令 和 ２ 年 度
京 都 大 学 大 学 院 理 学 研 究 科

Ｄ Ｃ ３ 回 生 研 究 発 表 会
要旨集

２０２０年１１月２０日（金）物 理 学 第 一 分 野

物理学第一分野ＤＣ３回生研究発表会
場所：理学研究科５号館 ５階・第四講義室
発表：２０分（別に質問１０分程度）
２０２０年１１月２０日（金）９：００～

目

開始

次

１．Quantum spin transport using ultracold ytterbium atoms with orbital degrees of freedom
Koki Ono（ ９：００）
・・・・・ １２．Turbulent relative dispersion in two-dimensional energy inverse-cascasde turbulence
Tatsuro Kishi（ ９：３０）・・・・・ ２
３．Transport properties of photoexcited carriers and excitons in ultrapure diamond
Kazuki Konishi（１０：００）
・・・・・ ３
４．Unconventional quantum oscillations and thermal metallic state ofcharge-neutral fermions in Kondo insulator YbB12
Yuki Sato（１０：３０）
・・・・・ ４
１１：００～１１：１０ 休憩
５．Controlling the polarization of high-order harmonics in solids bymanipulating the trajectory of Bloch electrons
Yasuyuki Sanari（１１：１０）
・・・・・ ５
６．Study on the characteristic motion of swimming droplets
Saori Suda（１１：４０）
・・・・・ ６
７．Nonequilibrium quantum phenomena and topologicalsuperconductivity in atomic layer materials
Hiroomi Chono（１２：１０）
・・・・・ ７
１２：４０～１３：４０ 昼休み
８．Anisotropic response of spin susceptibility on a spin-triplet superconductor UTe2
Genki Nakamine（１３：４０）・・・・・ ８９．Structure of single Xe nanoparticles upon crystallization
Akinobu Niozu（１４：１０）
・・・・・ ９１０．Injection Locking of Resonant-Tunneling-Diode Terahertz Oscillator
Tomoki Hiraoka（１４：４０）
・・・・・１０１５：１０～１５：２０ 休憩
１１．Identifying exogenous and endogenous activity in social media
Kazuki Fujita（１５：２０）
・・・・・１１
１２．Theoretical study of nonlinear transport in parity-violating magnets
Hikaru Watanabe（１５：５０）
・・・・・１２

Quantum spin transport using ultracold ytterbium atoms
with orbital degrees of freedom
Quantum Optics Group

Koki Ono

Abstract We report on quantum spin transport experiments using ytterbium atoms in the ground state 1S0
induced by the localized atoms in the metastable state 3P0. We succeeded in the observation of theinterorbital spin-exchange dynamics, and the demonstration of the novel quantum spin transport.
© 2020 Department of Physics, Kyoto University

Ultracold atomic gases in an optical lattice have reproduced paradigmatic models of condensed matter physics
such as the Hubbard model, demonstrating the ability of quantum simulation [1]. Recently, alkaline-earth-likeatoms have attracted considerable research interest due to the existence of the long-lived metastable states 3P0 and
3P coupled to the ground state 1S . This intriguing internal structure can provide the platform for the quantum
20
simulation with orbital degrees of freedom [2].
We present two experiments with the two-orbital system consisting of the 1S0 atoms and the 3P0 atoms. The firstwork is motivated by the quantum simulation of the Kondo effect, which is a prominent example of the quantum
many-body phenomenon highlighting a relevant role of the orbital and spin degrees of freedom. Several schemes
for cold-atom quantum simulation of the Kondo effect have been proposed. In particular, the two-orbital systemusing the fermionic isotope 171Yb has received much attention as a promising candidate for the quantum
simulation of the Kondo effect since our high-resolution laser spectroscopy reveals that the spin-exchangeinteraction between the 1S0 atom and the 3P0 atom is antiferromagnetic [3], one of the key ingredients for the
emergence of the Kondo effect. In order to realize the Kondo system, we implement the two-orbital lattice system
consisting of a one dimensional (1D) state-dependent optical lattice (SDL) with a wavelength of 650.7 nm and a
2D state-independent optical lattice with a wavelength of 759.4 nm, which is called the magic wavelength. The
1D SDL gives a strong confinement to the 3P0 atom and is superimposed along the axis of the 2D array of the tube
traps created by the 2D magic optical lattice, realizing the quasi (0+1) D system where the 1S0 atom and the 3P0
atom are itinerant and localized, respectively. We successfully observe the interorbital spin-exchange dynamics of
171Yb in the two-orbital lattice system, opening the route to the quantum simulation of the Kondo effect.
Secondly, we report on the demonstration of a novel type of localized-impurity-induced quantum spin transport
using the two-orbital system. So far, most studies of quantum transport in condensed matter systems have focusedon a nonequilibrium steady state coupled to external incoherent reservoirs. Ultracold atomic gases allow one tofocus on intrinsic transport properties because the system can be regarded as isolated from the external
environment. In order to emulate a mesoscopic system with the cold atoms, however, a high degree ofcontrollability to engineer a narrow conducting channel could be required [4]. Alternatively, the system consistingof a multi-component Fermi gas and a localized impurity provides a more feasible scheme of the quantum
transport experiment, where the current can be created in the spin space by introducing the spin-dependentinteraction with the impurity [5]. In this work, we observe the decoherence of the nuclear spin precession of the
1S atoms in the presence of the localized 3P atoms, exhibiting the quantum spin transport induced by the00localized impurities. Our work paves the way to the quantum simulation of quantum transport, which is called
atomtronics.
References[1] R. P. Feynman, Int. J. Theor. Phys. 21, 467–488 (1982).
[2] A. V. Gorshkov et al., Nat. Phys. 6, 289-295 (2010).
[3] K. Ono et al., PRA 99, 032707 (2019).
[4] J.-P. Brantut et al., Science 337, 1069 (2012).
[5] J.-S. You et al., PRB 99, 214505 (2019).

Turbulent relative dispersion in two-dimensional energy
inverse-cascasde turbulence
Fluid Physics Group

Tatsuro Kishi

Abstract Turbulence quickly disperses substances, which is one of the basic features of turbulence. We
propose the scaling relation of statistics on relative separation of particle pairs via two-time Lagrangian
velocity correlation function and confirm it by direct numerical simulations of two-dimensional energy
inverse-cascade turbulence.
© 2020 Department of Physics, Kyoto University

C L (t1, t2)

Transport phenomena are ubiquitous in nature. Especially, turbulence efficiently transports and promptly disperses substances such as pollutants, chemical and biological agents. The turbulent diffusion is a very importantphenomenon with various applications from cosmology, meteorology and biology to environmental problems,chemistry and engineering. Furthermore it is the extraordinary capability to disperse substances that characterizes
turbulence itself. Therefore, it is also significant for fundamental physics to understand the nature of turbulence.
The concentration of substances exhibits complex structures over broad scales of time and space because of thenonlinearity of turbulent flows. Thus, one-particle statistics such as the root-mean-square distance is insufficient tounderstand the multiscale nature of turbulent diffusion. Space-time correlations of particle positions are intrinsic
characteristics. The most fundamental one is the correlation between a particle pair [1].
We consider the simplest case, that is, turbulent diffusion of Lagrangian particles, which have no inertia, no backinfluence on the fluid velocity and no effects of molecular diffusion. Furthermore, we only deal with the statisticallystationary, homogeneous and isotropic turbulence. In such a case, we can deal with the relative distance, r(t) insteadof each position of particle pairs, x1 (t) and x2 (t), and thus the number of variables is reduced from particle-pairstatistics. We call the statistics of relative distance turbulent relative dispersion.
The turbulent relative dispersion can be dimensionally analyzed
by Kolmogorov’s phenomenology, which successfully describes the
0.30
Eulerian velocity statistics of turbulence, though they are Lagrangian
0.25
0.20
statistics [2]. The square mean separation of particle pairs is given
0.15
2
3
by hr (t)i ' gεt , exhibiting super-diffusivity. Here ε is the en0.10
0.05
ergy dissipation rate of turbulence and g is a universal constant called
0.00
the Richardson constant. However, there is no consistent theoretical
20
15
derivation from the Navier-Stokes equations, and also there is no clear
10
5
20
15
10
observation by both laboratory and numerical experiments [3].
0
5
0
t1/Tη
We discuss the turbulent relative dispersion in terms of the initial
separation dependence via the two-time Lagrangian velocity correla- Fig. 1: Two-time Lagrangian velocity corretion function C L (t1 , t2 ) as shown in Fig.1, which is directly related to lation function, C L (t1 , t2 ). Tη is a time scaleof energy dissipation.
the mean square separation of particle pairs as follows:
Z tZ thr2 (t)i ' r02 +C L (t1 , t2 ) dt1 dt2 ,
(1)
Tη

t 2/

0

0

where r0 is an initial separation of particle pairs. Here C L (t1 , t2 ) ≡ hδv(t1 ) · δv(t2 )i, where δv(t) is a relative
velocity between a particle pair. We propose that the two-time Lagrangian velocity correlation function has aself-similar form with scaling exponents which can not be determined by the dimensional analysis. Furthermore,
we confirm the proposed scaling relation by direct numerical simulation of two-dimensional energy inverse-cascade
turbulence. We discuss finite Reynold number effects on the scaling exponents and show the improved scaling law
for the turbulence relative dispersion.

References[1] L. F. Richardson, Proc. R. Soc. A Math. Phys. Eng. Sci. 110, 709 (1926).
[2] J. P. L. C. Salazar and L. R. Collins, Annu. Rev. Fluid Mech. 41, 405 (2009).
[3] T. Kishi, T. Matsumoto, and S. Toh, Physical Review Fluids 5, 054601 (2020).

Transport properties of photoexcited carriers and excitons
in ultrapure diamond
Solid State Spectroscopy Group

Kazuki Konishi

Abstract We have investigated transport properties of photoexcited carriers and excitons in asemiconductor diamond of extremely high purity. A clear correlation between the exciton diffusion
coefficient, lifetime, and residual strain was found from 4 to 300 K for the first time. We propose
an extended surface recombination model, which successfully explains the temperaturedependence of the carrier and exciton lifetimes and luminescence quantum efficiency of excitons.
© 2020 Department of Physics, Kyoto University

Lifetime (ns)An electron-hole system in a photoexcited semiconductor exhibits various complexes of carriers, such as
excitons, polyexcitons, and electron-hole droplets [1]. Even in a simple mixture of free carriers and excitons, thefraction of exciton density to the total carrier density largely affects the transport properties. In the recent study
using a monolayer semiconductor [2], the density dependence of the diffusion coefficient was investigated,revealing intriguing phenomena that occur due to many-body interactions. However, measurements at low
temperatures have not been performed because trapping at defects becomes prominent. In order to elucidatechanges in the transport properties as a function of temperature, we focus on diamond, in which the excitons arestabilized owing to the large exciton binding energy. Thanks to the high-quality diamond samples available today,
relationships between diffusion, lifetime, and residual strain have been systematically elucidated without impurity
trapping.
In this study, we used three single crystals of ultrapure diamond (Element Six, Ltd.) to ensure that the exciton
decay was not affected by trapping at impurities. Figure 1 shows the temperature dependence of the exciton
lifetime measured at different excitation spots by the time-resolved photoluminescence method [3]. By combining
information gained from the diffusion coefficient measurements, exciton fraction determined by terahertz
time-domain spectroscopy [4], and strain distribution evaluated by birefringence imaging, we revealed that
excitonic diffusion has a significant impact on the2lifetime in diamond. We proposed a surface
1000
recombination model extended for nonradiative
6
recombination at dislocations after diffusion (see the4
Inset), and the temperature dependence was successfully
2
explained from 4 to 300 K (solid lines). We also showedPLthat this model enables the prediction of internal
Pulse
laser
Diamond100
quantum efficiency of exciton luminescence, which is a6
crucial parameter for the optoelectronic applications of
4diamond toward realization of efficient devices working
2+ in the deep ultraviolet region useful for denaturation of+ Dislocation
10
viruses.
4
6
8
2
4
6
8
2
4
Furthermore, we evaluated the carrier lifetime and10100
mobility by the time-resolved cyclotron resonance andEffective temperature (K)time-of-flight (ToF) methods on the same samples from
2 to 300 K [5]. We demonstrated that the temperature Fig. 1 Exciton lifetimes measured at different spot
dependent electron lifetime is also explained by the positions (circles) in comparison with values
simple model proposed above. Therefore, our findings calculated according to the surface recombination
are generally applicable to a wide range of excitonic model (solid lines). Inset: Schematic of the exciton
diffusion measurements and recombination atmaterials in which excitons and free carriers coexist.
dislocations. Adapted from Ref. [3].
References[1] J. Omachi et al., Phys. Rev. Lett. 111, 026402 (2013).
[2] J. Zipfel et al., Phys. Rev. B 101, 115430 (2020).
[3] K. Konishi, I. Akimoto, J. Isberg, N. Naka, accepted for publication in Phys. Rev. B.
[4] T. Ichii, Y. Hazama, N. Naka, and K. Tanaka, Appl. Phys. Lett. 116, 231102 (2020).
[5] K. Konishi et al., accepted for publication in Appl. Phys. Lett.

Unconventional quantum oscillations
and thermal metallic state ofcharge-neutral fermions in Kondo insulator YbB12
Quantum Condensed Matter Group

Yuki Sato

Abstract The presence of Fermi surface, which is an unambiguous signature of metals, is manifested byquantum oscillations, and finite linear T-dependent terms in specific heat and thermal conductivity. Here,
we report notable observations of those “metallic” behaviors in Kondo insulator YbB12.
© 2020 Department of Physics, Kyoto University

Strong electronic correlation in many-body systems often results in a wide variety of ground states, such as
heavy Fermi liquid, unconventional superconductivity, and strongly correlated insulator. Understanding of thoseexotic electronic phases and excitations (i.e. emergent quasiparticles) from these ground states is the mostfundamental and important subject in condensed matter physics. Kondo insulator (KI) is a typical example of suchstrongly correlated insulators, where hybridization between conduction c-electron and localized f-electron opens
up charge gap across Fermi level. KIs have recently attracted much interest due to the observation of quantum
oscillations (QOs) in magnetization of SmB6 [1], which is a conflicting evidence of the Fermi surface of metals.
Here we present high field torque and transport measurements of another KI YbB12. Remarkably, YbB12
exhibits distinct QOs not only in magnetization but also in resistivity (Shubnikov-de Haas effect) even in theinsulating phase (Fig. a). The angular dependence of the oscillatory frequency captures 3-dimentional geometry,
indicating that the signals derive from the insulating bulk. The temperature dependence of the oscillatory
amplitude follows the conventional Fermi liquid theory of metals with a large effective mass, suggesting thepresence of an unconventional Fermi surface in an insulator [2].
We also show low temperature heat-transport measurements to discuss low energy excitations in the groundstate of YbB12. At zero field, sizeable linear T-dependent terms in specific heat and thermal conductivity areclearly resolved (Figs. b, c), indicating the presence of gap-less fermionic excitations with an itinerant character.
Remarkably, linear T-dependent thermal conductivity leads to a spectacular violation of the Wiedemann–Franzlaw: the Lorenz ratio is 104–105 times larger than that expected in conventional metals, indicating that YbB12 iselectrically insulating but thermally metallic. Moreover, we find these fermions couple to magnetic fields, despite
their charge neutrality. Our findings expose novel quasiparticles in this unconventional quantum state [3].

Fig. (a) Observed Shubnikov-de Haas effect. (b) Specific heat and (c) thermal conductivity data at low temperatures.

References[1] B. S. Tan et al., Science 346, 1208 (2014).
[2] Z. Xiang, Yuki Sato et al., Science 362, 65 (2018).
[3] Yuki Sato et al., Nature Physics 15, 954 (2019).

Controlling the polarization of high-order harmonics in solids bymanipulating the trajectory of Bloch electrons
Nanophotonics Group

Yasuyuki Sanari

Abstract The mechanism of high-order harmonic generation (HHG) in solids was investigated by using a
two-color excitation scheme. We manipulated the intraband and interband components of nonlinear
currents in GaSe and found that the polarization selection rules for HHG can be well explained byconsidering the two-dimensional trajectory of Bloch electrons.
© 2020 Department of Physics, Kyoto University

Recent developments in the generation of intense and ultrashort laser pulses have enabled us to study extremely
nonlinear optical phenomena, which cannot be treated in a perturbative manner [1]. One technically important
example of such extremely nonlinear light–matter interactions is high-order harmonic generation (HHG), where amaterial emits a broad spectrum consisting of many harmonics of the excitation frequency [2]. While the HHG in
atomic gas systems can be accurately explained by the so-called three-step model [3], the mechanisms of HHG insolids are still under debate. In the case of solids, it is considered that high-order harmonics (HHs) are generated
by the nonlinear currents induced by an incident laser field, that is, the current due to the motion of Blochelectrons in a band (intraband current) and the current that describes the nonlinear polarization between different
bands (interband current) [4]. Therefore, it can be expected that the manipulation of the trajectory of Blochelectrons allows us to control the HH polarization. Since a two-color excitation scheme using orthogonallypolarized laser fields enables a flexible manipulation of the trajectory of Bloch electrons, such two-color
excitation experiments should be very useful in clarifying the details of HHG in solids. For the polarization
control, it is also important to understand the relation between the HH polarization state and the crystal orientation,
because the nonlinear interband polarization is dominated by the symmetry of the system. In this work, we studied
the mechanism of the HHG in a GaSe crystal under excitation with orthogonally polarized laser fields at different
wavelengths, and discussed the relation between the HH polarization state and the crystal orientation.
In our experiments, a GaSe crystal was simultaneously excited by two orthogonally polarized optical pulses
with photon energies of ℏω1 = 0.517 eV (electric field amplitude |E1| ~10 MV/cm) and ℏω2 = 0.954 eV (|E2| ~1
MV/cm). We observed a broadband emission of HHs; the observed HH spectra exhibited multiple peaks at photon
energies equal to mℏω1 + nℏω2 where m and n are integers that define the harmonic order. Since ℏω2 was not an
integral multiple of ℏω1, it was possible to investigate the different orders independently. We found that theharmonic orders that obey the restriction m + n = odd are governed by the intraband contribution and thoseobeying m + n = even are governed by the interband contribution. The orthogonally polarized components of the
HHs generated by the interband current exhibited a strong crystal angular dependence, and thus the polarization
direction of these harmonic orders can be controlled by the crystal angle. On the other hand, we also observedorders whose polarization components along the direction of the weak incident electric field E2 exhibited highintensities at any crystal orientation, and these were assigned to components generated by the intraband current.
Because the angle dependence of the intraband-related components cannot be explained by conventional nonlinear
optics, we considered the electron trajectory in k-space that is manipulated by the weak electric field E2. Our
calculation results reproduced the experimental data well, suggesting that two-dimensionally driven Blochelectrons in the first conduction band generate a strong nonlinear current in the direction of E2. These results showthat a manipulation of the intraband and interband currents by controlling the laser fields and the crystal
orientation, enables the control of the polarization of HHs [5,6].
References[1] Y. Sanari et al., Phys. Rev. Lett. 121, 165702 (2018).
[2] S. Ghimire et al., Nat. Phys. 7, 138 (2011).
[3] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993).

[4] Y. Sanari et al., Phys. Rev. B 102, 041125(R) (2020).
[5] Y. Sanari et al., Nat. Commun. 11, 3069 (2020).
[6] Y. Sanari et al., under preparation for submission.

Study on the characteristic motion of swimming
droplets
Laboratory of Dissipative and Biological Physics

Saori Suda

Abstract The transition from linear to curvilinear motion of a spherical swimming microdroplet wasstudied experimentally and theoretically. The experimental results and the model's analysis show that the
motion transition depends on the droplet's sensitivity to external perturbations in the direction of motion.
© 2020 Department of Physics, Kyoto University

Over the last few decades, there has been a growing interest in active matters. The
knowledge of physical phenomena specific to self-propelled objects has potential
applications in various fields such as statistical physics, chemical technology, and life
sciences [1]. Swimming droplets have received much attention due to their relative ease ofchemical control. Furthermore, various life-like motions have been observed in individual
droplets as well. However, the mechanism of their motion transitions is not yet clearly
understood.
There exist several reports of motion transitions in swimming droplets. In liquid crystal
droplets, the extraction of specific modes of motion is possible via control of theinternal symmetry of the liquid crystal phase [2]. For two-dimensional droplets, studies
have experimentally and theoretically investigated spontaneous symmetry breaking andstraight-to-curvilinear motion transitions [3]. Recently, it has been reported byexperimental research that an increased concentration of external surfactants initiates
persistent random motion of swimming spherical droplets [4]. Although the straight-tocurvilinear motion transition is one of the most fundamental spontaneous symmetry breaking
phenomena observed in a self-propelled system, the mechanism behind the transition has notbeen explicated for a three-dimensional system of droplets.
In this study, the motion and internal flow of spherical swimming water microdroplets aremeasured, and the transition from straight to curved motion, including randomly steered
one, is quantitatively investigated in a three-dimensional system. A three-dimensional
theoretical model is constructed to clarify the motion transition mechanism, using theadvection-diffusion equation, the source and removal terms, and the Stokes equation (Eq.
(1)).
∂c
∂t

1

π

+ 𝐮 ⋅ ∇c = 𝑅2 DΔsphere c − αc + αβδ (θ − 2 , ϕ − ϕ0 (𝐯)),(1)

The model was validated quantitatively by comparing it with the measured internalconvection of the droplet. The model's analysis reveals that the motion transition depends
on the perturbation-sensitivity, which varies with the size of the droplet.
References[1] T. Vicsek and A. Zafeiris, Physics Reports 517, 71 (2012).
[2] M. Suga, S. Suda, M. Ichikawa, and Y. Kimura, Physical Review E 97, 062703 (2018).
[3] K. H. Nagai, F. Takabatake, Y. Sumino, H. Kitahata, M. Ichikawa, and N. Yoshinaga, Physical Review E 87,
013009 (2013).
[4] A. Izzet, P. G. Moerman, P. Gross, J. Groenewold, A. D. Hollingsworth, J. Bibette, and J. Brujic, Physical Review
X 10, 021035 (2020).

Nonequilibrium quantum phenomena and topologicalsuperconductivity in atomic layer materials
Condensed Matter Theory Group

Hiroomi Chono

Abstract We proposed a way to realize topological s-wave superconductivity with the application of circularly
polarized laser light in transition metal dichalcogenides bilayer. We believe that our study is an important firststep to search for topological superconductivity in various atomic layer materials with a weak spin-orbit
coupling.
© 2020 Department of Physics, Kyoto University

In the last two decades, motivated by the breakthrough of the laser experimental techniques, controlling ofphysical properties of materials by terahertz wave and intense short-pulse lasers have been attracting attention.
The photo-induced phase transition has been also significantly garnered attention. As a further development,
photo-induced topological phase transitions, where the topology of an electronic system is changed by laser light,
are studied intensively. Floquet theory is a method for theoretically describing such nonequilibrium steady-state,
and the attempts are called Floquet engineering. Floquet engineering has attracted interests as one of the strategies
for realizing topological phases of matter which is difficult to be stabilized in equilibrium. For example,
photo-induced anomalous quantum Hall states were theoretically proposed in graphene [1], and the observation ofthis phenomenon was experimentally confirmed recently [2]. It was also proposed that laser light can induce thetopological superconducting phase by opening the band gap in thin films of cuprates, which are nodal d-wave
superconductors [3]. Such nonequilibrium steady-state and relaxation processes induced by laser light are thestage for the realization of physical properties that cannot be realized in the equilibrium state. Furthermore, theseattempts have been extended to the superconducting phases.
On the other hand, the recent inauguration of nanotechnology and atomic layer materials research has createdan experimental environment for two-dimensional superconductivity with atomically thin materials. Theirlow-dimensionality has the merit of enabling us to control the superconducting properties. For making insulating
materials superconductive, a high-density electric carrier is required. The solution is to use an electric
double-layer transistor (EDLT), where ionic liquids and electrolytes are used as the gate electrodes. With thismethod, two-dimensional electron systems can be transferred to the superconducting phase by electric field
induction. In the previous experiments, transition metal dichalcogenides (TMD) such as MoS2 [4] and NbSe2, andiron-based superconductors FeSe have been focused.
Motivated by these situations, we have studied the nonequilibrium quantum phenomena and proposed possible
schemes to realize topological superconductivity (TSC) with the application of circularly polarized laser light in
two-dimensional TMD layered materials [5]. In contrast to a previous proposal of laser-induced TSC, the TSC in
this study does not need SOC and it is robust against the SOC with a realistic magnitude.
References[1] T. Oka and H. Aoki, Phys. Rev. B, 79, 081406 (2009).
[2] J. W. McIver, B. Schulte, F.-U. Stein, T. Matsuyama, G. Jotzu, G. Meier, and A. Cavalleri, Nat. Phys., 16, 38(2020).
[3] K. Takasan, A. Daido, N. Kawakami, and Y. Yanase, Phys. Rev. B, 95, 134508 (2017).
[4] J. Ye, Y. Zhang, R. Akashi, M. Bahramy, R. Arita, and Y. Iwasa, Science, 338, 1193 (2012).
[5] HC, K. Takasan, and Y. Yanase, arXiv:2007.11812 (2020).

Anisotropic response of spin susceptibility ona spin-triplet superconductor UTe2
Quantum Materials Laboratory

Genki NAKAMINE

Abstract UTe2 is a promising candidate for a spin-triplet superconductor due to its extremely high upper
critical field. To investigate the superconducting (SC) properties, we performed 125Te-NMR measurements.
Slight decrease of the Knight shift (ΔK) in the SC state, and anisotropic magnetic field response of ΔK
support the spin-triplet scenario.
© 2020 Department of Physics, Kyoto University

There are two types of superconducting (SC) spin states: spin-singlet (S = 0) and spin-triplet (S = 1). Since theSC order parameter of the spin-triplet pairing state has the spin degrees of freedom, unusual behaviors such as
multiple SC phases, and magnetic field (H) boosted superconductivity are expected. However, there are few
examples of spin-triplet superconductors, and thus the physical properties of the spin-triplet superconductivity
have not been well understood.
Uranium (U) -based ferromagnetic (FM) superconductors UGe2, URhGe and UCoGe are promising candidates
for a spin-triplet superconductor, as their SC phases are inside or overlapped with the FM phase, and their upper
critical field Hc2 is far beyond the Pauli-limiting field (HP)[1]. Unfortunately, the investigation of pure spin-triplet
SC properties is difficult in these systems because the spin-triplet SC state is affected with the FM ordered state.
In 2018, S. Ran et al. discovered superconductivity in UTe2 with Tc = 1.6 K[2]. UTe2 also exhibits very large
and anisotropic Hc2 exceeding HP along the three crystalline principal axes[2, 3] as observed in U-based FM
superconductors[1]. Particularly, in H || b, the Tc shows an upturn around 15 T as shown in Fig.1 and thesuperconductivity remains up to 35 T until it is terminated by ametamagnetic transition[4]. UTe2 has also been considered to be a
promising candidate for a spin-triplet superconductor as the spin-singlet
SC state could not survive in such a high-H region. An important featureof UTe2 is the absence of any magnetic order at ambient pressure, incontrast to U-based FM superconductors. Therefore, we can investigate
pure SC properties in UTe2.
We performed 125Te-nuclear magnetic resonance (NMR) measurements
to investigate the SC state of UTe2. The decrease in the spin part of theKnight shifts (ΔKspin) is much smaller than the decrease expected in the Fig.1. H-T phase diagram of UTe2 in
spin-singlet state[5]. Furthermore, the H response of ΔKspin for H || b and || H || b. The dotted line represents thecritical field where the SC properties
c is anisotropic as shown in Fig.2.; ΔKspin for H || b is almost H are expected to be changed.
independent up to 6.5 T while |ΔKspin| for H || c decreases with increasing
H and is almost zero at 5.5 T. The anisotropic H response and theunchanged Knight shift in H || c strongly support the spin-triplet scenario.
In addition, from the estimation of the critical field Hcr with ΔKspin, wesuggest that the SC properties are changed at Hcr ~ 13 T as shown in
Fig.1[6].
References[1] D. Aoki et al., J. Phys. Soc. Jpn. 88, 022001 (2019).
[2] S. Ran et al., Science 365, 684 (2019).
[3] D. Aoki et al., J. Phys. Soc. Jpn. 88, 043702 (2019).
Fig.2. H dependence of the decrease[4] G. Knebel et al., J. Phys. Soc. Jpn. 88, 063707 (2019).
in the spin part of Knight shift at the[5] G. Nakamine et al., J. Phys. Soc. Jpn. 88, 113703 (2019).
lowest temperature.
[6] G. Nakamine et al., submitted to Phys. Rev. Lett.

Structure of single Xe nanoparticles upon crystallization
Physics of Disordered Systems Group

Akinobu Niozu

Abstract We investigated the structure of single Xe nanoparticles crystallized in adiabatically cooled
Xe gas jet by single-shot X-ray diffraction using an X-ray free-electron laser. The single-particle structureof Xe nanoparticles indicates that crystallization kinetics play an essential role in determining thestructure.
© 2020 Department of Physics, Kyoto University

Crystallization is one of the most ubiquitous physical phenomena in nature. Nevertheless, atomic- andnanoscale structural dynamics upon crystallization is still a subject of controversy. Classical nucleation theory
assumes that crystallization starts from a small spherical nucleus having the structure of the stable phase in bulk.
On the other hand, more than a hundred years ago, Ostwald [1] proposed his famous step rule, stating that phasetransition can proceed via intermediate metastable phases. Recently, theoretical and computational studies haveprovided novel insights into the structural dynamics of crystallization [2]. In contrast, experimental observationsof the crystallization dynamics have been so far mostly restricted to slow dynamics, such as the crystallization ofcolloidal systems [3].
In this work, we overcome the restriction with the recently available structure analysis method using X-ray
free-electron lasers (XFELs). We investigated the structure of single Xe nanoparticles crystallized in asupercooled Xe gas jet. The experiments were carried out at SACLA [4] beamline 3. The single-particle structureof unsupported Xe nanoparticles was probed by XFEL pulses several hundred microseconds after the growth. The
diffraction signals were recorded on a shot-by-shot basis with a two-dimensional X-ray detector.
The experimental powder X-ray diffraction pattern indicates that the Xe nanoparticles form not only theface-centered cubic (fcc) structure (the bulk stable phase) but also a structure composed of randomly stacked
close-packed layers, the random hexagonal close-packed (rhcp) structure (Fig. 1). The existence of the rhcp
structure was further supported by a newly developed analysis method for the
single-shot diffraction patterns [5], where the three-dimensional structure
factor was deduced from positional correlations of two Bragg spots insingle-shot diffraction patterns. Furthermore, through an analysis of thesingle-particle diffraction patterns, we revealed that the fcc and rhcp phases
coexist in single Xe nanoparticles. The experimental results are consistent withthe scenario of structure aging, in which crystallization initially occurs at themetastable rhcp phase, and the structure later transforms into the stable fcc
structure. The observations are quite analogous to those in the colloidal
hard-sphere system [3] and also supported by a recent computational study on
ice nucleation [2]. The scenario is potentially applicable to the crystallizationof a wide variety of materials. The present work paves the way for the Fig. 1. Schematic of the random
real-time observation of various non-equilibrium dynamics in atomic systems hexagonal close-packed (rhcp)
using XFELs.
structure.
References[1] W. Ostwald, Zeitschrift Für Phys. Chemie 22, 289 (1897).
[2] L. Lupi et al., Nature 551, 218 (2017).
[3] V. J. Anderson and H. N. W. Lekkerkerker, Nature 416, 811 (2002).
[4] T. Ishikawa et al., Nat. Photonics 6, 540 (2012).
[5] A. Niozu et al., IUCrJ 7, 276 (2020).

Injection Locking of Resonant-Tunneling-Diode
Terahertz Oscillator
Solid State Spectroscopy Group

Tomoki Hiraoka

Abstract We investigated injection-locking properties of the resonant-tunneling-diode terahertz oscillator
in the small-signal injection regime. The locking range was proportional to the injection amplitude andconsistent with Adler's model. The noise-reduction behavior due to injection locking was qualitatively
explained by Maffezzoni's model of the noise reduction in general limit-cycle oscillators.
© 2020 Department of Physics, Kyoto University

Resonant tunneling diode (RTD) oscillator is a semiconductor device which can oscillate in terahertz (THz)frequency region at room temperature [1]. RTD works as a gain component due to a negative differential
conductance originating from the resonant tunneling effect. The dynamics of RTD THz oscillator is not obvious
because the conduction properties of RTD are affected by several quantum effects: Under strong THz field, aphoton-assisted tunneling occurs and a current-voltage curve of an RTD changes [2]. It is also known that the
shot-noise suppression or enhancement occurs in an RTD depending on a bias voltage [3].
One of the major concerns of the RTD THz oscillator is its large linewidth, which is typically 10 MHz for an
oscillator operating around several hundred GHz. To stabilize the oscillation frequency, we focused on theinjection locking [4, 5], where a noisy oscillator is synchronized to a narrow-band injection signal. To understandthe injection-locking mechanism, it is important to clarify the injection-locking properties in a small-signal
injection regime. However, experiments in the small-signal injection regime have not been conducted so far. Thisis because even the slightest return light from surrounding objects acts as an optical feedback, hiding the intrinsic
properties of injection locking in the small signal regime.
In this study, we studied the injection-locking properties in the small-signal injection regime without an opticalfeedback effect. We developed an isolator for THz wave [6] to eliminate minute return light from the detection
system. We succeed in the injection locking and dramatically reduced the line width of the emission spectrum
from 4.4MHz in the free-running state to less than 120mHz. Figure 1 shows emission spectrum for various
injection frequencies. The locking range wasproportional to the injection amplitude and consistent
with Adler's model [4]. We also observed the noise
suppression under injection signal of various amplitudes
at the free-running frequency. As increasing theinjection amplitude, the noise component in the power
spectrum decreased, and the injection-locked component
alternatively increased. The noise-reduction behavior
was qualitatively consistent with Maffezzoni's model [5]for general limit-cycle oscillators.
References[1] M. Asada and S. Suzuki, J. Infrared Millim.
Terahertz Waves 37, 1185 (2016).
[2] H. Drexler et al., Appl. Phys. Lett. 67, 2816 (1995).
[3] G. Iannaccone, et al, Phys Rev Lett 80, 1054 (1998).
[4] R. Adler, Proc. IRE 34, 351 (1946).
[5] P. Maffezzoni and D. D’Amore, IEEE Trans.
Circuits Syst. I Regul. Pap. 58, 2498 (2011).
[6] M. Shalaby et al., Nat. Commun. 4, 1558 (2013).

Fig. 1. Emission power spectrum of the RTD THz
oscillator under various injection frequencies. The top
blue trace is a free-running spectrum. The narrow
peaks in other traces show the injection frequencies.
The red traces show injection-locked state.

Identifying exogenous and endogenous activity
in social media
Nonlinear Dynamics Group

Kazuki Fujita

Abstract Existing time series data are full of non-stationarity. This fluctuation may have been caused byexternal stimulation, or/and internal self-excitation. We developed a statistical model to make theinference for the occurrence of events, and apply it to a real time series of online social network data.
© 2020 Department of Physics, Kyoto University

Highly clustered event sequences are observed in certain types of real data, such as earthquakes, financial transaction,
crimes, neuron activity, and tweets[1]. The occurrence of new events in a system is typically driven by externalcauses and by previous events taking place inside the system. For instance, Hawkes processes are linear selfreinforced processes, where the occurrence of an event increases the likelihood of future events[2][3].
Distinguishing between exogenous and endogenous forces is critical for understanding the mechanisms that drive
dynamics of systems and has important practical applications, such as the quantification of marketing or external
factors that may manipulate the social system[3], however, this assumption has not been examined in any detail.
This is a general statement, applying to a range of situations including, more recently, to the activity of users inonline social networks (OSNs).
Here we develop a method for extracting from a series of posting times the relative contributions that are exogenous,e.g., news media, and endogenous, e.g., information cascade. The method is based on the fitting of a generalized
linear model (GLM) equipped with a self-excitation mechanism[5]. We assume the rate of event occurrence 𝜆(𝑡) =exp (𝛾(𝑡) + 𝛼 ∑0 ℎ(𝑡 − 𝑡0 )), where 𝛾(𝑡) and 𝛼 represent the time-varying external environment and the degreeof internal self-excitation, respectively. ℎ(𝑡) represents the time course of internal excitation. As a priordistribution of 𝜆(𝑡), we assume that external modulation 𝛾(𝑡) is slow, by penalizing the large gradient, |𝑑𝛾/𝑑𝑡|.
With the maximum a posteriori (MAP) estimate, we can estimate external modulation and the degree of internal
self-excitation.
We test the method with synthetic data generated by a nonlinear Hawkes process, and apply it to a real time seriesof tweets with a given hashtag. In the empirical dataset, the estimated contributions of exogenous and endogenous
volumes are close to the amounts of original tweets and retweets respectively.
abFig. 1. Original tweeting rate estimation for the one week samples of tweets between Jan 9 and Jan 16,
2017. The solid blue line shows the rate of all tweets, red line below shows the rate of original tweets,
black curve represents the inferred exogenous activity and green is the total rate given by the GLM.

References[1]R. Crane and D. Sornette, Proc. Natl. Acad. Sci. U.S.A. 105, 15649 (2008).
[2]A. G. Hawkes, Biometrika 58, 83 (1971).
[3]T. Onaga
and S. Shinomoto, Phys. Rev. E 89, 042817 (2014).
c[4]E. Lewis, G. Mohler, P. J. Brantingham, and A. L. Bertozzi, Security J. 25, 244 (2012).
[5]K.Fujita, A. Medvedev, S.Koyama, R. Lambiotte, and S. Shinomoto, Phys Rev. E 98, 052304 (2018).

Theoretical study of nonlinear transport
in parity-violating magnets
Hikaru Watanabe (Condensed Matter Theory Group)
Abstract This work investigates the nonlinear current generation induced by AC/DC electric field byutilizing the magnetic parity-violation, by which both of the parity and time-reversal symmetries arebroken. Combining the symmetry analysis and microscopic calculations, we demonstrate the uniqueproperty of these responses.
© 2020 Department of Physics, Kyoto University

It is well known that the parity violation in solids gives rise to various emergent responses and exotic quantum
phases. For instance, the coupling between spin and charge degrees of freedom has been used to manipulate themicroscopic magnets. The importance of parity breaking has now been widely shared in fields of condensed
matter physics. Usually, the parity-violation is satisfied by the asymmetry in crystal structure and by the external
electric field. On the other hand, we can notice another type of parity-violation, that is magnetic parity-violation.
The magnetic parity-violation breaks not only the parity (P) but time-reversal (T) symmetries while the crystal
parity-violation preserves the T-symmetry. The fundamental differences between the magnetic and crystal
parity-violations are clarified by the group-theoretical classification making use of multipole degree of freedom insolids [1]. Interestingly, the magnetic parity-violation is highly tunable since it is induced by the magnetic order.
Such high controllability is contrasting to the fact that the crystal asymmetry is less controllable.
We theoretically investigate the magnetic parity-violation by working on non-reciprocal current generations
via AC and DC electric field, which are photocurrent response and (non-reciprocal) nonlinear conductivity,
respectively [2,3]. Although these phenomena can be implemented by the crystal parity-violation as well as
magnetic parity-violation, they should be distinguished by the preserved symmetry. The T and PT-symmetries (PToperation is the combination of the P and T-operations) preserved in the crystalline and magnetic parity-violating
system play important roles in classifying the emergent responses [1,4]. Comparing with the non-reciprocal
responses in the T-symmetric systems, we systematically study the striking properties arising from the magnetic
parity-violation as follows.
1. Field-free non-reciprocal Hall effect in metallic antiferromagnets [2]Making use of the symmetry analysis and microscopic calculations, we demonstrate the field-free Hall effect in a
metallic antiferromagnet, doped BaMn2As2, and propose a potential application to antiferromagnetic spintronics[5].
2. Giant chiral photocurrent generation in a topological antiferromagnet [3]Following the systematic classification with the T and PT-symmetries, we discover the current generation
response induced by circularly-polarized-lights. This response is explained by the shift-current mechanism andclosely related to the quantum geometry in the electronic band structure. Moreover, the strongly-enhanced
photocurrent response is demonstrated by the microscopic analysis of the antiferromagnetic compounds hosting
the topologically-nontrivial electronic structure.
References[1] H. Watanabe and Y. Yanase, Phys. Rev. B 96,064432 (2017); ibid 98, 245129 (2018); S. Hayami et al., Phys.
Rev. B 98, 165110 (2018).
[2] H. Watanabe and Y. Yanase, Phys. Rev. Research 2, 043081 (2020).
[3] H. Watanabe and Y. Yanase, arXiv.2006.06908.
[4] J. Železný et al., Phys. Rev. B 95, 014403 (2017).
[5] V. Baltz et al., Rev. Mod. Phys. 90, 328 (2018).