1902.00122 (2019).
[5] M. Hirayama, T. Misawa, T. Ohgoe, Y. Yamaji, and M. Imada, arXiv:1901.00763 (2019).
[6] T. Misawa et al., Comput. Phys. Comm. 235, 447 (2019).

112

P46

Resummation of diagrammatic series with zero convergence radius
for the unitary Fermi gas
R. Rossi1, T. Ohgoe2, K. Van Houcke3, and F. Werner4
1

Center for Computational Quantum Physics, Flatiron Institute2
Department of Applied Physics, The University of Tokyo
3
Laboratoire de Physique Statistique, Ecole Normale Superiéure
4
Laboratoire Kastler Brossel, Ecole Normale SuperiéureFeynman diagrams are powerful tools for studying various fields of physics. Still, theanalysis usually involves approximations, because only some types of diagrams or
low-order diagrams are considered there. However, the Monte Carlo method forunbiased sampling of Feynman diagrams has been recently developed[1-3]. On theother hand, the diagrammatic series sometimes have zero radius of convergence. The
question is whether it is still possible to make accurate predictions by summing up
Feynman diagrams.
Here we report high-precision results obtained by the bold-line diagrammatic Monte
Carlo method for the unitary Fermi gas with zero convergence radius[4]. We derive thelarge-order asymptotic behavior of the diagrammatic series, and we give mathematicalarguments and numerical evidence for the resummability of the series by a specificallydesigned conformal-Borel method that incorporates the large-order behavior.
Combining this new resummation method with diagrammatic Monte Carlo evaluation
up to order 9, we obtain new results for the equation of state, which agree with theultracold-atom experimental data, except for the 4-th virial coefficient for which ourdata point to the theoretically conjectured value. I will also report our accurate results of
Tan's contact[5].
References[1] N. Prokof'ev, and B. Svistunov, Phys. Rev. Lett. 99, 250201 (2007)[2] K. Van Houcke et al., Nat. Phys. 8, 366 (2012)[3] K. Van Houcke, F. Werner, T. Ohgoe, N. Prokof'ev, and B. Svistunov,Phys. Rev. B 99, 035140 (2019)[4] R. Rossi, T. Ohgoe, K. Van Houcke, and F. Werner, Phys. Rev. Lett. 121, 130405 (2018)[5] R. Rossi, T. Ohgoe, E. Kozik, N. Prokof’ev, B. Svistunov, K. Van Houcke, and F. Werner,
Phys. Rev. Lett. 121, 130406 (2018)

113

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Effect of Coulomb Interaction in Seebeck Coefficient ofOrganic Dirac Electron System α-(BEDT-TTF)2I3
Daigo Ohki1, Yukiko Omori2 and Akito Kobayashi1
12Department of physics, Nagoya University, Nagoya, Aichi 464-8602, Japan
National institute of technology, Toyota College, Toyota, Aichi 471-8525, Japan- (BEDT - TTF) 2I3 (a-ET) has a massless Dirac electron(massless DE) phase in its pressure-temperature phase diagram at high temperature andhigh pressure. In massless DE phase, a-ET shows unique physical properties particular
to Dirac electron (e.g. quantum conduction) [1]. When the inversion symmetry is
broken below critical temperature TCO and pressure PC, a-ET becomes a charge ordered(CO) phase due to the nearest neighbor coulomb interaction [2]. In massless DE phase,a velocity renormalization and excitonic fluctuation due to the long range coulombinteraction have been observed by NMR measurement [3]. Thus, electronic correlation
effect plays significant roles in each phase in a-ET.
In this study, we focus on an anomalous pressure-temperature dependence of a
Seebeck coefficient S(T) in a-ET [4, 5]. S(T) in P > PC has a positive value, and shows
peak structure at T = 50K, and linearly decreases toward T = 0K as temperature
decreases. On the other hand, in P < PC, S(T) being positive in massless DE phaseshows a peak at T ~ TCO, and sharp drop of the value with sign inversion. The
mechanism of these unique temperature dependences in S(T) have not been elucidated.
In the present study, we will calculate the Seebeck coefficient S(T) using
Nakano-Kubo formula and semi classical formula, and aim to elucidate the mechanismsof these unique behaviors. The electronic correlation in S(T) is investigated using
extended Hubbard model which describes the electronic structure of a-ET. We treat thechemical potential carefully, since the chemical potential in Dirac electron system issensitive to the carrier doping and electron-hole asymmetry [6], and S(T) is sensitive tothe chemical potential [7].
[1] N. Tajima et al. J. Phys. Soc. Jpn. 69, 543 (2000).
[2] H. Seo, J. Phys. Soc. Jpn. 69, 805 (2000).
[3] M. Hirata et al. Science 358, 1403 (2017).
[4] T. Konoike et al. J. Phys. Soc. Jpn. 82, 073601 (2013).
[5] R. Kitamura et al. JPS Conf. Proc. 1, 012097 (2014).
[6] A. Kobayashi et al. J. Phys. Soc. Jpn. 77, 064718 (2008).
[7] S. G. Sharapov et al. Phys. Rev. B 86, 035430 (2012).
The organic conductor

114

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Time-reversal symmetry breaking superconductivity
in hole-doped monolayer MoS2
Rikuto Oiwa1 , Yuki Yanagi2 , Hiroaki Kusunose1
1

Department of Physics, Meiji University, Kawasaki 214-8571, Japan
2
IMR, Tohoku University, Sendai 980-8577, Japan

Time-reversal symmetry (TRS) breaking superconductors have attracted much attention in theirintriguing features. Although no TRS breaking spin-singlet states have been observed so far, possible
TRS breaking spin-singlet state has been proposed in several candidate materials, such as Fe-based
superconductors [1]. These TRS breaking states are commonly characterized by their multi-bandfeatures with potential frustration in the relative phase among their Cooper pairing components, leading to an intermediate phase in between the range, [0, π]. Consequently, spontaneous TRS breaking
superconductivity emerges [2].
In recent years, the layered transition-metal dichalcogenides(TMDCs) have extensively been investigated both experimentally and theoretically as a new platform for exotic superconductivity in context of spin-orbit physics. In particular, it has
been suggested that the hole-doped TMDCs have a richer variety of pairing states than the electron-doped one, because thevalence bands have much characteristics originating from Mo dorbitals with relatively strong spin-orbit coupling (SOC) [3,4].
We examined theoretically the Cooper-pair symmetries withins waves that can be realized in hole-doped monolayer MoS2 [4],in which we introduced the multi-orbital gap functions; the conventional spin-singlet ψ in the Γ pocket, spin-triplet orbitalsinglet dz , and spin-singlet orbital-triplet Dz in the K and K′
pockets.
In the present study, we have further investigated the natureof the pairing states in the superconducting phase. Figure 1(a)shows the doping (x = 2 − n, n being the electron density perunit cell) dependence of T c and the Cooper-pair components for
U = 0.5, J = −U/1.7, and λ = 0.073 eV. The red, green andblue arrows represent ψ, Dz and dz in the complex plane, respectively, and ϕ1 is the relative phase between ψ and dz . The Figure 1: (a) Phase diagram of the dopchange of the Fermi-surface topology and the relative phase of ing x vs. temperature T for U = 0.5,
the parings near x ∼ 0.1 is shown in Fig. 1(b). The TRS break- J = −U/1.7, and λ = 0.073 eV. ϕ1 is the reling phase appears inside the TRS preserving phase, in which ative phase between ψ and dz . (b) Change
ϕ1 , 0, π. This pairing is triggered by the Lifshitz-transition of the Fermi-surface topology and the relrealized at x ∼ 0.1 (Fig. 1(b)). Note that this intriguing pairing ative phase of the pairings near x ∼ 0.1.
can emerge only in the presence of SOC, because without SOC,
ψ, Dz , and dz cannot hybridize with each other by symmetry. We will also discuss the densities ofthe finite spin and orbital angular momentum in the TRS breaking phase in the momentum space.
References[1] For example, J. Garaud, M. Silaev, and E. Babaev, Physica C 533, 63 (2017).
[2] S. Maiti and A. V. Chubukov, Phys. Rev. B 87, 144511 (2013).
[3] Y.-T. Hsu, A. Vaezi, M. H. Fischer, and E.-A. Kim, Nat. Commun. 8, 14985 (2017).
[4] R. Oiwa, Y. Yanagi, and H. Kusunose, Phys. Rev. B 98, 064509 (2018).

115

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Spin-charge coupled phenomena in a chiral soliton lattice
Shun Okumura, Yasuyuki Kato, and Yukitoshi Motome
Department of Applied Physics, The University of Tokyo
Chiral magnets often show interesting properties associated with their peculiar spin
structures. A typical example is a chiral soliton lattice (CSL), which was found inmonoaxial chiral magnetic conductors CrNb3S6 [1] and Yb(Ni1-xCux)3Al9 [2]. These
compounds show a helimagnetic state at zero field andturn into the CSL in a magnetic field perpendicular tothe helical axis (Fig. 1). As the magnetic field
increases, the spatial period of the CSL increases andfinally the system relaxes into a forced ferromagnetic
state (FFM). While itinerant electrons might play animportant role in the electronic and magnetic properties
in these chiral states, most of the previous theoreticalstudies were focused on effective localized spin models.
In this study, we investigate a minimal itinerant
electron model, a one-dimensional Kondo lattice modelwith the Dzyaloshinskii-Moriya interaction, byvariational calculations for the ground state and byMonte Carlo simulations at finite temperatures. We
show that the model exhibits peculiar behavior in an
applied field, which was not found in the localized spinmodels. In particular, we find that the spatial period of
CSL can be locked at particular values dictated by the
Fermi wave number (Fig. 2) [3]. The result suggests
the possibility of spontaneous formation of the CSL
even at zero field. We also show that the CSL exhibits
nonlinear negative magnetoresistance, which is closely
related to the soliton density (Fig. 3) [4].
[1] Y. Togawa et al., J. Phys. Soc. Jpn. 85, 112001 (2016).
[2] T. Matsumura et al., J. Phys. Soc. Jpn. 86, 124702 (2017).

Fig. 1: Schematic pictures of (a)a chiral helimagnetic state, (b)chiral soliton lattices (CSLs).

Fig. 2: Lock-in of the spatial
period of CSL in a magnetic field.

Fig. 3: Nonlinear negative
magnetoresistance which is
proportional to the soliton density.

[3] S. Okumura, Y. Kato, and Y. Motome, J. Phys. Soc. Jpn. 87, 033708 (2018).
[4] S. Okumura, Y. Kato, and Y. Motome, J. Phys. Soc. Jpn. 86, 063701 (2017).

116

P50

Numerical study of magnetic hedgehog crystals
in itinerant chiral magnets
Shun OkumuraA, Satoru HayamiB, Yasuyuki KatoA, Yukitoshi MotomeAA

Department of Applied Physics, The University of Tokyo
B
Department of Physics, Hokkaido University

Chiral magnets have attracted much attention due to the interesting properties associatedwith their peculiar spin structures originating from the antisymmetric exchangeinteraction called the Dzyaloshinkii-Moriya interaction. For example, among the
B20-type compounds, MnSi and MnGe show 2D skyrmion and 3D hedgehog crystals,
respectively, both of which are characterized by three wave numbers (3Q). These peculiar
spin structures induce an interesting quantum transport called a topological Hall effect [1].
In addition, a new chiral spin texture characterized by four wave numbers, the 4Q
hedgehog crystal, has been reported in MnSi1-xGex [2]. Although such systems aremetallic and the coupling between spin and charge should be important, there is no
theoretical study including the effect of itinerant electrons for the chiral multiple-Q states.
In the present study, in order to clarify the stability of the chiral multiple-Q states in the
3D systems, we study an effective spin model including the effect of itinerant electronsand spin-orbit interaction by variational calculations and Monte Carlo simulations. We
find several 3D chiral spin textures, including the skyrmion and hedgehog crystals withthe 3Q and 4Q vectors shown in Fig. 1. We also discuss the behaviors of such multiple-Q
spin textures in a magnetic field.
-.

(a)!* = (−%, −%, %)!* = (0, 0, %)
-0

-.

(b)

!) = (0, %, 0)-/!" = (%, 0, 0)!, = (%, %, %)-/-0!" = (%, −%, −%)!) = (−%, %, −%)Fig. 1: Examples of the ordering vectors for (a) 3Q and (b) 4Q states.
[1] N. Kanazawa et al., Adv. Mater. Rev. 29, 1603227 (2017).
[2] Y. Fujishiro et al., in press.

117

P51

Symmetry Indicators for Topological Superconductors
S. Ono1, Y. Yanase2 and H. Watanabe3
1Institute for Solid State Physics, The University of Tokyo, Japan2
Department of Physics, Kyoto University, Japan2
Department of Applied Physics, The University of Tokyo, Japan
Many people are making efforts to investigate the relationships between topology andsymmetry, especially searching for topological crystalline insulators (TCI) and topologicalsuperconductors (TSC). L. Fu and C. L. Kane proposed the first efficient way to discover TI,
called Fu-Kane formula [1], which is applicable to inversion symmetric systems. Recently,
there have been fundamental advances in the method of symmetry indicators [2] and in asimilar formalism, which provide an efficient way to diagnose the topology of band insulatorsand semimetals based on the representations of valence bands at high-symmetry momenta.
This scheme can be understood as a generalization of the Fu-Kane formula that computes the
topological index in terms of inversion parities to arbitrary space groups and a wider class of
topologies including higher-order ones. It formed the basis of recent extensive material
searches based on the density functional theory (DFT) calculation by several groups that
resulted in the discovery of an enormous number of new topological materials [3-5]. Up to thismoment, however, symmetry indicators are applicable only to insulators and semimetals. If
one wants to apply this method to SCs, one must examine the representations in the band
structure of the Bogoliubov-de Gennes (BdG) Hamiltonian including a gap function. In fact,
this is the approach taken in Ref. [6] that recently extended the symmetry indicators to the 10
Altland-Zirnbauer symmetry classes. However, this is not ideal because such a band structure
is not available in the standard DFT calculation. Furthermore, in this way, the total number ofbands that have to be taken into account can be huge unless one uses an effective tight-binding
model.
In this work, we further develop the theory of symmetry indicators exclusively designed forSCs. It enables us to determine the topology of SCs based on the representations of a finite
number of bands below the Fermi surface in the normal phase, although one still has to assume
a symmetry transformation property of the gap function [7].
[1] L. Fu and C. L. Kane, Phys. Rev. B, 76, 045302 (2007).
[2] H. C. Po, A. Vishwanath, and H. Watanabe, Nat. Commun 8, 50 (2017)[3] T. Zhang, Y. Jiang, et al., arXiv:1807.08756.
[4] F. Tang, H. C. Po, et al., arXiv:1807.09744.
[5] M. G. Vergniory, L. Elcoro, et al., arXiv:1807.10271.
[6] S. Ono and H. Watanabe, Phys. Rev. B. 98, 115150 (2018).
[7] S. Ono, Y. Yanase, and H. Watanabe, arXiv:1811.08712.

118

P52

Anomalous energy transfer between electrons and phonons inphotoexcited metals
Shota Ono
Dept. of Electrical, Electronic and Computer Engineering, Gifu University, JapanThe energy relaxation dynamics of photoexcited metals has been usually interpreted
based on the two-temperature model (2TM), where the electron and phonon
temperatures are assumed to be defined at any time. However, the assumption behindthe 2TM is questionable because the effects of electron-electron (e-e) andphonon-phonon (ph-ph) collisions would not be strong enough to keep thequasiequilibrium distribution and that of electron-phonon (e-ph) collisions would bestrong enough to disturb the quasiequilibrium.
We thus study the relaxation dynamics in electron-excited metals by using theBoltzmann equation with e-e, e-ph, and ph-ph collisions [1]. Figure 1(a) shows thetime-evolution of the excess energies of electron, LA phonon, and TA phonon. Most of
the electron energy is transferred to the LA phonon system through e-ph collisions in theinitial relaxation. In a meanwhile, the energy transferred is redistributed through ph-ph
collisions to reach thermal equilibrium, yielding an overshoot of the LA phonon energy
at 0.2 ps. Interestingly, the electron energy decays quite slowly before reaching thermal
equilibrium (Fig. 1(b)). This anomaly can occur as a result of the backward energy flowfrom LA phonons to electrons (Fig. 1(c)). Similar slow dynamics has been observed in
the final stage of relaxation for other systems [2,3]. The present study suggests that theappearance of backward energy flow would be an indicator for thermalization [4].
Electron(1) Energy transfer(Initial relaxation)(2) Backward energy flow
(Final relaxation)

LA phonon(2) Energy redistribution(Final relaxation)

TA phonon(c)Fig. 1: (a) The time-evolution of the excess energies in an electron excited metal. (b) The
magnified view of the electron energy. (c) Schematic illustration of the relaxation processes.
[1] S. Ono, Phys. Rev. B 97, 054310 (2018). [2] Y. Ishida et al., Phys. Rev. B 93, 100302(R) (2016).
[3] S. Ono, Phys. Rev. B 96, 024301 (2017). [4] S. Ono, arXiv:1901.04652.

119

P53

Strong-coupling formula of momentum-dependent susceptibilities
in dynamical mean-field theory
J. Otsuki1, K. Yoshimi2, H. Shinaoka3, Y. Nomura4
1

Research Institute for Interdisciplinary Science, Okayama University
2
Institute for Solid State Physics, University of Tokyo
3

4

Department of Physics, Saitama University
Department of Applied Physics, University of Tokyo

The dynamical mean-field theory (DMFT) and its combination to the density
functional theory (DFT+DMFT) have been extensively applied to strongly correlated
models and materials. For computing two-particle responses (q), one needs to solve theBethe-Salpeter equation, which demands large computational cost compared to thesingle-particle excitations. For this reason, it is difficult to perform susceptibility
calculations for realistic materials with d and/or f electrons.
Recently, we derived a simplified formula for (q) that can be solved at a considerably
lower cost [1]. The formula presents the features listed below:
 The formula has an RPA-like form, and calculations are quite easy.
 The derivation of the formula exploits a strong-coupling feature of the vertex
function, but numerical calculations demonstrate surprisingly wider applicability
including weak-coupling region.
Momentum dependence is characterized by an effective intersite interactions I(q).
In the strong-coupling limit, I(q) is reduced to the well-known formulas such as the
kinetic exchange interaction and the RKKY interaction. Hence, our formula alsooffers a way to estimate I(q) in first-principles calculations.
In the presentation, we present the details of the formula and its application tomultiorbital models.
[1] J. Otsuki, K. Yoshimi, H. Shinaoka, Y. Nomura, arXiv:1812.10918

120

P54

Quantum oscillations in topological Kondo insulator
R. Peters1, T. Yoshida2 and N. Kawakami1
1

2

Kyoto University, Kyoto, Japan
University of Tsukuba, Tsukuba, JapanOne of the most puzzling recent experimental discoveries in condensed matter physics
has been the observation of quantum oscillations in insulating materials SmB6 and YbB12[1,2]. Our understanding of quantum oscillations is rooted in the existence of a Fermi
surface; electron bands, which form the Fermi surface, form Landau levels in a magnetic
field. When the magnetic field strength is changed, the energy of these Landau levels
changes which lead to an oscillatory behavior in almost all of the observable properties.
However, SmB6 and YbB12 are strongly correlated f electron systems for which a gap
develops due to a hybridization between conduction electrons and strongly correlated felectrons, and thus a large resistivity at low temperatures can be measured. Thus, we can
expect that SmB6 and YbB12 do not possess a Fermi surface, thus there are no electrons,
which can form Landau levels close to the Fermi energy. On the other hand, SmB6 andYbB12 are both good candidates for topological Kondo insulator. Naturally, the question
arises, if these quantum oscillations can be due to the interplay between non-trivial
topology and strong correlations.
We here answer this question by showing results of dynamical mean field theory in amagnetic field for a two dimensional topological Kondo insulator. We demonstrate thatthe gap closing, described for a noninteracting
continuum model with momentum dependent
hybridization [3], persists for a topological
Kondo insulator on a two dimensional (2D)
lattice, see Fig. 1. Furthermore, we demonstratethat the amplitude of quantum oscillations isstrongly enhanced due to correlations, whichmakes them easily observable in quantities likemagnetization and resistivity over a wide range ofmagnetic fields before the magnetic breakdown
occurs.
Figure 1: Comparison of energy levels in
noninteracting and interacting model fordifferent magnetic fields.

References: Times, 11pt
[1] Tan et al. Science 349, 287 (2015)[2] Z. Xiang et al. in Science (2018)[3] Long Zhang et al. Phys. Rev. Lett. 116, 046404 (2016)
For more information see: arXiv:1901.05099

121

P55

Quantum spin liquids in the pyrochlore S=1/2 Heisenberg model with
Dzyaloshinskii-Moriya interactions
R. Pohle, Y. Yamaji, M. Imada
Dept. of Appl. Phys., University of Tokyo
The discovery of novel quantum states of matter such as spin liquids stimulates theinvestigation of new materials and model systems. Recently it has been suggested that a
new quantum spin liquid phase might occur in pyrochlore iridates [1], materials withnotably strong spin-orbit coupling. In order to quantify this statement, we numericallyinvestigate the quantum S=1/2 Heisenberg model with Dzyaloshinskii-Moriya
interactions on the pyrochlore lattice. By starting from a projection of the pyrochlore
lattice along the (001) direction (the 2 dimensional checkerboard lattice), we benchmark
our many-variable variational Monte Carlo (mVMC) [2, 3] results to exact
diagonalization (ED) [4] and iPEPS [5] data. By extending the lattice into the third
dimension we allow for a transition between the 2 dimensional checkerboard and the 3
dimensional pyrochlore physics, and open the door for detailed investigations of their
ground state properties.

[1] D. Pesin and L. Balents Nature Physics 6, 376-381 (2010)[2] Tahara et al. J.Phys. Soc. Jpn. 77, 114701 (2008)[3] Misawa et al. Comp. Phys. Commun. 235 (2019) 447-462[4] Kawamura et al. Comp. Phys. Commun. 217 (2017) 180-192[5] T. Okubo, private communication

122

P56

The role of exchange interactions on spin reorientation transition of
Nd0.5Dy0.5FeO3: A first principle study
Sarita Rajput1 , Padmanabhan B1,2, V.K.Malik1,
1

T.Maitra1

Department of Physics. Indian Institute of Technology Roorkee, Roorkee 247667, India
2

Department of Allied Sciences, Graphic Era University, Dehra Dun,248002, India

The rare-earth orthoferrites (RFeO3) which are known for their spin-reorientation and other interesting
properties like magneto-dielectric effect show a large Neel temperature of 700 K, below which the Fe3+
spins order as G-type antiferromagnet in the configuration Γ4 (Gx,Ay,Fz). Magnetic properties andelectronic structures of rare-earth orthoferrite Nd0.5Dy0.5FeO3 have been studied by performing accurate
first principle calculation based on density functional theory. Among the various possible magnetic
configurations of Fe3+ spins with the frozen Nd/Dy 4f states, the G-type ordering emerges as the groundstate. Both, NdFeO3 and DyFeO3, order in entirely different magnetic configurations. From ourcalculations it is found that C-type magnetic structure emerges as the lowest energy state of Nd3+/Dy3+,
which is well in agreement with experiments. The spin-reorientation and the ground state of Fe3+
moments are evaluated using non-collinear calculations with spin-orbit coupling. With inclusion of 4f
states of Nd/Dy, the Γ2 structure emerges as lowest energy in the Fe3+ spins which is thereby consistent
with experiment and symmetry analysis. Estimation of exchange interactions by mapping the system to
Heisenberg model yields a competing Nd-Fe and Nd-Dy and Dy-Dy exchange interactions in comparison
to much smaller Dy-Fe and Nd-Nd interactions. Hence, it is clear that the Nd3+ -Fe3+ and Nd3+ -Dy3+
exchange interactions play a dominant role in large polarization of the rare earth ions which prevent
independent ordering of the Nd3+ - Dy3+ moments.

123Poster Presentations – Section C

P57

Inversion and Magnetic Quantum Oscillations in Kondo Insulators
Panch Ram and Brijesh Kumar
Jawaharlal Nehru University, New Delhi
The occurrence of magnetic quantum oscillations in Samarium hexaboride (SmB6), a Kondo insulator,
caused a stir in condensed matter community because it is not expected to occur in insulators [1, 2].
The periodic oscillations in magnetization with respect to the (inverse) magnetic field is known as deHaas-van Alphen (dHvA) effect, which is considered as a hallmark of the metallic systems and a
direct probe to the Fermi surface [3, 4]. The dHvA oscillations in SmB6 forced us to reexamine andreconsider the underline working principle. In this regard, some interesting theoretical studies areproposed but still, this recalls for clear understanding.
In this poster, I would present our theoretical study of quantum oscillations in Kondo insulators byconsidering basic models, Kondo lattice model and symmetric periodic Anderson model, all at halffilling on bipartite lattices. Using a canonical representation of electrons invented by Kumar [5], weconstruct a self-consistent theory of spin and charge dynamics that appropriately describes the Kondo
insulating state and gives the quantum oscillations of magnetization as a general bulk property. We
get these oscillations due to inversion of charge quasiparticles dispersion whose effective chemicalpotential surface they measure. These quasiparticles are gapped and occupy half of the bulk BZ [6,7].
References:[1] G. Li et al, Science 346, 1208 (2014).
[2] B. S. Tan et al, Science 349, 287 (2015).
[3] W. de Haas, J. de Boer, and G. van dën Berg, Physica 1, 1115 (1934).
[4] L. Onsager, Phil. Mag. 43, 1006 (1952).
[5] B. Kumar, Phys. Rev. B 77, 205115 (2008).
[6] P. Ram and B. Kumar, Phys. Rev. B 96, 075115 (2017).
[7] P. Ram and B. Kumar, arXiv:1809.04654 (2018).

127

P58

Dynamics of the magnetic polaron
Roman Rausch1 , Norio Kawakami1 , and Michael Potthoff2
1 Kyoto University
2 University of Hamburg

The magnetic polaron is a quasiparticle that appears in the low-density
limit of the Kondo Lattice Model. It is an electron dressed by a cloud ofmagnons and can be thought of as an opposite limit to the Kondo problem (where a single localized spin is screened by electrons). We discuss the
eigenstates of the Correlated Kondo Lattice Model in the low-density limit
and show that additional states appear: a flat doublon band without any
dispersion; and a triplet-coupled polaron band. The doublon is a boundstate of two electrons otherwise known from the Hubbard model, but appears here even for U = 0, acquiring a dispersion for U > 0. We discuss theconsequences of these states for real-time dynamics and spectroscopies using
the time-dependent density matrix renormalization group (t-DMRG) in one
dimension.

128

P59

Superconductivity in quasiperiodic systems
Shiro Sakai
Center for Emergent Matter Science, RIKEN, Wako, Saitama 351-0198, Japan
Quasicrystal can be a platform of novel electronic properties because of its self-similar
but aperiodic crystal structure. Early studies on quasiperiodic systems focused on
noninteracting electrons, which indeed show various anomalous properties distinct fromthose in periodic systems, while recent experiments [1,2] have advanced our interests tointeracting electrons in quasiperiodic systems.
In this work, we theoretically study quasiperiodic superconductors, which were indeed
discovered in recent experiment [2]. Because of the lack of the Fermi surface,fundamental questions are how Cooper pairs are formed and what property differs fromthat of periodic superconductors. We studied these issues by introducing a simple
theoretical model, the attractive Hubbard model on a two-dimensional Penrose
structure.
Through numerical simulations on this model, we find a superconductivity at low
temperatures. Reflecting the aperiodicity of the Penrose structure, the superconducting
state exhibits a spatial inhomogeneity, which however conforms with the five-foldrotational symmetry and self-similarity. [3] We further study the effect of magnetic fieldon the quasiperiodic superconductors, where we find a novel superconducting state witha spatially alternating sign of the superconducting order parameter. [4]Fig.: Spatial pattern of onsite
superconductingorderparameter for various choicesof the average electron density
n andattraction U, calculated

on a 4181-site Penrose cluster.
Large(small)amplitude

colored by red (yellow).

[1] K. Deguchi et al., Nature Materials 11, 1013 (2012).
[2] K. Kamiya et al., Nature Commun. 9, 154 (2018).
[3] S. Sakai, N. Takemori, A. Koga, and R. Arita, Phys. Rev. B 95, 024509 (2017).
[4] S. Sakai and R. Arita, in preparation.

129

is

P60

Magneto-optic study of thermally driven ferrimagnet-helimagnet
transition in a chiral-polar magnet
Tatsuki Sato, Nobuyuki Abe, Shojiro Kimura1, Yusuke Tokunaga andTaka-hisa Arima
Department of Advanced Materials Science University of Tokyo, Kashiwa, Chiba, Japan1
IMR Tohoku University, Sendai, Miyagi, Japan
A chiral polar magnet Ni2InSbO6 (NISO), which belongs to space group R3, was first
synthesized in 2013 [1,2]. It has a long-periodic helimagnetic (HM) ground state with amodulation vector perpendicular to crystallographic axis [1]. Recently, we havesynthesized Ni2In0.9Cr0.1SbO6 (NICSO), where magnetic Cr3+ ions are partiallysubstituted for nonmagnetic In3+ ions. HM ground state in NICSO becomescommensurate ferrimagnetic structure in a magnetic field of a few tesla, while HM stateis stable up to around 20 T in NISO [3, 4].
Here we present a magneto-optic study of the magnetic phase transition in NICSO. We
have observed that a magnetic field suppresses the absorption intensity around 1.55 eV
at 4.2 K. This negative magnetochromism arises from the spin-dependent oscillator
strength of intra-atomic d-d excitation from 3A2 ground state to 1E excited state of Ni2+
ion with d8 electronic configuration. The magnetochromism is found to diminish around
50 K (see Figure), below the Neel temperature 84 K. It indicates that the FM state at
low temperatures is thermally destabilized and turns to HM state, similar to the case ofskyrmion crystal phase in some chiral or polar noncentrosymmetric helimagnets.
Temperaturedependenceofmagnetochromism around 1.55
eV in NICSO. Vertical axis
corresponds to the change in theabsorption intensity around 1.55
eV. PM denotes paramagnet.
Insetsshow

schematicallyillustrated magnetic structure.
References[1] S. A. Ivanov et al., Chem. Mater. 25, 935 (2013) [2] M. Weil et al., Cryst. Res. Technol. 49,
142 (2014) [3] Y. Araki et al., in preparation [4] T. Sato et al., in preparation

130

P61

Quantum Criticality in the Two-Dimensional Periodic Anderson Model
T. Schäfer1,2,3, A. A. Katanin4, M. Kitatani1, A. Toschi1, and K. Held1
1

Institute of Solid State Physics, TU Wien, 1040 Vienna, Austria
Collège de France, 11 place Marcelin Berthelot, 75005 Paris, France
3
CPHT, CNRS, Ecole polytechnique, IP Paris, F-91128 Palaiseau, France
4
Institute of Metal Physics, 620990, Kovalevskaya str. 18, Ekaterinburg, Russia
2

Despite the fascinating physical phenomena accompanying a quantum critical point, e.g.
the exhibition of non-Fermi liquid behavior, a general theory for quantum phasetransitions is still lacking.
In my poster presentation, I will show a step forward by analyzing results from the
dynamical vertex approximation (DГA), a cutting-edge quantum field theoreticalmethod including temporal as well as spatial correlations. Within this framework, I will
analyze the fundamental model of strongly correlated heavy fermion compounds, theperdiodic Anderson model, in two dimensions. By varying the hybridization strength of
localized f-electrons and itinerant d-electrons, and a careful analysis of response
functions, one can trace the change in the ground state from an antiferromagnet at low
hybridizations to a paramagnetic Kondo insulating phase, resembling the famous
Doniach phase diagram. Eventually, I will show the evolution of the (classical andquantum) critical exponents of the magnetic susceptibility, which are changing from theone of free spins γ = 1 to γ = 2 in the quantum critical regime.

References:
T. Schäfer, A. A. Katanin, M. Kitatani, A. Toschi, and K. Held, arXiv:1812.03821.
T. Schäfer, A. A. Katanin, K. Held, and A. Toschi, Phys. Rev. Lett. 119, 046402 (2017).

131

P62

Two-dome superconductivity in FeS induced by a Lifshitz transition
M. Shimizu A, N. TakemoriB , D. GuterdingC and H. O. JeschkeB
A Department of Physics, Okayama University
B Research Institute for Interdisciplinary Science, Okayama University
C Technische Hochschule Mittelhessen

Recently, two superconducting domes in the temperature-pressure phase diagram for
FeS have been observed [1]. FeS has a tetragonal structure and superconducts with thecritical temperature Tc = 4.7K at ambient pressure. The superconductivity is suppressed
at approximately 4 GPa, and the second dome appears from 5 GPa to 22 GPa. Since it
has been reported that a structural phase transition from tetragonal into hexagonal occurs
around 7 GPa, it has been unclear what causes the reentrant superconductivity.
In this study, we calculate the superconducting order parameter with density functional theory and spin fluctuation theory calculations using the structure obtained fromexperiment [2]. We find that the pairing strength decreases up to 4.6 GPa and increases
again from 4.7 GPa to 5.5 GPa, in agreement with experiment. The superconducting gap
symmetry changes at 4.7 GPa from d wave to nodeless sign changing s wave. Detailed
analysis of the spin susceptibility allows us to link the change of order parameter to theadditional Fermi surface presents after the Lifshitz transition.

Fig. 1: Leading gap functions for FeS at (a) ambient pressure and (c) P = 4.7 GPa. (b)Leading eigenvalues λ of the linearized gap equation as function of pressure.

References[1] J. Zhang et al., npj Quantum Mater. 2, 49 (2017).
[2] M. Shimizu et al., Phys. Rev. Lett. 121, 137001 (2018).

132

P63

Dynamical susceptibility in DMFT: a sparse QMC sampling approach
Hiroshi Shinaoka1, Dominique Geffroy2,3, Jan Kuneš3, Junya Otsuki4,
Markus Wallerberger5, Emanuel Gull5, and Kazuyoshi Yoshimi61) Department of Physics, Saitama University, 2) Department of Condensed Matter
Physics, MU Brno, 3) Institute for Solid State Physics, TU Wien, 4) Research Institutefor Interdisciplinary Science, Okayama University, 5) University of Michigan, 6)
Institute for Solid State Physics, University of Tokyo
Computing dynamical susceptibility in dynamical mean-field theory (DMFT) is
challenging due to large computation time and massive memory consumption in dealing
with two-particle Green’s function. Recently, some of us and co-workers have proposed acompact representation of two-particle Green’s functions [1] based on the "intermediate
representation" (IR) basis [2]. In this compact representation, the data size of two-particle
Green’s functions grows only logarithmically with inverse temperature.
In this talk, we discuss a new technique for the efficient quantum Monte Carlo (QMC)
sampling of two-particle Green’s functions in the framework of DMFT. The sampling is
performed only at a sparse set of Matsubara frequencies, dramatically reducing thecomputational costs and memory footprint. We demonstrate that the correct
low-/high-frequency structures can be recovered from the sampling data.
The sparse sampling can be used for the calculation of dynamic susceptibilities, after
inversion of the Bethe-Salpeter equation. We present results in the context of the BEC to
BCS crossover [3, 4] in the vicinity of the excitonic condensation in the two-band
Hubbard model [5]. We also discuss future applications of this technique to ab-initio
calculations of dynamical susceptibility and diagrammatic extensions of DMFT.
References[1] H. Shinaoka, J. Otsuki, K. Haule, M. Wallerberger, E. Gull, K. Yoshimi, M. Ohzeki,
PRB 97, 205111 (2018).
[2] H. Shinaoka, J. Otsuki, M. Ohzeki, K. Yoshimi, PRB 96, 035147 (2017); N. Chikano
et al., PRB 98, 035104 (2018); N. Chikano et al., arXiv:1807.05237.
[3] B. Zenker, D. Ihle et al., PRB 85, 121102(R) (2012).
[4] J. Kuneš, J. Phys.: Cond. Mat. 27, 333201 (2015).
[5] D. Geffroy, J. Kaufmann et al., arXiv 1808.08046.

133

P64

Machine learning study of photoexcited states in the half-filled
one-dimensional extended Hubbard model
Kazuya Shinjo1), Shigetoshi Sota2), Seiji Yukoki2), and Takami Tohyama1)
1)Department of Applied Physics, Tokyo University of Science, Tokyo, JAPAN2)
RIKEN Center for Computational Science, Kobe, Hyogo, JAPAN

Photoinduced nonequilibrium states can provide new insights into the dynamical
properties in strongly correlated electron systems. One of the typical and extensively
studied systems is the half-filled one-dimensional extended Hubbard model (1DEHM).
Here, we propose that the supervised machine learning (ML) can give useful
information for characterizing the photoexcited state in 1DEHM [1]. Using
entanglement spectra as a training dataset, we construct the neural network. Judging
from the trained network, we find that the quantum state driven by driving pulse has
bond-spin-density wave (BSDW) order for U ≲ 2V , where U (V ) is the on-site(nearest-neighbor) Coulomb interaction. We separately calculate the time evolution oforder parameters and find that the order parameters of BSDW are actually enhanced byphotoexcitation as predicted by ML. Interestingly, the enhancement of the BSDW orderin the photoexcited states for the 1DEHM has never been reported previously, despite
the extensive studies so far, thus demonstrating the advantage of ML to assist
characterizing photoexcited quantum states.

[1] K. Shinjo, S. Sota, S. Yunoki, and T. Tohyama, arXiv:1901.07900.

134

P65

Possible Enhancement of Coercivity in Rare-Earth Permanent
Magnets on the Verge of a Valence Transition
Hiroaki Shishido1, 2, Munehisa Matsumoto3, 4, Masato Takamura1,
Tetsuro Ueno4, 5, 6, Kotaro Saito4, 5, 7
1

Department of Physics and Electronics,Osaka Prefecture University
2
NanoSquare Research Institute, Osaka Prefecture University
3
Institute for Solid State Physics, University of Tokyo
4
Elements Strategy Initiative Center for Magnetic Materials (ESICMM), National
Institute for Materials Science (NIMS)
5
Institute of Materials Structure Science, High Energy Accelerator Research
Organization
6
Quantum Beam Science Research Directorate, National Institutes for Quantum andRadiological Science and Technology
7
Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute
CeCo5 and CeCu5 crystalize CaCu5-type hexagonal structure, which is the samestructure as a prototypical commercial rare-earth permanent magnet SmCo5. CeCo5
orders ferromagnetically below the Curie temperature TC = 649 K, and Ce is tetravalent[1]. CeCu5 becomes antiferromagnet below the Néel temperature TN = 3.9 K, and is aheavy fermion compound [2]. Thus, Ce is trivalent. Therefore, one can expect that avalence transition in Ce would emerge induced by Cu substitution for Co in CeCo5. To
investigate the influence of valence fluctuations to the ferromagnetic property, we grew
poly-crystalline samples of Ce(Co1-xCux)5. We measured magnetization by SQUID
magnetometer (MPMS3, Quantum Design) in the temperature range 2 -300 K.
Reflecting a ferromagnetic ordering, magnetization curves for x ≤ 0.6 exhibit clear
hysteresis loops. Ferromagnetic phase as a function of x is consistent with the previous
report [3]. No extrinsic pinning centers are observed in our samples. Intrinsic coercivity
Hc is strongly enhanced by Cu substitution and has a maximum at x = 0.3. Moreinterestingly, Hc for x ~ 0.5 exhibits steep increase at low temperatures, whereas that forx ~ 0.3 is monotonically enhanced with decreasing temperature. These results imply theenhancement of Hc on the verge of valence transition induced by Cu substitution ortemperature in Ce(Co,Cu)5. Implications on variants of crystal lattices are discussed.
[1] A. V. Korolov et al., Phys. Met. Metallogr. 40 (1975) 191.
[2] E. Bauer et al., J. Mag. Mag. Mater. 63-64 (1987) 37.
[3] D. Girodin et al., J. Less-Common Mat. 110 (1985) 149.

135

P66

Quantized Excitation Spectra by Confinement in(Quasi-)One-Dimensional S=1 Quantum Spin Systems
Sei-ichiro Suga and Takafumi Suzuki
University of Hyogo, Shosha, Himeji, Hyogo 671-2280, Japan
The infinite time-evolving block decimation algorithm is applied to calculate thedynamical spin structure factor (DSF) of (quasi-)one-dimensional (q1D) S=1 quantum
spin systems. We first consider the q1D S=1 antiferromagnetic Heisenberg spin systemwith single-ion anisotropy and bond alternation [1]. We find that the staggered fieldarising from the weak interchain interaction changes the excitation continuum
originating from magnons into the quantized spectra. When the single-ion anisotropy isnegatively strong, the energies of the quantized spectra agree well with the negative
zeros in the Airy function (NZAF). This quantized magnon spectra is a counterpart of
the quantized excitation spectra caused by the spinon confinement in the q1D S=1/2
antiferromagnets, which has been recently observed in the inelastic neutron scattering(INS) experiments [2,3]. We further show that the quantized excitation spectra appear
near the phase boundary between the Haldane and Neel phases in the phase diagram.
We next consider the S=1 ferromagnetic (FM) Ising spin chain with negative single-ion
anisotropy in magnetic fields [4]. When thetransverse magnetic field is applied, both theΔS=2 excitation continuum and the one-magnon
mode appear in the low-lying excitation. When thelongitudinal magnetic field is further applied, theΔ S=2 excitation continuum changes into thequantized excitation spectra as shown in Fig. 1.
The quantized ΔS=2 excitation spectra originate
from confinement of two domain walls, each ofwhich carries Δ S=1. The quantized excitationenergies agree with the NZAF. We expect theobtained quantized excitation spectra to be detectedby the INS experiments.

Fig. 1: DSF of the S=1 FM Ising
spin chain in magnetic fields.
Quantized

excitation

spectrabelow one-magnon mode.

[1] T. Suzuki and S. Suga, Phys. Rev. B 98, 180406 (R) (2018). [2] B. Grenier, et al., Phys. Rev.
Lett. 114, 017201 (2015). [3] Q. Faure, et al., Nat. Phys. 14, 716 (2018). [4] T. Suzuki and S.
Suga, arXiv: 1902.05169.

136

P67

Topological Transition in Hard-Core Bosonic Haldane Model
Takuya Susa, Takanori Sugimoto, and Takami Tohyama
Department of Applied Physics, Tokyo University of Science.

The topological Haldane model on two-dimensional honeycomb lattice is well known as thefirst example where it can exhibit quantum Hall effect without magnetic fields [1]. This model
breaks the time-reversal and sublattice symmetries due to the second-neighbor hopping with aphase and the alternating chemical potential, leading to the non-trivial Chern number. This
non-zero Chern number indicates the presence of the chiral edge state and the non-zero Hall
conductance. Inspired by this study, recent theoretical studies on topological states have been
extended to no-fermion systems, e.g., bosonic systems [2]. Interestingly, the recent study has
shown that even the bosonic systems exhibit non-trivial topological number.
Here, we consider a hard-core boson instead of fermion and boson as a quantum object on the
Haldane model. The hard-core boson has the same commutation relation as one-half spin, i.e.,
on-site anti-commutation and inter-site commutation. The Jordan-Wigner transform of one-halfspin to fermion causes the many-body interaction in two-dimensional systems. Therefore, we
can clarify not only the quantum statistical effects but also the effects of interaction on the
topological state in this model. To break the time-reversal symmetry, we introduce asecond-neighbor hopping term with a phase depending on number operator at the neighboringsites, which corresponds to the scalar chirality of three spins. Additionally, we also introduce ananisotropy in the lattice to break the sublattice symmetry. As the result, we have found thesignature of topological transition in this model. Our study also indicates a possibility of new
topological states in quantum spin systems.
References[1] F. D. M. Haldane, Phys. Rev. Lett. 61, 18 (1988).
[2] H. Kondo, Y. Akagi, and H. Katsura, Phys. Rev. B 99, 041110(R) (2019)

137

P68

Large-scale dynamical simulation of Hubbard model
Hidemaro Suwa1,2, Gia-Wei Chern3, Kipton Barros4, Cristian D. Batista2,5Department of Physics, The University of Tokyo, Tokyo
2
Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN
3
Department of Physics, University of Virginia, Charlottesville, VA
4Theoretical Division and CNLS, Los Alamos National Laboratory, Los Alamos, NM
5
Quantum Condensed Matter Division and Shull-Wollan Center,
Oak Ridge National Laboratory, Oak Ridge, TN
1

Weak Mott insulators emerge from 5d electron systems with strong spin-orbit couplings.
Coulomb repulsion (U) between electrons in 5d orbitals is not strong enough to suppress
charge degrees of freedom completely. For example, the charge gap is comparable to themagnon-band width for strontium iridates. Such electron systems can be described by the
Hubbard model with an intermediate U, which would be in the crossover regime between
Slater and Mott insulators. Reliable calculation for the intermediate-U region istheoretically challenging because of the lack of a small parameter in the model. We havedeveloped a new numerical approach that enables large-scale calculation (with more than
104 sites) of dynamical quantities at finite temperatures in the broad-U region (Fig. 1). In
our approach, the sampling of auxiliary vector
fields from the Boltzmann distribution and thereal-time dynamics of the density matrix are
successfully combined. As an application of the
newly developed method, we calculated the
dynamical spin structure factor, targeting 5delectron systems. Our calculation accounts forexperimental results of relevant materials suchas iridates. Importantly, our approach isapplicable to various systems without any
restrictions on the type of hopping, the type oflattice, and electron filling. The broad U region Fig. 1: Target region and valid
is accessible up to the temperature comparable approaches in the parameter spaceof interaction strength (U/t) andto the charge gap.
temperature (T).

[1] G.-W. Chern, K. Barros, Z. Wang, H. Suwa, and C. D. Batista, Phys. Rev. B 97, 035120 (2018)138

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P69

Ground-state phase diagram of the extended Kitaev-G model on ahoneycomb lattice
Takafumi Suzuki and Sei-ichiro Suga

University of Hyogo, Shosha, Himeji, Hyogo 671-2280, Japan

Since the features of the Kitaev spin liquid (KSL) have been reported in the inelastic

neutron scattering experiments on a honeycomb-lattice magnet, a-RuCl3[1,2], thiscompound has been the focus of intensive research. The proper model for a-RuCl3 havebeen much discussed so far. It has been pointed out that not only the Kitaev interaction(K) but also the symmetric off-diagonal interaction (G) acting on the nearest-neighboreffective spins is important to explain the characteristic features of a-RuCl3[3,4]. The
recent numerical calculations for the K-G model on a honeycomb lattice have pointed
out that a quantum spin liquid for arbitrary ratio of G/K for ferromagnetic Kitaev
interaction probably exists [5]. Motivated by the above works, we study theground-state phase diagram of the extended K-G model whose lattice connects thehoneycomb lattice, isolated dimers and spin chains. The Hamiltonian is written as,

hijix

H =(1
2 )

+
X

+
hijiy

X
hijiz

X

cos
cos Siz Sjz + sin

Six Sjx + sin(Six Sjy + Siy Sjx )(a)(Siy Sjz + Siz Sjy )cos Siy Sjy + sin (Six Sjz + Siz Sjx ) ,Rev. B 97, 075126 (2018).

139
(c)
b
c

Jy Siy Sjy



Jz Siz Sjz

a

Sx

(b)



Jx Six Sjx

Sz

Fig.1 Honeycomb lattice and*the Kitaev interactionSy

Y K’ 

where l is the parameter for the anisotropy. At l = 1/2, the model describes
M a spin chainthat includes only the x- and y- bond interactions. At l =1/3 thebecomeK X
 interactions
isotropic (honeycomb-lattice), and at l = 0 the system is equivalent to the isolated dimer
model. From the numerical exact diagonalization, we find that there are at least fivekinds of phases. Interestingly, one of the five phases is continually connected with theTomonaga-Luttinger liquid phase at l = 1/2. This phase and the dimerized phase adjoin
each other at l = 1/3 in a wide range of f.

[1] A. Banerjee, et al., Nature Mater. 15, 733-740 (2016). [2] A. Banerjee, et al., npj Quantum

Materials 3, 8 (2018). [3] L. Janssen, E. C. Andrade, and M. Vojta, Phys. Rev. B 96, 064430(2017). [4] S. M. Winter, et al., Phys. Rev. Lett. 120, 077203 (2018). [5] M. Gohlke, et al., Phys.

P70

Superconducting fluctuations in 𝑺 = 𝟏 one-dimensional Kondo latticemodel under transverse magnetic fields
Kohei Suzuki and Kazumasa HattoriTokyo Metropolitan University, Hachioji 192-0397, Japan
We study superconducting fluctuations in the 𝑆 = 1 one-dimensional Kondo latticemodel (spin-1 KLM) under transverse magnetic fields. Previously, we have studied theeffects of the transverse magnetic fields on the spin-1 KLM [1], keeping URhGe in mind.
URhGe shows superconductivity inside its ferromagnetic (FM) state and superconducting(SC) transition temperature is strongly enhanced by transverse magnetic fields around
𝐻~12 T [2]. We have found various phases such as FM phases, Kondo plateau phases,
and Tomonaga–Luttinger liquids (TLL) in the ground state of the spin-1 KLM.
In this study, we clarify the relation between SC fluctuations and the transverse magnetic
fields ℎ by analyzing SC correlation functions, using the density matrix renormalizationgroup [3]. Since Cooper pairs couple to the 𝑆 = 1 local spins, all the possible SC
correlations within the nearest neighbors are taken into account. For example, local paircorrelation functions are given by
3,4,5,6

<3,4

5,6

3,4

4

𝜒-. ,-0 ,-1 ,-2 (𝑟) = 〈∆-. ,-0 (𝑁/2)∆-1 ,-2 (𝑁/2 + 𝑟)〉 , ∆-. ,-0 (𝑟) = 𝑆A3 𝑐-. A 𝑆A 𝑐-0 A .
Here, 𝑐-. A is the annihilation operator of the conduction electron with spin 𝜎D at 𝑟 site,
𝑆A3 is the 𝛼-component of the 𝑆 = 1 local spin,
and 𝑁 is the system size. The maximum eigenvalue(𝑟) corresponds to the leading
𝜒(𝑟) for 𝜒-3,4,5,6. ,-0 ,-1 ,-2
SC fluctuations. We find that 𝜒(𝑟) shows a powerlaw decay ~𝑟 HI and the SC fluctuations areenhanced in the TLL phases as shown in Fig. 1.
We will also discuss the nearest-neighbor SC
correlations and the effects of transverse magnetic
fields on the 𝑆 = 1/2 one-dimensional Kondo
lattice model with Ising-type interaction between
the local spins.

5
4FM

Kondo Plateau

3
2
0

0.5

1

1.5

2

2.5

3

Fig. 1: ℎ dependence of 𝜃, where 𝜃
is defined by 𝜒(𝑟) ~ 𝑟 HI .

[1] K. Suzuki and K. Hattori, J. Phys. Soc. Jpn. 88, 024707 (2019).
[2] D. Aoki and J. Flouquet, J. Phys. Soc. Jpn. 81, 011003 (2012).
[3] S. R. White, Phys. Rev. Lett. 69, 2863 (1992); Phys. Rev. B 48, 10345 (1993).

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Ab initio derivation of effective Hamiltonian for
La2CuO4/La1.55Sr0.45CuO4 heterostructures
Terumasa Tadano1, Yusuke Nomura2, and Masatoshi Imada2
12

National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Department of Applied Physics, The University of Tokyo, Bunkyo-Ku, Tokyo, Japan

Superconductivity at interfaces is one of the hottest research topics, where interface
atomic layers often show properties superior to the bulk in terms of the criticaltemperature and its stability [1,2]. One of the most successful examples is the interface
between the overdoped La2-xSrxCuO4 and Mott insulating La2CuO4, where the pinning of
Tc at 40 K, which is the optimum Tc of the bulk, was observed in a wide range of 0.2 < x <0.5 [2]. Previous variational Monte Carlo study based on the single-band Hubbard
Hamiltonian attributed the pinning to the interlayer phase separation [3], but the layer
dependence of the effective Hamiltonian was considered only through the difference ofthe onsite energy level adjusted empirically.
To develop more robust and quantitative understandings of the superconductivity at
the LCO/LSCO interface, we have recently derived the layer-dependent two-band (Eg)
Hamiltonian by performing large-scale constrained RPA calculations of the
La2CuO4/La1.55Sr0.45CuO4 superlattice [4]. We have found that the parameters of thesuperlattice differ considerably from those for the bulk La2CuO4, particularly significant
in the partially-screened Coulomb parameters and the level offset between the dx2−y2 anddz2 orbitals. In addition, we have investigated the effect of the lattice relaxation on the Eg
Hamiltonian by carefully comparing the parameters derived before and after the structure
optimization. We then found that the CuO6 octahedra distort after the relaxation as aconsequence of the Madelung potential difference between the insulator and metal sides,
by which the layer dependence of the hopping and Coulomb parameters becomes more
gradual than the unrelaxed case. We will discuss the mechanism of the gradual layer
dependence of the parameters realized by the lattice relaxation and its potential impact inthe superconductivity at the interface.
References:[1] I. Bozovic and C. Ahn, Nature Phys. 10, 892 (2014).
[2] J. Wu et al., Nature Mater. 12, 877 (2013).
[3] T. Misawa, Y. Nomura, S. Biermann, and M. Imada, Sci. Adv. 2, e1600664 (2016).
[4] T. Tadano, Y. Nomura, and M. Imada, arXiv:1902.03743.

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Field-Angle Dependence of Interlayer Magnetoresistancein Organic Dirac Electron System
Takehiro Tani 1, Naoya Tajima 2, Akito Kobayashi 1
1
2

Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan
Department of Physics, Toho University, Funabashi, Chiba 274-8510, Japan

Nowadays, 2-D organic conductor, α-(BEDT-TTF)2I3 (α-ET), has attracted muchattention because α-ET has the massless Dirac fermions under high pressures. The
massless Dirac fermions have many interesting properties, especially in a perpendicularmagnetic field. It is known that the N=0 Landau level with the spin splitting emerges atthe Dirac point which coincides with the Fermi energy in α-ET. This effect has been
detected in interlayer magnetoresistance [1] and the spin lattice relaxation rate [2].
In the present study, we revealed the mechanism of the anomalous field-angle
dependence of the interlayer magnetoresistance with a combination of theory andexperiment. The enhancement of the effective g-factor obtained in this work is consistent
with the results of the Nernst effect [3, 4] and the spin lattice relaxation rate [2].

(b) The experimental results on
B-dependence of Rzz.

(a) The theoretical results on

B-dependence of ρzz.
[1] T. Osada, J. Phys. Soc. Jpn. 77, 084711 (2008)[2] M. Hirata, Ph. D. Thesis, The University of Tokyo (2012)[3] I. Proskurin and M. Ogata, J. Phys. Soc. Jpn. 82, 063712 (2013)[4] T. Konoike, M. Sato, K. Uchida, and T. Osada, J. Phys. Soc. Jpn. 82, 073601 (2013)

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Characterization of quantum chaos by two-point correlators
Masaki Tezuka
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Two criteria are widely used currently to characterize quantum chaos: therandom-matrix-like universality of the fine-grained energy spectrum and theexponential Lyapunov growth of an out-of-time-order correlation function (OTOC). The
Sachdev-Ye-Kitaev (SYK) model, a model of N fermions with all-to-all random
interactions, is solvable in the large-N limit and offers a possibility to study a maximallychaotic quantum system. However, the numerical study of the SYK model is limited tosmall values of N and it remains challenging to observe the exponential growth of
OTOC. We have previously generalized the growth exponent of the OTOC to define aspectrum of quantum Lyapunov exponents, which exhibits random matrix behavior
when the system is chaotic. [1]In this work, [2] we propose a simple characterization of quantum many-body chaos. We
consider a collection of simple operators and the set of all possible two-point correlation
functions, in real time or in imaginary time, between these operators can be organized
into a matrix.
We find that the spectral statistics of the singular values of this matrix exhibits universal
features characteristic of a random matrix, if the system is in a quantum chaotic phase. We
demonstrate this by numerically studying models including the SYK model and a
one-dimensional spin chain with random magnetic field. When these models are brought
outside the chaotic regime, the spectral statistics significantly deviate from those of
known random matrix ensembles and approach those of an uncorrelated distribution. We
expect that this correspondence of the behavior of the singular value spectrum of thecorrelators to the phase of the system can be extended to various types of phasetransitions in condensed matter physics.
This work has been done in collaboration with Hrant Gharibyan (Stanford), Masanori
Hanada (Southampton), and Brian Swingle (Maryland).
References:[1] H. Gharibyan, M. Hanada, B. Swingle, and M. Tezuka, “Quantum Lyapunov Spectrum,”arXiv:1809.01671 (quant-ph).
[2] H. Gharibyan, M. Hanada, B. Swingle, and M. Tezuka, preprint.

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Canceled

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Spin freezing crossover and SYK strange metal
Naoto Tsuji
RIKEN Center for Emergent Matter Science (CEMS)
Multiorbital systems offer a playground of strong correlation physics, including orbitalselective Mott transitions, non-Fermi liquids, and unconventional superconductivity.
Those physics can be successfully described by the multiorbital Hubbard model, whichfinds various material applications such as Sr2RuO4 and iron-based superconductors. In
the multiorbital Hubbard model with Hund’s coupling, there is a competition between the
Kondo effect (screening of local magnetic moments) and the Hund effect (formation of
local moments), between which emerges a highly moment-fluctuating regime called thespin freezing crossover (Fig. 1). The spin freezing crossover regime shows the anomalousTfrequency dependence of the self-energy (Σ(𝜔)~𝜔&/( ) and slow relaxation of the spincorrelation (⟨𝑆+ (𝜏)𝑆+ (0)⟩ ∼ 𝜏 0& ).
Recently, it has been pointed out that such a spin freezing crossover state has a striking
similarity to the Sachdev-Ye-Kitaev (SYK) model [1], which shows a maximally chaotic
behavior as probed by out-of-time-ordered correlators (OTOCs), and is expected to be aholographic dual to black holes. Motivated by this observation, we study OTOCs for the
Hubbard model with a new technique using an imaginary-time four-point function withunusual time ordering [2]. The results
show that in the spin freezing crossover
regime a spin-related OTOC rapidly
decays without oscillations in a shortFermi liquidfrozen moment
time, and relaxes more slowly as a
power law in a long time. This is in agood agreement with the behavior of
OTOCs in the SYK model, providing a
SYK strange metal?
firm evidence that the spin freezing
0
0.25
0.5
nσ
crossover regime in the multiorbital
Hubbard model effectively realizes theFig. 1: A schematic phase diagram of the multiSYK strange metal state.
orbital Hubbard model with Hund’s coupling.

[1] P. Werner, A. Kim, and S. Hoshino, Europhys. Lett. 124, 57002 (2018).
[2] N. Tsuji and P. Werner, arXiv:1812.04217.

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Dynamics of fractional excitations in quantum spin ice
Masafumi Udagawa1 and Roderich Moessner2
Dept. of physics, Gakushuin University, Japan1,
Max Planck Instisute for the Physics of Complex Systems2
Fractional excitation is one of the surprising manifestations of topologically orderedsystems. As a typical example, quantum spin ice, which is known as U(1) quantum spin
liquid, hosts fractional excitations, called magnetic monopoles. The monopole
excitations draw considerable attention due to their possible roles in the dynamical andtransport properties of several candidate materials of quantum spin ice [1]. However,
due to the fractional nature of excitations, magnetic monopoles show quite different
behaviors from conventional particles, such as magnons, and their character still
remains quite elusive.
To understand the nature of magnetic monopoles, we adopt the spin-1/2 XXZ model
defined on a pyrochlore lattice. We focus on the region where quantum fluctuation is
sufficiently small, and treat the transverse exchange in the first-order degenerate
perturbation theory, and obtain the dynamical spectrum of fractional excitations withexact diagonalization (ED) of 32 site cluster. We further construct a hopping model ofmonopoles by mapping to the three-leaf Husimi cactus, and found that this effectivemodel reproduces the numerical result of ED quite well. The obtained spectrum shows asteep edge at low energy as shown in Fig. 1, which is attributed to van Hove singularityof monopole spectrum, enhanced by the coupling to background “gauge field" [2]. We
will discuss the relevance of this result to an actual material, such as Pr Zr(Sn, Hf) O ,and propose an experiment to identify the U(1) quantum spin liquid, through theobservation of fractional excitations.
22

7

References:[1] Y. Tokiwa, T. Yamashita, D. Terazawa, M. Udagawa,S. Kittaka, T. Sakakibara, Y. Shi- moyama, T.
Terashima, Y. Yasui, T. Shibauchi, and Y. Matsuda, Nat.
Commun. 7, 10807 (2016).
[2] M. Udagawa and R. Moessner, to appear in Phys.
Rev. Lett.
Fig. 1: Two-monopole density of stateswith ED and the effective model

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Topological phase transitions induced by pressure, band-filling, andmagnetic field in strongly-correlated pyrochlore iridates
Kentaro Ueda
Department of Applied Physics, the University of Tokyo
Recent widespread research in the field of topological quantum states has been exploring thepotential role of strong electron correlation which can open a wide range of possible realizationsof emergent topological phases. Pyrochlore iridate R2Ir2O7 (R=rare-earth ions) is one of the mostpromising candidates. The forte of this system is that the metal-insulator transition (MIT) can be
finely tuned by changing R ions via the modulation bandwidth; the R=Pr compound isparamagnetic metal down to the lowest temperature, while others with smaller R-ionic radius
thermally turn into an insulator concomitantly with or separately from the antiferromagnetic(all-in all-out) order at low temperatures [1], reminiscent of a Mott transition system such as
vanadates and nickelates. Recent intensive study unveils that the ground state of the R=Pr
compound is so-called Luttinger semimetal where a quadratic band touching point exists at thecenter of Brillouin zone as guaranteed by the lattice symmetry [2]. Interestingly, a symmetry
breaking is suggested to turn this state into versatile emergent states including magnetic Weyl
semimetal and topological Mott insulator, which remains to be studied.
Here we present several results regarding phase transitions in pyrochlore iridates, which are
induced by pressure, band-filling, and magnetic field. (i) Magnetic-field induced MIT isobserved in (Nd,Pr)2Ir2O7 by electric transport measurements. We suggest that multiple
topological electronic states show up near the quantum critical point via modulations ofmagnetic structures. (ii) A sizable spontaneous Hall effect is accompanied by a vanishingly
small magnetization only within the narrow temperature window in the band-width controlled(Nd,Pr)2Ir2O7. This is attributable to the emergence of Weyl points in the course of the MIT. (iii)
The hole-doping (substitution of R ions with Ca ones) drives the transition from
antiferromagnetic insulator to paramagnetic metal in (Eu,Ca)2Ir2O7. The thermopower reaches aremarkable value in a wide range of doping level and shows the unique temperature dependence.
Combined with the LDA+DMFT calculation, we suggest that the quadratic band touching is acommon and robust feature in the paramagnetic metallic phase of hole-doped pyrochlore
iridates even in proximity of the Mott transition.
This work is performed in collaboration with Y. Tokura, J. Fujioka, R. Kaneko, N. Nagaosa, R.
Arita, B.-J. Yang, and H. Ishizuka.
[1] K. Matsuhira et al., J. Phys. Soc. Jpn. 80, 094701 (2011); K. Ueda, J. Fujioka, and Y. Tokura, Phys.
Rev. B 93, 245120 (2016).

[2] T. Kondo et al., Nat. Commun. 6, 10042 (2015).

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Disorder-enhanced quantum fluctuations in a quasi-two-dimensionalorganic Mott system
M. Urai,1 T. Furukawa,2 Y. Seki,1 K. Miyagawa,1 H. Taniguchi,3 T. Sasaki,4and K. Kanoda1
1

2

Department of Applied Physics, University of Tokyo, Japan
Department of Applied Physics, Tokyo University of Science, Japan3
Department of Physics, Saitama University, Japan
4
IMR, Tohoku University, JapanThe layered organic conductor k-(ET)2Cu[N(CN)2]Cl (abbreviated to k-Cl) is an
antiferromagnetic (AFM) Mott insulator at ambient pressure, and it exhibits apressure-induced first-order Mott transition [1,2]. Recent studies of k-Cl have shown that
X-ray irradiation causes drastic changes of charge transport [3] and magnetic properties[4], casting new light on the interplay between electron correlation and disorder.
In the present work, we have performed the transport measurements of a successively
X-ray irradiated k-Cl crystal under continuously controlled He-gas pressure. The mostnoticeable irradiation effect is the suppression of the Mott critical endpoint, which
decreases much more rapidly than the transition temperature of the disorder-sensitive
non-s-wave superconductivity, indicating that the Mott critical endpoint is fragile todisorder. In contrast, the quantum critical scaling of resistivity observed in the pristine
k-Cl [5] holds with a similar critical exponent zn after the irradiation of 70 hours. The
invariant value of zn indicates that the system is still in the regime of interaction-driven
metal-insulator transition.
As for the magnetism, the 1H NMR measurements revealed the extreme fragility of thecommensurate AFM order; it collapses into a randomly-frozen spin state even when thedisorder is so weak that the mean distance between the disorder centers is several
hundred Å. A further increase in disorder reduces the frozen moments, leading to theemergence of a quantum disordered spin state with gapless excitations.
The present results demonstrate that disorder enhances quantum fluctuations in both the
charge and spin degrees of freedom in a quasi-two-dimensional Mott transition system.
References:[1] P. Limelette et al., PRL 91, 016401 (2003).
[2] F. Kagawa et al., PRB 69, 064511 (2004).
[3] T. Sasaki, Crystals 2, 374 (2012).
[4] T. Furukawa et al., PRL 115, 077001 (2015).
[5] T. Furukawa et al., Nat. Phys. 11, 221 (2015).

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High-throughput search for rare-earth-free permanent magnets
Alena Vishina, Heike C. Herper, Olga Yu. Vekilova
Uppsala University, Uppsala, SwedenHigh performance permanent magnets, or materials with large uniaxial magnetic
anisotropy, are needed for a large number of applications, such as electric motors andgenerators, wind mills, and many more. At the same time these days most highperformance magnets contain rare-earth (RE) materials which makes them expensive,
while some of the RE elements (like Dy) are rapidly decreasing in availability. According
to the assessment of European Union in 2014 not only the RE elements, but also Pt andCo, and phase stabilizers like Ge and In, used in permanent magnets, have been classified
as critical raw materials.
Going through a large number of known structures (ICSD database [1]) and using afull-potential linear muffin-tin method with relativistic formulation in RSPt electron
structure code [2] to calculate magnetic anisotropy and Curie temperature, we are looking
for the materials with high magnetization >1 T, uniaxial anisotropy >1 MJ/m3, and
Tc>300 K to identify the suitable replacement for hazardous materials, without a loss of
performance in permanent magnets.

References[1] http://www2.fiz-karlsruhe.de/icsd_home.html[2] John M Wills, Olle Eriksson, Mebarek Alouani, David L. Price, arXiv:cond-mat/9912173(1999)

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Thermoelectric effects in a doped spin liquid candidate κ-(ET)4Hg2.89Br8
K. Wakamatsu1, T. Fujii2, Y. Suzuki1, K. Miyagawa1, H. Taniguchi3 and K. Kanoda1
1

Department of Applied Physics, University of Tokyo, 2 Cryogenic Research Center, the University ofTokyo, 3 Department of Physics , Saitama University

κ-(ET)4Hg2.89Br8 is a quasi-two-dimensional organic conductor that possibly host a doped spin liquid[1]. The previous studies revealed a pressure-induced crossover or transition from a non-Fermi liquid(NFL) to a Fermi liquid (FL) in resistivity behavior (Fig.1) and from a low to high carrier-density statesin the Hall coefficient [2]. The superconductivity that emerges at low temperatures was suggested to
exhibit a crossover from local to extended paring with increasing pressure by the evaluation of thecoherence length [3]. These results imply that a crossover from NFL/BEC to FL/BCS is driven by thechange of electron correlation.
In the present work, we investigated thermoelectric effects, which are useful to reveal the electronic
states, for κ-(ET)4Hg2.89Br8 under pressure. We found that, in the crossover pressure region, the Seebeckcoefficient divided by temperature, S/T, are enhanced at low temperatures (Fig.2) and its temperaturedependence is logarithmic. Similar logarithmic enhancement of S /T is also reported in other systems
situated in the vicinity of quantum phase transitions, suggesting that the present observation originatesfrom NFL-FL quantum critical fluctuations. Regarding the superconductivity, the Nernst voltage is
generated at higher temperature than Tc and keeps large values at higher magnetic fields in the
low-pressure regime than in the high-pressure regime, indicating that superconducting fluctuations arestrongly enhanced in the doped spin liquid state at low pressures.
The present results of thermoelectric studies support a quantum phase transition or crossover between acorrelated metal and a doped spin liquid, followed by a BEC-BCS crossover at low temperatures.

Fig. 1: Phase diagram of κ-HgBr [4]Fig. 2: Color plot of |S/T| in κ-HgBr[1] H. Oike, et al., Nat. Commun. 8, 756 (2017). [2] H. Oike et al., Phys. Rev. Lett. 114, 067002 (2015). [3] Y.
Suzuki, Master Thesis (2017). [4] H. Oike, Doctoral Thesis (2013).

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Two Competing Superconducting Phasesin Molecular Conductors k-(BEDT-TTF) X
2

Hiroshi Watanabe1, Hitoshi Seo1,2, and Seiji Yunoki1,2,3
1
2

RIKEN Cluster for Pioneering Research (CPR), Saitama 351-0198, JapanRIKEN Center for Emergent Matter Science (CEMS), Saitama 351-0198, Japan
3

RIKEN Center for Computational Science, (R-CCS), Hyogo 650-0047, Japan

Unconventional superconductivity (SC) is often observed around the Mott transition instrongly-correlated electron system. In the family of molecular conductors
k-(BEDT-TTF) X with a 3/4-filled energy band, the application of external pressure or
chemical substitution of X to the so-called dimer-Mott insulators induces SC whose gap
2symmetry is still controversial [1]. Although k-(BEDT-TTF) X is often compared withthe high-T cuprates with several similarities [2], there are also differences which should2c

be carefully examined. In k-(BEDT-TTF) X, SC appears next to the nonmagnetic Mott
insulators as well as the AF Mott insulators. It is not clear whether the AF spin
fluctuation mechanism discussed in the cuprates can be directly applied to2k-(BEDT-TTF) X. Furthermore, the intradimer charge degree of freedom, which is acharacter of molecular materials, plays important roles as suggested in the anomalous
dielectric responses and Raman spectroscopies.
2

In this study, we investigate the extended Hubbard model for k-(BEDT-TTF) X where
the intradimer charge degree of freedom and the intersite (long-range) Coulombinteractions are explicitly considered. The variational Monte Carlo method is used forthe analysis of the ground-state properties. We obtain the ground-state phase diagram
for both bandwidth-control [3] and filling-control [4] cases. There appear various
competing phases: dimer-type AF phase, polar charge-ordered phase, two 3-foldcharge-ordered phases, and two SC phases with different gap symmetries. The“extended-s+d ”-wave SC is favored for the electron-doped side and the “d ”-wave SC,
which is similar to the symmetry in the cuprates, is favored for the hole-doped side. We
show the origin of two SC phases from the viewpoint of spin/charge fluctuations and the
2

2

x -y

2

xy

geometrical frustration unique to k-(BEDT-TTF) X.
2[1] K. Kanoda, J. Phys. Soc. Jpn. 75, 051007 (2006).
[2] R. H. McKenzie, Science 278, 820 (1997).
[3] H. Watanabe, H. Seo, and S. Yunoki, J. Phys. Soc. Jpn. 86, 033703 (2017)[4] H. Watanabe, H. Seo, and S. Yunoki, arXiv: 1811.09035.

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Theory of Proton-Driven Quantum Spin-Dipole Liquids
Masahiko G. Yamada and Yasuhiro Tada
Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan
We present a theory of a hybrid quantum liquid state, quantum spin-dipole liquid(QSDL), in a hydrogen-bonded electron system, by combining the concepts of aquantum proton ice and Anderson's resonating valence bond (RVB) theory [1],motivated by the recent experimental discovery of a proton-driven QSDL in
κ-H3(Cat-EDT-TTF)2 called H-Cat [2]. In this material, it was observed experimentallythat quantum spin liquid is strongly coupled to quantum proton motion. In our theory, anelectron spin liquid and a proton dipole liquid are realized simultaneously, while none ofthem can be established independently. Analytical and numerical evaluations ofentanglement entropy reveal that this state is far beyond the crude Born-Oppenheimer
approximation, showing a volume-law entanglement entropy between spins and dipoles.
We also examine the stability of QSDL against perturbations and discuss implications
for the experiments in H-Cat and its deuterated analog D-Cat.

References[1] P. W. Anderson, Mater. Res. Bull., 8, 153 (1973).
[2] M. Shimozawa et al., Nat. Commun., 8, 1821 (2017).

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Phase transitions in non-Hermitian fermionic superfluidity undercomplex-valued interactions
Kazuki Yamamoto and Norio Kawakami
Dept. of Phys. Kyoto Univ.
In recent years, non-Hermitian (NH) quantum systems have been actively studied bothexperimentally and theoretically. It has been revealed that non-Hermiticity drasticallyalters the properties of a number of quantum phenomena that have been established in
the Hermitian physics [1,2]. Such theoretical predictions have been confirmed
experimentally by using optical systems and ultracold atoms. However, since most of
the previous studies dealt with single-particle physics, understanding of many-body
physics in NH systems is yet in its infancy.
Motivated by recent experimental advances in ultracold fermionic atoms, we analyze a
non-Hermitian (NH) BCS Hamiltonian with complex-valued interactions arising frominelastic scattering between fermions. We develop a mean-field theory to obtain a NH
gap equation for order parameters, which are similar to but different from the standard
BCS ones because of the inequivalence of left and right eigenstates in the NH physics.
We find unconventional phase transitions unique to NH systems: the superfluidity
breaks down and reappears with increasing dissipation, featuring non-diagonalizable(exceptional) points, lines, and surfaces in the quasiparticle Hamiltonian for weak
attractive interactions. As for strong attractive interactions, the superfluid gap nevercollapses but is enhanced by dissipation due to an interplay between the BCS-BEC
crossover and the quantum Zeno effect. Our results lay the groundwork for studies offermionic superfluidity of ultracold atoms under inelastic collisions.
Acknowledgement: This work is done in collaboration with Masaya Nakagawa(RIKEN), Kazuaki Takasan (Kyoto Univ.), Kyosuke Adachi (Kyoto Univ.), Masahito
Ueda (Univ. of Tokyo &RIKEN).
References:[1] Yuto Ashida, Shunsuke Furukawa, Masahito Ueda, Nature Commun. 8 (2017) 15791.

[2] Masaya Nakagawa, Norio Kawakami, Masahito Ueda, Phys. Rev. Lett. 121 (2018)
203001.

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Non-Hermitian perspective of strongly correlated systems
Tsuneya Yoshida1, Robert Peters2,
Norio Kawakami2, and Yasuhiro Hatsugai1
1

University of Tsukuba, 2 Kyoto University

Recently, a variety of new topological phenomena have been reported for
non-Hermitian systems. So far, such phenomena are supposed to be observed for
non-equilibrium systems. Interestingly, however, it became clear that non-Hermitianphenomena may emerge even in equilibrium systems because of finite lifetime ofquasi-particles. The analysis based on the perturbation theory elucidates the emergenceof exceptional points accompanied by bulk Fermi arcs [1].
Inspired by the above results, we analyze strongly correlated systems where the effectsof quasi-particle damping are important. Specifically, we address the following two
issues.
(i) Numerical observation of the exceptional points beyond the perturbation theory [2].
We address this issue by analyzing the heavy-fermion systems with DMFT+NRG.
In addition to the emergence of exceptional points we observe that the lattice
periodicity results in the fusion of exceptional points and the bulk Fermi loop.
(ii) Interplay of symmetry and exceptional points: emergence of symmetry-protected
exceptional rings and torus [3].
In the second part, we analyze the interplay of exceptional points and symmetry
which plays an important role for Hermitian topological systems. We analyze thesystems with chiral symmetry as an example and observe that thesymmetry-protection results in novel topological degeneracies which we callsymmetry-protected exceptional rings and torus. The former and the latter are
observed for two and three dimensions, respectively. In addition, we propose ahoneycomb lattice model hosting the above topological degeneracies.

[1] V. Kozii and L. Fu arXiv: 1708.05841.
[2] Tsuneya Yoshida, Robert Peters, and Norio Kawakami, Phys. Rev. B 98 115147 (2018).
[3] Tsuneya Yoshida, Robert Peters, Norio Kawakami, and Yasuhiro Hatsugai arXiv:1810.06297(to appear in PRB Rapid Communication)

154

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Constructing Neural Stationary States for open quantum many-body systems
Nobuyuki Yoshioka and Ryusuke Hamazaki
Dept. of Physics, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.
The dramatic development of machine learning techniques has inspired physicists to invent new
numerical algorithms that further explore the frontier of condensed matter physics. Successful
applications include the phase classification using the well-established algorithms such as the deeplearning [1,2] and the representation of the quantum many-body states using the high expressive
power of the neural networks [3].
Despite its rapid progress, however, machine-learning-based algorithms have yet to be applied to
one of the most challenging problems in modern physics -- open quantum many-body systems. Dueto the growth of the number of parameters in proportion to the square of the Hilbert space dimension,
description of the quantum states by density matrices requires additional computational resourcecompared to the closed system.
In this work, we present a new scheme for simulating the stationary states of open quantum
many-body systems by employing the ansatz based on the neural-network quantum states [4]. See
Fig.1 for a schematic illustration of our method. Using the high expressive power of the variational
ansatz described by the restricted Boltzmann machines (RBM), the stationary states of the Lindblad
dynamics is computed. After mapping the stationary-state search problem into finding a zero-energy
ground state of an appropriate Hermitian operator, we apply the conventional variational Monte
Carlo method for optimization. Our method is shown to simulate various spins systems efficiently,
i.e., the transverse-field Ising models in both one and two dimensions and the XYZ model in one
dimension.

Fig. 1: Schematic illustration of our method for the case with two spins.
(a) (Left) A spin model with dissipations. (Right) The vector representation of the Lindbladian ason the doubled Hilbert space.
(b) Neural stationary state represented by the RBM, which is composed of the physical spins σi,
fictitious spins τi, and hidden spins hj.
References:[1] J. Carrasquilla and R. G. Melko, Nat. Phys. 13, 431 (2017).
[2] N. Yoshioka, Y. Akagi, and H. Katsura, PRB 97, 205110 (2018).
[3] G. Carleo and M. Troyer, Science, 355, 602 (2017).
[4] N. Yoshioka and R. Hamazaki, arXiv:1902.07006 (2019).

155