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京 都 大 学 大 学 院 理 学 研 究 科

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要旨集

２０１７年１１月２７日（月）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：１５分（別に質問１０分程度）
２０１７年１１月２７日（月）９：００～

目

開始

次

1. Observation of terahertz-photon dressed exciton in sub-cycle regime
内田 健人（

９：００）

2. Topological and dynamical properties of Majorana fermions in one dimension
太田 卓見（ ９：２５）
3. Dynamic mechanism of the microorganism swimming near a non-slip boundary
大村 拓也（
4. Ultracold Ytterbium Atoms in a Tunable Non-Primitive Optical Lattice
小沢
１０：４０～１０：５０

９：５０）

秀樹（１０：１５）

休憩

5. Concurrency-induced transitions in epidemic dynamics on temporal networks
翁長 朝功（１０：５０）
6. Interplay of Superconductivity and Spin texture: Application to spintronics andtopological phase髙嶋 梨菜（１１：１５）
7. Ultrafast disordering of local structurein Xe clusters exposed to intense NIR laser
西山 俊幸（１１：４０）

8. Renormalization Group Analysis of Nonequilibrium Phase Transitions in Driven
Disordered Systems
芳賀 大樹（１２：０５）
１２：３０～１３：３０

昼休み

9．Antiperovskite Oxide Sr3-xSnO:
Discovery of Superconductivity and Deficiency Phase Diagram
オーダ，モーハメッド（１３：３０）
10．Uniaxial-Strain Control of Hc2 Anisotropy of Nematic Superconductor SrxBi2Se3
コスティレフ，イワン（１３：５５）

11．Swelling effect on twist elastic constant in the smectic C liquid crystal
畑 加奈子（１４：２０）
12．Magnetoelectric response in electric octupole ordered state
人見尚典（１４：４５）

13．77Se-NMR Studies on Iron-based Superconductors Fe(Se1-xSx)
シ，アンロ（１５：１０）

Observation of terahertz-photon dressed excitonin sub-cycle regime
Solid State Spectroscopy Group

Kento Uchida

Abstract Nonlinear optical response of exciton under intense terahertz electric field has been investigatedby using phase-stable terahertz waves and femtosecond laser pulses. We found modulation of absorption
spectrum with even-multiples of the terahertz wave frequency, indicating the formation ofterahertz-photon dressed state of excitons.
© 2017 Department of Physics, Kyoto University

Strong light-matter interaction gives us an opportunity to access far from equilibrium, and manipulate thematerial properties with ultrafast time-scale. In particular, under strong light field, electronic state forms
non-trivial energy structure different from bare-state and might emerge novel material properties. This state iscalled photon dressed state, and can be controlled by laser properties. In order to realize ultrafast control ofmaterial properties, it is important to understand the response of photon-dressed state in ultrafast time scale.
Excitons in semiconductor, electron-hole pairs bounded together with Coulomb interaction, are good candidates
for investigating ultrafast optical response of photon-dressed states. The intra-excitonic transition lies in terahertz(THz) region, and thus, interaction between exciton and THz-photon easily enters highly nonlinear regime withmoderate electric field (< 10 kV/cm) [1]. In addition, exciton has a strong absorption near the bandgap ofsemiconductor, and can be probed by near-infrared (NIR) light through interband transition. THz-photon-dressed
excitons have been observed with time-averaged measurements [2,3], however, optical response within the cycleof THz wave (sub-cycle optical response) is unclear.
In this study, we investigated the optical response of exciton driven by phase-stable and strong THz wave withfield strength from 0.4 to 6.4 kV/cm [4]. We accessed the sub-cycle response of them by using NIR laser pulse,
whose pulse width (0.1 ps) is much shorter than the cycle of THz wave (1.7 ps), as a probe. Figure 1(a) shows
time-averaged absorption spectra with and without THz excitation at 10 K. The salient peak at 1s = 1.55 eV isattributed to 1s-heavy-hole exciton, and shows bleaching under THz wave driving. In time domain, according to
the THz driving as shown in Fig. 1(b), the absorption change at excitonic resonance 1s shows oscillations witheven-multiples of THz frequency THz/2as shown in Fig. 1(c) (black solid line). In addition to absorptionchange at 1s, those at 1s + 2THz (red dashed line) and 1s + 4THz (blue dotted line) also show strong
modulations. Probe energy dependence of modulations
showed that the dominant frequencies of 2THz/2 (red)and 4THz/2 (blue) in each modulation correspond to theenergy differences from 1s. The calculation of absorption
induced by THz-photon dressed exciton revealed that thisphoton energy dependence of modulation reflects
ladder-like energy levels separated by THz photon energy,
which are essential in photon dressed state [4]. Our study
elucidated the link between photon-dressed state and theultrafast optical modulation, and provides way to access Fig. 1. (a) Absorption  spectra with (black line) andwithout THz excitation (grey area). Absorption electronic states strongly driven by light field.
with THz excitation is averaged over between time
References
delay of -1.5 ps and 2.5 ps. (b) Temporal profile of[1] B. Zaks, R. B. Liu, and M. S. Sherwin, Nature 483, THz electric field. (c) Absorption changes  as a
580 (2012).
function of time delay between THz wave and NIR
pulse. Black solid, Red dashed and blue dotted lines[2] J. Kono et al., Phys. Rev. Lett. 79, 1758 (1997).
[3] M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010). respectively correspond to photon energies indicatedby the arrows in Fig. 1(a). For clarity, absorption[4] K. Uchida et al., Phys. Rev. Lett. 117, 277402 (2016).
changes are vertically offset.

Topological and dynamical properties of Majorana fermions in
one dimension
Physics of Matter: Condensed Matter Physics (YITP)

Takumi Ohta

Abstract We study the topological and dynamical properties of the one-dimensional Kitaev model of
spinless p-wave superconductor with competing interactions. We first map out ground-state phase
diagram of the model. Then we study the time evolution of the string correlation function and theentanglement spectrum during the interaction sweep.
© 2017 Department of Physics, Kyoto University

Understanding phases of matter has been the central topic in condensed matter physics. For a long time, theLandau theory based on symmetry breaking and the corresponding local order parameters had been the canonicalframework. However, since the discovery of the quantum Hall effect, new phases which we call topological havebeen theoretically predicted and experimentally realized. These phases cannot be characterized by the Landau
theory, but by non-local string order parameters or emergent modes at the boundaries of the system. One of thesimplest models which show the topological phases would be the Kitaev model of spinless p-wave superconductor.
[1] In the topological phase, there appears a Majorana edge mode with zero energy at the ends of the system. It
has recently been recognized that quantum entanglement is useful to characterize the phases and that theentanglement spectrum which quantifies entanglement between subsystems reflects the edge modes.
Since the proposal of the use of edge modes to the quantum computation, theorists and experimentalists havemuch interest in the dynamics of topological phases. It has been recently reported that the dynamics of the
Majorana edge mode in the Kitaev model depends on the topological properties of the system. [2] Thus it would
be interesting to study the dynamics of topological phases from the perspective of entanglement.
In this study, we study the ground-state phase diagram and dynamics of one-dimensional model of Majorana
fermions with several competing interactions. We first map out the ground-state phase diagram of the model andcharacterize the phases by the correlation functions and the entanglement spectrum. We then investigate thedynamical properties during interaction sweeps using the time-dependent Bogoliubov theory. When the sweep
speed is slow compared to the typical time scale of the system, both correlation function and entanglement
entropy exhibit spatially periodic structures after the system passes the critical interaction strength. [3] On top ofthis, we find that the entanglement spectrum changes qualitatively in time during the sweep. [4] By explicitly
calculating the above quantities for excited states, we attribute these behaviors to the Bogoliubov quasiparticles
generated near the critical points. We also show that the entanglement spectrum reflects the pattern of the
Majorana correlation even for the excited states.
References[1] A. Y. Kitaev, Physics-Usp. 44, 131 (2001).
[2] A. Bermudez, L. Amico, and M. A. Martin-Delgado, New. J. Phys. 12, 055014 (2010).
[3] T. Ohta, S. Tanaka, I. Danshita, and K. Totsuka, J. Phys. Soc. Jpn. 84, 063001 (2015).
[4] T. Ohta, S. Tanaka, I. Danshita, and K. Totsuka, Phys. Rev. B 93, 165423 (2016).

Dynamic mechanism of the microorganism
swimming near a non-slip boundary
Dissipative Structure and Biological Physics Laboratory

Takuya Ohmura

Abstract We investigate the microorganism swimming near a non-slip boundary by both experiment andhydrodynamic numerical calculation. As a result, the novel biological sensing system and the effect of the
body shape have been figured out as factors of the non-trivial swimming motion near the wall.
© 2017 Department of Physics, Kyoto University

Can we realize complex cell behavior under simple physical rules? If the mechanism of the cell behavior could
be quantified by physics, it would be beneficial in not only biological viewpoint but also engineering and medical
applications for micro-machineries. As one of the biophysical approach for motion of swimming cell,hydrodynamic analysis is generally used. When a micrometer-sized cell swims in fluid, the governing equation of
the system obeys Stokes equation in low-Reynolds hydrodynamics. After Lighthill firstly has proposed a very
simple mathematical model describing swimming microorganisms, which is called the squirmer model [1], many
researchers have analyzed the model. In the squirmer model, swimming microorganisms are categorized byexternal flow filed into three forms, a pusher (e.g. sperm, Escherichia coli), a puller (e.g. Chlamydomonas,
Euglena) and a neutral swimmer (e.g. ciliates, Volvox). While this model well reproduces long-range
hydrodynamic interactions between swimming cells in bulk water, the motions with short-range interactions are
sometimes non-trivial and contradict actual biological behavior. One of the most fundamental interactive
problems is interaction with a flat non-slip boundary, which has been reported about microorganisms described asa pusher and a puller have been investigated [2].
Ciliates categorized to a neutral swimmer are known to accumulate near a solid-fluid boundary, where they
can get a lot of food and a stable environment. In contrast, a neutral swimmer is repelled away from the wall [3].
To identify the mechanism of the accumulation, we investigated the difference between experiment andhydrodynamic model. Observing one of ciliates, Tetrahymena pyriformis, the single cell slid on a solid wall after
colliding the wall (Fig. 1(a)), which contradicts the numerical result of a neutral swimmer (Fig. 1(b)). In otherexperiments, we found the biological feature of cilia, which generate swimming thrust force while attaching thewall. Numerical results showed that the biological sensing system and the anisotropic shape of cell body are
dynamically essential for the sliding motion (Fig. 1(c)) [4].

Fig. 1. (a) Snapshots of the ciliate sliding adjacent to a wall. (b) The isotropic neutral swimmer is repelledaway from the wall. (c) The anisotropic swimmer with a novel boundary condition slides on the wall.

References[1] M.J. Lighthill, Comm. Pure Appl. Math. 5, 109–118 (1952).
[2] V. Kantsler, J. Dunkel, M. Polin, & R.E. Goldstein, Proc. Natl. Acad. Sci. U.S.A, 110(4), 1187-1192 (2013).
[3] G.J. Li & A.M. Ardekani, Phys. Rev. E, 90(1), 013010 (2014).
[4] T. Ohmura, Y. Nishigami, A. Taniguchi, S. Nonaka, J. Manabe, T. Ishikawa & M. Ichikawa, submitted.

Ultracold Ytterbium Atoms in a
Tunable Non-Primitive Optical Lattice
Quantum Optics Group

Hideki Ozawa

Abstract We present a series of experiments with ultracold ytterbium atoms in a Lieb lattice and dimerized
lattice geometry, realized by optical means. In a Lieb lattice experiment, we study the property of a flatband. In a dimerized lattice, we demonstrate the Pomeranchuk effect on the quantum magnetism.
© 2017 Department of Physics, Kyoto University

Ultracold atoms trapped in optical lattices offer a clean and highly flexible experimental
platform to investigate exotic phases of condensed matter. Recently, nonprimitive
optical lattices with multiple lattice sites per unit cell have been realized, revealingyB
nontrivial ordering and dynamics arising from the orbital degrees of freedom [1]. In ourCAwork, we present a series of experiments with ultracold ytterbium (Yb) atoms in a Lieb
Unit
cell
lattice (Fig.1) and dimerized cubic lattice geometry (Fig.2).
x
In a Lieb lattice, a flat band appears due to destructive interference of the tunneling. It
Fig. 1. Lieb lattice
is a fascinating question whether a Bose-Einstein condensate (BEC) is stable in a flatband. Experimental study for this question is hindered by the fact that a flat band is in an excited state in the case of
Lieb lattice. We have developed a method to coherently transfer a BEC of 174Yb into the flat band of Lieb lattice,and studied the stability of the BEC loaded in the flat band. We also investigate the inter-sublattice dynamics of thesystem by projecting the sublattice population onto the band population. This measurement shows the formation of
the localized state in the flat band [2]. Furthermore, we measure the lowest three bands of an optical Lieb lattice fora BEC in a momentum-resolved manner. A BEC, which is initially prepared around zero quasimomentum in thelattice, is transported to a desired quasimomentum by applying a constant force. The energy dispersion of the lowest
band is reproduced by integrating measured group velocities. The excited band energy is reconstructed by measuring
the gap from the lowest band with the same quasimomentum, which can be extracted from the oscillation of thesublattice populations after preparing a superposition of the band eigenstates. It is revealed that the second band,
which should be flat in a single-particle description, is shifted and distorted around the Brillouin zone edge as theinteraction strength increases [3].
A Lieb lattice system has a mathematical analogy to a three-level system with Λ-type transition. Most interestingly,
a localized state in a flat band of the Lieb lattice corresponds to the dark state in the three-level system. By
adiabatically changing the tunneling amplitudes in an counter-intuitive order, we coherently transfer atoms fromone sublattice (B) to another (C) without populating the intermediate sublattice of A, which can be regarded as a
spatial analogue of stimulated Raman adiabatic passage [4].
173
Yb is characterized by SU(N=2I+1) symmetric repulsive interaction for nuclear
spin I=5/2. For this large-spin system, Pomeranchuk cooling is enhanced; large-spin
degrees of freedom can effectively cool down the system by absorbing the entropyyfrom motional degrees of freedom. The precise control of the spin degree of freedom
provided by optical pumping technique enables us a straightforward comparisonz
between the cases of SU(2) and SU(4). Our important finding is that larger singletx
triplet imbalance is observed in a dimerized cubic lattice for SU(4) spin system
Fig. 2. Cubic latticecompared with SU(2) as a consequence of Pomeranchuk cooling effect. This is an
dimerized along thex-axis.
important step towards the realization of novel SU(N>2) quantum magnetism.
References[1] P. Soltan-Panahi et al., Nature Phys., 8, 71-75 (2012). [2] S. Taie et al., Science Advances 1, e1500854 (2015).
[3] H. Ozawa et al., Phys. Rev. Lett. 118, 175301 (2017). [4] S. Taie et al., arXiv:1708.01100 (2017).

Concurrency-induced transitions in epidemic dynamics on
temporal networks
Nonlinear Dynamics Laboratory

Tomokatsu Onaga

Abstract: We theoretically investigate the effects of concurrency, the number of neighbors that anode has at a given time point, on the epidemic threshold in the stochastic
susceptible-infected-susceptible dynamics on temporal network models. We show that network
dynamics can suppress epidemics when the node’s concurrency is low, but can also enhance
epidemics when the concurrency is high. We analytically determine different phases of thisconcurrency-induced transition.
© 2017 Department of Physics, Kyoto University

Social contact networks underlying epidemic processes in humans and animals are highly dynamic. The
spreading of infections on such temporal networks can differ dramatically from spreading on static networks. For
example, temporal properties of networks, such as long-tailed distributions of inter-contact times, temporal andcross-edge correlation in inter-contact times, and entries and exits of nodes, considerably alter how infections
spread in a network [1].
In the present study, we focus on a relatively neglected component of temporal networks, i.e., the numberof concurrent contacts that a node has. Even if two temporal networks are the same when aggregated over a time
horizon, they may be different as temporal networks due to different levels of concurrency. Concurrency is alongstanding concept in epidemiology, in particular in the context of monogamy or polygamy affecting sexuallytransmitted infections. The recent numerical works suggest that a level of high concurrency enhances epidemics [2,3]. However, this finding lacks theoretical underpinning.
In this study, we use the analytically tractable activity-driven model of temporal networks to explicitly
modulate the size of the concurrently active network with the structure of the aggregate network fixed. With thismachinery, we reveal non-monotonic effects of link concurrency on spreading dynamics [4]. We show that the
dynamics of networks can either enhance or suppress infection, depending on the amount of concurrency that
individual nodes have. We analytically determine different phases of this concurrency-induced transition (Fig. 1).
10100enhancedpp5βcdie out

su

m

re

ss

ed

50

mc

10

1

5
0

0.2

0.4

τ

0.6

0.8

1

Fig. 1. Phase diagrams for the epidemic threshold, βc. m denotes concurrency, and τ denotes the time scaleof network dynamics. Note that βc takes the same value for all m at τ = 0. In the “die out” phase, infection
eventually dies out for any finite β. In the “suppressed” phase, βc is larger than the βc value at τ = 0. In the“enhanced” phase, βc is smaller than the βc value at τ = 0. The color bar indicates the βc values. In the grayregions, βc > 100.
References[1] P. Holme and J. Saramäki, Phys. Rep. 519, 97 (2012).
[2] C. Bauch and D. A. Rand, Proc. R. Soc. B 267, 2019 (2000).
[3] K.Y. Leung and M. Kretzschmar, AIDS 29, 1097 (2015).
[4] T. Onaga, J. P. Gleeson, and N. Masuda, Phys. Rev. Lett. 119, 108301 (2017).

Interplay of Superconductivity and Spin texture:
Application to spintronics and topological phase
Condensed matter theory Group

Rina Takashima

Abstract We study the interplay of superconductivity and magnetism in correlated systems in proximity
to a superconductor. We propose that a supercurrent can exert spin-torques on localized spin, topologicalstructures can be realized in Bogoliubov quasiparticle spectrum, and noncollinear spin textures can be
induced by a supercurrent.
© 2017 Department of Physics, Kyoto University

Recently, the interplay of superconductivity and magnetism receives renewed interest in a wider range offields such as spintronics applications [1] and topological phases [2]. The progress in these fields is accelerated
not only by exotic theories but also recent experimental advances. For example, the singlet and triplet
superconducting proximity effects inside ferromagnets are experimentally observed, and magnetic adatoms or ananowire on top of a superconductor realize the topological phase in superconductors.
We study such interplay in correlated systems with superconductivity. Firstly, we study supercurrent-induced
spin torques that act on localized moments in a metallic magnet with superconductivity. We show that a triplet
supercurrent gives rise to a spin-transfer torque, and it can realize an efficient domain wall manipulationcompared to the conventional spin-transfer torques [3]. In the presence of the relativistic spin-orbit
coupling in an inversion asymmetric system, a singlet supercurrent can also render a spin torque. Considering a
cubic chiral magnet, we study the dependence of the spin torque on magnetization directions and the magnitude of
the relativistic spin-orbit coupling [4].
Secondly, we study the topological states realized in Bogoliubov quasiparticle spectrum [4]. Weshow that the above model, a cubic chiral magnet with superconductivity, hosts the Weyl node
structures. Depending on the magnitude of chemical potential and spin-orbit coupling, type-I ortype-II Weyl quasiparticles are realized.
Thirdly, we propose a way to induce and switch noncollinear spin textures by an applied
supercurrent in a correlated metal with superconductivity [5]. Noncollinear spin textures are
crucial to engineer topological superconductors and generate triplet Cooper pairs. We show that asupercurrent can lead to phase transitions from a paramagnetic state to noncollinear magnetic
phases with helical or vortexlike spin textures.

Fig. 1. (a) Domain wall dynamics in a ferromagnet (FM) driven by a triplet supercurrent. (b) Type-II Weyl conein Bogoliubov quasiparticle spectrum. (c) Noncollinear magnetic order induced by a supercurrent.

References[1] J. Linder and J. W. A. Robinson, Nat. Phys. 11, 307 (2015).
[2] J. Alicea, Rep. Prog. Phys. 75, 076501 (2012).
[3] R. Takashima, S. Fujimoto, and T. Yokoyama, Phys. Rev. B 96, 121203 (R) (2017)[4] R. Takashima, and S. Fujimoto, Phys. Rev. B 94, 235117[5] R.Takashima, Y. Kato, Y. Yanase, and Y. Motome, arXiv: 1710.11349

Ultrafast disordering of local structurein Xe clusters exposed to intense NIR laser
Physics of Disordered Systems Group

Toshiyuki Nishiyama

Abstract Time-resolved wide-angle x-ray scattering experiments on giant Xe clusters have been
performed to investigate structural changes at the time of interaction between intense near-infrared laser
and matter. Ultrafast changes in local structure were observed as a significant decrease of intensity of
Bragg spots and broadening of their width. © 2017 Department of Physics, Kyoto University
Noble gas cluster has been used as an ideal model for study of laser-matter interaction due to its characteristics
of isolated many-body system. It is well known that a cluster exposed to intense laser undergo highly ionized andcan form a nanoscale high-dense plasma, nanoplasma [1]. Elucidation of mechanism of nanoplasma formation isimportant for understanding of laser-matter interaction, and numerous researches of nanoplasma have been
conducted [2, 3]. Recently, small angle x-ray scattering study using intense short pulses delivered from X-ray freeelectron laser (XFEL) has been revealed the ultrafast morphology change of giant xenon cluster which is triggeredby irradiation of intense near-infrared (NIR) laser [4]. Preceding works rise the question that how the local atomicorders changes in the nanoplasma formation process. Information of atomic scale structural changes will give insightfor nanoplasma physics, but experimental information is still lacking.
In this work we present results of time-resolved wide-angle x-ray scattering experiments on xenon (Xe) clusters
highly excited by NIR laser. The experiments were carried out at the experimental hutch 2 of the beamline 3 at
SACLA in Japan. Xe clusters composed of around 10 million atoms were generated by expanding Xe gas througha conical nozzle with a 200 μm diameter and a 4˚ half-opening angle. The NIR pulses (800 nm, 30 fs FWHM) were
irradiated to the Xe clusters. The intensity of the NIR pulses were up to 4.2×1016 W/cm2. For each NIR shot, a single
XFEL pulse (11.2 keV, 10 fs FWHM) was scattered by the heated Xe clusters. The intensity of the XFEL was
3.5×1017 W/cm2. The scattered x-rays were recorded with a multi-port charge-coupled device (MPCCD) octal sensor[5] installed 100 mm downstream of the interaction point.
We recorded scattered x-ray from single Xe clusters in shot-to-shot manner by using the MPCCD detector. Bragg
spots corresponding to reflections from (111), (200) and (220) planes of fcc lattice of Xe were found at the
momentum transfer of 1.79, 2.07, 2.93 Å-1 in the MPCCD images of giant Xe clusters. When we irradiated intense
NIR pulses to clusters, Bragg spots immediately decreased within several hundred fs, which indicated ultrafast
disordering of fcc structure in Xe clusters. We found clear NIR laser intensity dependence in its time scale. Delaydependence of intensity and width of Bragg spots revealed that the intensity decreased significantly in the time scaleof a few hundred fs after irradiation of NIR, accompanied with the broadening of width of spots. Delay dependenceof spot width is well reproduced by the surface melting model [4], which relates the intensity of Bragg spot to its
width by well-known Scherrer equation. The present results suggest that a cluster exposed to NIR laser melts fromits surface to the inner core and the local structure of the inner core is maintained for several hundred fs after
interaction with NIR laser.

References[1] H. Wabnitz et al., Nature 420, 482 (2002)[2] Th. Fennel et al., Rev. Mod. Phys. 82, 1793 (2010)[3] K. R. Ferguson et al., Sci. Adv. 2: e1500837 (2016)[4] T. Gorkhover et al., Nat. Photon. 10, 93-97 (2016)[5] T. Kameshima et al., Rev. Sci. Instrum. 85, 033110 (2014)

Renormalization Group Analysis of Nonequilibrium Phase
Transitions in Driven Disordered Systems
Nonlinear Dynamics Group

Taiki Haga

Abstract We investigate the critical behavior of disordered systems driven out of equilibrium. From
intuitive arguments, we derive a dimensional reduction property which predicts that the critical exponentsof driven disordered systems are the same as those of lower dimensional pure systems. By developing arenormalization group formalism, we consider the condition that the dimensional reduction fails.
© 2017 Department of Physics, Kyoto University

The effect of quenched disorder on the large-scale structure of interacting systems remains an unsettled issue.
The peculiarity of disordered systems comes from the fact that the competition between disorder and interaction
leads to multiple meta-stable states, which are local minima of the mesoscopic free energy. One of the mostremarkable phenomena associated with meta-stable states is the breakdown of the so-called “dimensional
reduction property” in random field spin models. Standard perturbation theory predicts that the critical exponentsof the -dimensional random field spin models are the same as those of the-dimensional pure spinmodels [1]. However, it is known that this dimensional reduction breaks down in low enough dimensions due to a
non-perturbative effect associated with multiple meta-stable states. To clarify the mechanism of the breakdown of
the dimensional reduction has been a central issue in the statistical mechanics of disordered systems, and recently,
a remarkable progress has been achieved by applying a modern renormalization group formalism [2].
A more challenging problem concerning the effect of disorder is to understand the critical behavior ofdisordered systems driven out of equilibrium in the presence of an external force. One of the well-studied
examples is the driven vortex lattices in dirty superconductors [3]. In these systems, vortex lines are driven by
Lorentz force in the presence of a random pinning potential. To obtain a clear insight into the underlying physics
of such driven disordered systems, it is a natural starting point to introduce a nonequilibrium counterpart of thedimensional reduction property.
In this study, we first introduce a simple model of driven disordered systems, which is a random field spinmodel driven at a uniform and steady velocity. From intuitive arguments, we derive a novel type of dimensional
reduction property, which states that the critical exponents of the -dimensional driven random field spin model
are the same as those of the-dimensional pure spin model. Unfortunately, this dimensional reduction canbreak down in low enough dimensions due to a non-perturbative effect. Therefore, we develop a renormalizationgroup formalism for the driven random field spin model and its critical behavior is investigated. By comparing theactual critical exponents with their dimensional reduction values, we determine the region in the parameter space
wherein the dimensional reduction breaks down [4].
As a remarkable corollary of the dimensional reduction, it is expected that a driven disordered system with U(1)symmetry exhibits the Kosterlitz-Thouless transition in three-dimensions. We show that this expectation iscertainly correct. Concretely, the system exhibits a quasi-long-range order at weak disorder and a topologicalphase transition, which is caused by the structural changes in vortices. This is a first example of topological phasetransition wherein the interplay between disorder and driving plays a crucial role [5].
References[1] G. Parisi and N. Sourlas, Phys. Rev. Lett. 43, 744 (1979).
[2] G. Tarjus and M. Tissier, Phys. Rev. B 78, 024203 (2008); 78, 024204 (2008).
[3] G. Blatter, M. V. Feigelman, V. B. Geshkenbein, A. I. Larkin, V. M. Vinokur, Rev. Mod. Phys. 66, 1125 (1994).
[4] T. Haga, arXiv:1704.03644.
[5] T. Haga, arXiv:1703.09486.

Antiperovskite Oxide Sr3-xSnO:
Discovery of Superconductivity and Deficiency Phase Diagram
Quantum Materials Laboratory

Mohamed Oudah

Abstract We report on the discovery of superconductivity in the antiperovskite oxide Sr 3-xSnO,
and emphasize the effect of Sr deficiency on superconductivity in this material. We discuss theunexpected appearance of two superconducting transitions simultaneously and show theevolution of these two transitions as a function of deficiency x in Sr3-xSnO.
© 2017 Department of Physics, Kyoto University

Antiperovskite oxides A3BO (A = Mg, Ca, Sr, Ba, Yb, Eu; B = Si, Ge, Sn, Pb) have recently been demonstrated
to exhibit Dirac electronic dispersion near the Γ point in the Brillouin zone based on first-principles calculations[1,2]. Charge balance in this class of material, assuming a typical A2+ and O2- valences, leads to (A2+)3(B4-)(O2-),
where the B ions have an unusual 4- state. Such negative valance ions are consistent with theoretical calculations.
Furthermore, antiperovskite oxides with heavier elements have been predicted as topological crystalline insulators[3], due to the inversion of the conduction d band of A2+ and the valance p band of B4-. Finding ofsuperconductivity emerging in such an unusual electronic state would be quite interesting, and an important step
towards understanding of superconductivity in topologically nontrivial materials.
To this goal, we synthesized various antiperovskite oxides, including Ca 3SnO, Ca3PbO, Sr3SnO, and Sr3PbO.
These antiperovskite oxides are air sensitive, and this work required developing various handling andmeasurement techniques so that exposure to air is minimized. Finally, we discovered the first superconductivity
among the antiperovskite oxides in the hole-doped Sr 3-xSnO [4], with evidences of zero resistivity (Fig. 1(a)) anddiamagnetic Meissner signal below the critical temperature Tc of 5 K (Fig. 1(b)).
The initial synthesis method used suffered from uncontrolled evaporation of
Sr and it was difficult to quantify the exact composition needed forsuperconductivity, but we overcame this hurdle after some modifications. With
the improved synthesis method, we successfully tuned the amount of thestrontium deficiency, and the volume fraction of the superconductivity at 5 K
was increased to about 80% with the optimal deficiency of around x ~ 0.5 [5,6].
We calculated band structures of various “Sr 2.5SnO” and found that the shape ofthe bands are not altered significantly by the Sr deficiency, but the chemical
potential is shifted substantially [7]. Interestingly, this compound exhibits
double superconducting transitions with the Tc’s of about 5 and 1 K, as
demonstrated in the zero field AC susceptibility measurement in Fig. 1(b). We
found that the Tc’s at 5 and 1 K do not change significantly as a function of x.
References[1] T. Kariyado and M. Ogata, J. Phys. Soc. Jpn. 80, 083704 (2011).
[2] T. Kariyado and M. Ogata, J. Phys. Soc. Jpn. 81, 064701 (2012).
[3] T. H. Hsieh, J. Liu, and L. Fu, Phys. Rev. B 90, 081112(R) (2014).
[4] M. Oudah et al., Nat. Commun. 7, 13617 (2016).
[5] M. Oudah et al., in preparation.
[6] J.N. Hausmann, M. Oudah et al., in preparation.
[7] A. Ikeda, T. Fukumoto, M. Oudah et al. to be published in Physica B (2017).

Figure 1 (a) Resistivity ρ and (b)Real part of AC susceptibility
χ’AC as a function of temperatureof Sr3-xSnO under zero andvarious magnetic fields. The χ’AC
measurement under 0 T is cooled
down to ~0.15 K [4].

Uniaxial-Strain Control of Hc2 Anisotropy of NematicSuperconductor SrxBi2Se3
Quantum Materials Laboratory

Ivan Kostylev

Abstract Motivated by the recent discoveries of nematic superconductivity, characterized by therotational symmetry breaking in the amplitude of the superconducting wave function, we explore
the possibility of controlling nematicity by uniaxial strain. We plan to detect the superconducting
nematicity via measuring the upper critical field of Sr0.06Bi2Se3.
© 2017 Department of Physics, Kyoto University

Recently, it has been clarified that novel electronic states in strongly correlated systems can be induced orcontrolled by the application of uniaxial strain. In my PhD, attempting to control properties of various
superconductors by applying strain and measuring its superconducting transition temperature and upper critical field,
we utilized a piezo-stack-based device [1], capable of both tensile and compressive strains, for which I have devoted
much effort to establish various methods for sensing strain values, mounting samples, measuring sample properties,
and evaluating the actual strain using finite element analysis. Then we measured superconducting properties of Pb,
Nb-doped SrTiO3, and SrxBi2Se3, the latter of which is focused on in this presentation.
The search for new topological states of matter has motivated the discovery of superconductivity in the
topological insulator Bi2Se3 by Cu intercalation [2]. This compound has recently been attracting much attention as
the candidate for nematic superconductivity, being characterized by the rotational symmetry breaking of thesuperconducting wavefunction amplitude with respect to the underlying crystal lattice. Indeed, the trigonalrotational symmetry breaking in CuxBi2Se3 has been discovered by a two-fold field angular anisotropy in NMR andspecific heat experiments [3, 4]. These results indicate that CuxBi2Se3 is an odd-parity spin-triplet nematic
superconductor with either the so-called Δ4x or Δ4y order parameters. It was recently found that Sr intercalated Bi2Se3
also exhibits superconductivity [5, 6]. Interestingly, rotational symmetry breaking was also observed in the uppercritical field Hc2 measurements of SrxBi2Se3 [7]. Furthermore, a sample dependence of the Hc2 anisotropy wasreported [8], suggesting that Δ4x and Δ4y are nearly degenerate and might be switched by external stimuli.
We thus try to control the Δ4x/Δ4y nematicity by using uniaxial strain. For this goal, we investigate the anisotropyof Hc2 of SrxBi2Se3 under strain by measuring the AC magnetic susceptibility. We mount a SrxBi2Se3 single crystal(x=0.06), provided from Prof. Yoichi Ando’s group at Univ. Cologne in Germany, in the strain applying device asshown in Fig. 1. The strain is determined by monitoring the capacitance of a parallel plate capacitor system whosegap changes with applied strain. The device with the sample is then cooled by a dilution refrigerator equipped witha vector magnet capable of two-axis control of magnetic field direction. The susceptibility is measured by theconcentric pair of coils surrounding the sample.
The superconducting signal was successfully detected at 3 K, consistent with previous reports. At the maximum
applied strain, no measurable changes in the superconducting transition temperature at zero field were observed.
Nevertheless, although the quality of the data needs improvement, there is some evidence for change in the Hc2
anisotropy by uniaxial strain.
References[1] C. W. Hicks et al., Rev. Sci. Instrum. 85, 65003 (2014).
[2] Y. S. Hor et al., Phys. Rev. Lett. 104, 057001 (2010).
[3] K. Matano et al., Nat. Phys. 12, 852 (2016).
[4] S. Yonezawa et al., Nat. Phys. 13, 123 (2017).
[5] Shruti et al., Phys. Rev. B 92, 20506 (2015).
[6] Z. Liu et al., J. Am. Chem. Soc. 137, 10512 (2015).
[7] Y. Pan et al., Sci. Rep. 6, 28632 (2016).
[8] G. Du et al., Sci. China Phys., Mech. Astron. 60, 037411(2017).

Fig. 1. Strain applying device with a sample (S)attached to a holder to be mounted in the vector
magnet. The two outer (A) and one inner (B)piezoelectric elements apply tensile andcompressive strain, respectively.

Swelling effect on twist elastic constantin the smectic C liquid crystal
Soft Matter Physics Group

Kanako Hata

Abstract We introduced a solvent between layers in the smectic C (SmC) liquid crystal, and investigated
the swelling effect on interlayer steric interaction. We found that the twist elastic constant directly related
to the interlayer steric interaction monotonically decreases with the increase in the swelling ratio.
© 2017 Department of Physics, Kyoto University

Relaxation frequency t-1 [ms-1]

Recently, our group found that the repeat distance of the smectic layer can be greatly swollen to about threetimes thicker than that of original layers[1]. In this phase, a perfluorinated oil (FO) is introduced between layersby the chemical incompatibility of hydrocarbon and perfluorinated carbon chains. Then, we expected that thesteric interaction can be continuously tuned by the thickness of the solvent layer and the orientation order will belost at a certain limit. However, it is mysterious that the SmC phase can be well stabilized in whole concentration
range considering that the correlation of C-directors propagates between layers through disordered solvent layers.
In this study, we measured a twist distortion elasticity by dynamic light scattering (DLS) measurement, androtational viscosity by the electro optic switching behavior, because twist elasticity is directly related to the stericinteraction of the interlayer C-director correlation between the layers[1]. From this study, we clarified the role ofthe microscopic solvent molecules introduced between layers.
In this study, we mixed perfluorinated oil into a smectic liquid
0wt%
crystal, 11-(4’-cyanobiphenyl-4- yloxy) undecyl pentadeca40wt%
fluorooctanoate (BI) [2]. Figure 1 shows the dispersion relations for
50wt%
the twist mode of C-director in the mixtures. It is found that t-1 is
proportional to q2, as predicted by the hydrodynamic equations
t-1=B3q2/ [3] in pure and swollen SmC phases. Here is a rotational
viscosity coefficient. Then, in order to evaluate only twist elastic
constant, we substituted spontaneous polarizations Ps evaluated bypolarization switching current measurement and switching time
(Scattering vector )2 q2 [mm-2]
constants  evaluated by transmittance behavior in the electro-opticalmeasurement for equation of C-director motion   PsE (E:ElectricFig.1. Dispersion relations for the twistmode of C-director in the pure andfield) [4]. As a result, we found that  decreases by the swollen effect.
swollen SmC phases.
Then, we substituted  for B3/ estimated from dynamic lightscattering and evaluated swollen effect on B3. Figure 2 shows thedependence of B3 on the volume fraction of BI. As indicated in Figure
2, B3 monotonously decreases with increasing mixing ratio of theperfluorinated oil, and it becomes near 1 order weaker in the swollen
SmC phase as compared to that in the original SmC phase. We
concluded that intercalated solvent molecules prevent the direct
collisions between liquid crystal molecules, and weaken the stericinteraction to produce the orientation order across the smectic layers.
Reference[1] K. Hata, et al, submitted[2] M. Murase, et al, RSC Advances 5, 215-220 (2015)[3] I. Drevenšek, et al, Phys. Rev. A 41, 923 (1990).
[4] S. Kimura, et al, Jpn. J. Appl. Phys. 26, L255 (1987).

1 /   2 .0dSolvent(FO)d  1/ Fig.2. The dependence of B3 on the
inverse of the volume fraction of BIwith respect to the perfluorinated oil.

Magnetoelectric response in electric octupole ordered state
Condensed matter theory group

Takanori Hitomi

Abstract Motivated by a growing interest for magnetoelectric response in odd-parity multipole states, weinvestigate the magnetoelectric effect and spin Hall effect in an odd-parity electric octupole state andreveal several peculiar magnetoelectric properties. We propose a possible realization of the electric
octupole order in bilayer high-Tc cuprate superconductors.
© 2017 Department of Physics, Kyoto University

Transport property in noncentrosymmetric systems is one of the major topics in condensed matter physics. This
property is characterized by antisymmetric spin-orbit coupling (ASOC), which arises from an inversion symmetry
breaking. Most of previous studies considered noncentrosymmetric systems having a crystal structure with lack of
the inversion center. On the other hand, the odd-parity multipole order in locally non-centrosymmetric systems
has recently attracted a lot of attention. In such systems, a sublattice-dependent ASOC emerges due to the local
parity violation at each atomic site. The entanglement of the spontaneous inversion symmetry breaking due to the
odd-parity multipole and the sublattice-dependent ASOC leads to exotic magnetoelectric response, e.g.,magnetoelectric effect by magnetic quadrupole order in zigzag chains [1] and magnetic toroidal order inhoneycomb lattice [2].
In this study, we focus on the magnetoelectric response in the odd-parity electric octupole (EO) state, andinvestigate the magnetoelectric effect and spin Hall effect in a bilayer Rashba system. We construct a forward
scattering model by considering a layer-dependent Rashba ASOC (bilayer Rashba model). By analyzing the modelon the basis of the mean-field theory [3], we calculate the magnetoelectric coefficient and spin Hall conductivity
by using the Kubo formula. We obtain following consequences: (1) the magnetic moment characterized by thesymmetry of the EO moment is induced by the electric field, (2) the spin Hall conductivity is drastically enhancedin the EO state.
Furthermore, we study the bilayer cuprates, YBa2Cu3O7-x (YBCO), which is categorized into the high-Tc
superconductors. A change of nematicity has been detected by the magnetic torque measurement at a temperature
higher than the onset temperature of charge density wave state [4]. Moreover, the optical measurements, such as alinear dichroism [5], have pointed out the broken inversion symmetry. These experimental results indicate thepossibility of the EO order in the pseudogap regime. We discuss the “hidden order” in YBCO by analyzing thebilayer Rashba model with a weak two-fold anisotropy. From the calculation of the spin susceptibility, we obtainthe kink in the temperature dependence of the magnetic torque at the phase transition point and the reduction of
the spin susceptibility in the EO ordered phase, which are consistent with experimental observations. We further
show that the two-fold magnetic anisotropy is significantly enhanced in the superconducting state because the Van
Vleck susceptibility is remarkably anisotropic. Possible experimental test of magnetic anisotropy andcharacteristic magnetoelectric responses is proposed.
References[1] Y. Yanase, J. Phys. Soc. Jpn. 83, 014703 (2014).
[2] S. Hayami, H. Kusunose, and Y. Motome, Phys. Rev. B 90, 024432 (2014).
[3] T. Hitomi and Y. Yanase, J. Phys. Soc. Jpn. 85, 124702 (2016).
[4] Y. Sato et al., Nat. Phys. (2017), 10.1038/nphys4205[5] Y. Lubashevsky et al., Phys. Rev. Lett. 112, 147001 (2014).

77

Se-NMR Studies on Iron-based Superconductors Fe(Se1-xSx)
Quantum Materials Lab.

Anlu Shi

Abstract We have conducted 77Se-Nuclear Magnetic Resonance (NMR) on iron-based superconductors
Fe(Se1-xSx) to investigate the normal-state and the superconducting properties in various magnetic fields.
We found that nuclear spin-lattice relaxation rate divided by temperature (T1T)-1 is suppressed well above
Tc(H). The result is related to the theoretically-predicted preformed pairs.
© 2017 Department of Physics, Kyoto University

In the family of iron-based superconductors (FeSCs), FeSe (Tc ~ 9 K) has the simplest crystalline structure[1],but has some distinct properties from other FeSCs, e.g. it undergoes a structural phase transition below Ts ~ 90 K
without a static magnetic ordering and has an extremely small Fermi energy EF (EF ~10 meV). The characteristic
parameter, the ratio of superconducting gap () and Fermi energy /EF (~ Tc/Ts) in FeSe is then estimated to be
10-1 which is several orders of magnitude larger than that in conventional-metal superconductors (~ 10-4). Thus thesuperconductivity in FeSe is suggested to be in the
Bardeen-Cooper-Schrieffer (BCS) – Bose-Einstein condensate(BEC) crossover regime[2]. One of the most intriguing topics inthis regime is whether the theoretically-predicted preformed pairs
are present or not above Tc[3].
In my thesis, we performed 77Se-NMR measurements on FeSein various magnetic fields (H) to detect the trace of preformed
pairs microscopically. From the measurement of nuclear
spin-lattice relaxation rate 1/T1, we found that (T1T)-1 enhancedby antiferromagnetic (AFM) fluctuations below Ts is gradually
suppressed below T* ~ 15 K (Fig. 1), and exhibits a broad
maximum at Tp(H) above Tc(H) (Fig. 2). This is similar topseudogap (PG) behavior in the optimally-doped cuprate
superconductors YBa2Cu3O7. Since Tp(H) decreases in the same
Fig. 1. Temperature dependence of (T1T)-1 onmanner as Tc(H) with increasing H, the PG behavior in FeSe isFe(Se1-xSx) (x = 0, 0.08) in 0H = 12 T.
ascribed to superconducting fluctuations presumably originatingfrom the predicted preformed pairs above Tc[4].
We have also studied the chemical pressure effect of FeSe by
substituting S for Se. In particular, we performed NMR
measurements on Fe(Se0.92S0.08) (Tc ~ 10.1 K, Ts ~ 75 K). We
found that the AFM fluctuations below Ts are obviously enhanced
in Fe(Se0.92S0.08) (Fig. 1). It suggests that the increase of Tc is dueto this result. The PG behavior is also observed in this sample,
with the same ratio of Tp/Tc as that of FeSe. Based on these results,
we claim that the PG behavior is the intrinsic property of FeSe
superconductor, and is related to the formation of preformed pairs
above Tc.
References[1] F.-C. Hsu et al., Proc. Natl. Acad. Sci. 105, 14262 (2008).
[2] S. Kasahara et al., Proc. Natl. Acad. Sci. 111, 16309 (2014).
[3] Q. Chen et al., Phys. Rep. 412, 1 (2005).
[4] A. Shi et al., submitted to JPSJ.

Fig. 2. Temperature dependence of (a)diamagnetic-shielding signals and (b) (T1T)-1 on
FeSe in various H below 15 K.