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要旨集

２０１６年１１月２２日（火）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）
２０１６年１１月２２日（火）９：００～

目

開始

次

1. Visualization of Chiral Domain Structure in Chiral Superfluid 3He-A
笠井
純（

９：００）

2. Modifying Structure and Adhesion in Thin films and Membranes
GIBBONS, Andrew（

９：２５）

3. Membrane fluctuation and shape deformation with continuous flow of water
permeating lipid bilayer
鴻巣 正樹（ ９：５０）
4. Geometrical responses in topological materials

１０：４０～１０：５０

住吉浩明（１０：１５）

高畑

光善（１０：５０）

休憩

5. Nonlocal interaction of light with weakly confined excitons

6. Laser-induced Kondo effect and topological phases in ultracold alkaline-earth atoms
中川 大也（１１：１５）
7. Gap classification of multi-orbital Superconductors and non-symmorphic
Superconductors
野本 拓也（１１：４０）
8. Topological Aspects of Cold Atoms in Optical Superlattice松田
１２：３０～１３：３０

冬樹（１２：０５）

昼休み

9. Study of superconducting gap structure in prototypical heavy-fermion CeCu2Si2
山下 卓也（１３：３０）

10. Mechanism of excitonic valley coherence in monolayer transition metal
dichalcogenides
吉川 尚孝（１３：５５）
11. Quasiparticle excitations in FeSe in the vicinity of BCS-BEC crossover studied bythermal transport measurements
綿重 達哉（１４：２０）
12. Geometric Fluctuation Theorem
渡邉航太（１４：４５）

Visualization of Chiral Domain Structure
in Chiral Superfluid 3He-A
Low Temperature Physics Laboratory

Jun Kasai

Abstract We established a spectroscopic NMR imaging technique. By using this technique,
we visualized planar defects in superfluid 3He-A and identified them as chiral domain walls.
Those structures were stable for a long time in sufficiently low temperature.
© 2016 Department of Physics, Kyoto University

Superfluid 3He-A is well known as a chiral superfluid whose Cooper-pairs orient the orbital
angular momentum toward the same direction. In general, two ground states which have chiralitiesof +1 and -1 are degenerated. Around a chiral domain wall which exists between two domains of
opposite chiralities, the existence of edge mass current owing to intrinsic orbital angularmomentum is predicted but it has not been studied experimentally at all. Ikegami et al. reported anindication that chiral multi-domain are formed on a free surface of 3He-A [1]. However there haveever been no way to observe where chiral domain walls are.
We established an advanced NMR imaging technique, Magnetic Resonance Spectroscopic
Imaging (MRSI), which makes it possible to extract the local NMR spectra with a spatial resolutionof a few ten micrometers. With NMR imaging we discovered planar defects of 3He-A in a slab
geometry, shown in Figs.1. Topological defects which appear in superfluid 3He can be identified by
NMR frequencies. Detailed analysis of local NMR spectra tells that the planar defects had almost
the same frequency as the ground state, and locally broadened NMR line width. Comparison withnumerical simulation identified these planar defects as the dipole-locked soliton. It is topologicallyunstable in bulk volume, but in our experimental situation it is stabilized by chiral domain walls
pinned at the surface of the container. Therefore we succeeded in visualization of the chiral domain
structure in chiral superfluid for the first time. Those structures were naturally formed by cooling
into the superfluid phase and different patterns appeared in each cooling. They were metastable
but unchanged below T/Tc = 0.9 for a long time (> 1 day).

(a)
(b)
3
Fig. 1 MRI images of planar defects in He-A. (a) is the front view and (b) is the top viewof the slab. Several dark lines lying across the bright bar shapes which represent the
spatial distribution of 3He, are chiral domain walls.
References[1] H. Ikegami, Y. Tsutsumi and K. Kono, J. Phys. Soc. Jpn., 84, 044602 (2015)

Modifying Structure and Adhesion in Thin films and Membranes
Solid State Spectroscopy Group

Andrew Gibbons

Abstract Using phase separation of block copolymers, porous thin films membranes have beendeveloped for ultrafiltration. Additives have been used to tune the structure and adhesion properties of themembranes.
© 2016 Department of Physics, Kyoto University

Thin film polymer membranes are an attractive solution to the problem of making filter membranes with tunable
and flexible properties. Phase separation of block copolymers can be used to produce films with long range porous
structure [1]. A problem with many types of polymer membrane is the build-up and adhesion of organic materials.
This material is not easily removed and reduces the efficiency and longevity of filter membranes [2]. The topic of
this presentation is the modification of block copolymer membranes for membrane filtration applications.
Block copolymer membranes were developed with surfactant additives to reduce adhesion on the membranesurface. The block copolymer membranes of polystyrene-block-poly(methyl methacrylate) [PS-b-PMMA] weredeveloped using a self-assembly phase inversion technique to create porosity within the membrane. A wellcharacterized triblock copolymer of the Pluronic™ commercial series, made of blocks of polyethelene oxide andpolypropylene oxide, was used as an additive in the PS-b-PMMA film. Pluronic is a surfactant and its addition
increases the hydrophilicity of the surface resulting in a reduction of the adhesion of proteins on the surface [3].
Fig.1
Variationof
Pluronic
concentration in PS-b-PMMA film canresults in different film structures. The
optical microscope image (a) shows 10
wt % Pluronic producing crystalline
growth of Pluronic at crack interfaces.
SEM image (b) shows beginning of
crack development at the nanoscalewith 5 wt % Pluronic.

The effect of the Pluronic on the properties of the PS-b-PMMA membranes was examined in detail. Addition of
Pluronic results in an increase of the films hydrophilicity, as measured by contact angle. Above certain
concentrations the Pluronic also results in the formation of monolayers of dendritically crystallized Pluronic
surface layers, macrophase separation of the two mixed block copolymers and then much larger crack structures
within in the film, as shown in Fig.1 . The triple level of complexity in the resulting structure offers interesting
possibilities for altering nucleation and growth of adhesive layers during adsorption processes; therefore theresulting materials, through careful tuning of their structures via a range of parameters, offers an interesting
system for observing adhesion over multiple length scales.
References[1] H. Ahn, et al., ACS Nano, 8, 11745-11752 (2014).
[2] T. Nguyen et al., Membranes, 2, 804-840, (2012).
[3] Y.Q. Wang, et al., Journal of Membrane Science, 283, 440-447 (2006).

Membrane fluctuation and shape deformation with continuous
flow of water permeating lipid bilayer
Dissipative structure and Biological physics laboratory

Masaki Konosu

Abstract We investigate the influence of the continuous flow of water permeating lipid
membrane on shape deformation and fluctuation of giant vesicles. As a result, we find new
tubular morphology of lipid membrane. Furthermore, we discuss the kinetic model of amembrane in the context of soft matter physics on a lipid membrane and hydrodynamics on thesolvent. © 2016 Department of Physics, Kyoto University
Lipid bilayer membranes can exhibit a rich variety of shape transformation due to their mechanical flexibility,
which is adapted to a living system to express many cellular functions. Notably, shape deformation of lipid
membrane is known to be crucial in dynamic processes such as endocytosis, exocytosis, forming cell organelles
and cell death [1]. Giant unilamellar vesicles (GUVs) are suitable to observe their live deformations by optical
microscopy. Lipowsky and his co-workers have studied highly curved membrane structures such as budding andtubular structures of GUVs experimentally and theoretically, where such structures were known as the case driven
by the spontaneous curvature and thermal fluctuation of a membrane [2].
We studied the influence of the continuous osmotic flow of water permeating lipid membrane on shapedeformation and membrane undulation although the change in osmotic pressure results in the decrease in theinternal volume of the semi-permeable membrane of GUVs. The system that the concentration of sorbitol solution
in bulk linearly increases was developed using a perfusion chamber with syringe pumps and allowed to observe
shape deformation of each GUV with the continuous flow of water. Eventually, we made phase diagram of shapedeformation for the magnitude of permeation flow and found the tubular structure after a drastic fluctuation when
the water diffusion from the inner to the outer of a membrane was significant (Fig.1).
The goal of this study is to understand the role of mechanics in a growth of such highly curved membrane
structures. Physical properties gathered for GUVs with and without the permeation flow were power spectrum andrelaxation time of a flaccid membrane before the protrusion, both of which depend on the spatial frequency offluctuation. As a result, the dependencies on wave number in the case of insignificant efflux of water were thesame as the previous studies based on elastic energy minimum and thermal noise [3]. In the case of massive effluxof water, however, the dependencies deviated from ones without the permeation flow. We discuss why thephysical properties change and the equation of motion on unstable lipid bilayer.

Fig. 1. Osmotic permeation flow-induced protrusion of lipid bilayer
after perfusion of a hypertonic solution of sorbitol.

References[1] B. Alberts, et al., “Molecular Biology of the Cell” (Garland Science, New York, 2002).
[2] H.-G. Döbereiner, et al., Eur. Biophys. J. 28, 174 (1999).; R. Lipowsky, Biol. Chem. 395, 253 (2014).
[3] J. Pécréaux, et al., Eur. Phys. J. E 13, 277 (2004).; R. Rodríguez-García, et al., Phys. Rev. Lett. 102, 128101 (2009).

Geometrical responses in topological materials
Condensed matter theory group

Hiroaki Sumiyoshi

Abstract Using a technique of the theory of gravitation, we derive the Berry-phase formula for thethermal Hall effect in chiral superconductors and demonstrate that a new topological response referred toas the torsional chiral magnetic effect occurs in Weyl semimetals owing to a fictitious magnetic field due
to lattice defects.
© 2016 Department of Physics, Kyoto University

Recent both theoretical and experimental studies have revealed that topological materials suchas topological insulators and superconductors, graphene, and Weyl semimetals exhibit
characteristic transport properties associated with the geometrical structures of the wave functionsof the electrons in the real and momentum spaces. In addition to applying electromagnetic fields,
these responses are also caused by external forces, such as applying a temperature gradient,
twisting the sample, making dislocations in the lattice. Theoretically, they are described by thetechnique of the differential geometry. Based on these backgrounds, in this work, we theoretically,studied two geometrical transport phenomena in topological materials, using this technique.
First, we study on the thermal Hall effect in superconductors with broken time-reversal-symmetry[1]. The thermal Hall effect is the thermal analog of the Hall effect and the production of atemperature gradient by the transverse temperature gradient, and is important for detecting the
topological feature of the superconductors rather than the Hall effect, since the number of electrons
is not conserved and then electromagnetic responses hardly grasp it. We derive a formula for thethermal Hall coefficients, represented by the Berry-curvature of the Bogoliubov-de Gennes (BdG)
Hamiltonian. We also applied it to two-dimensional chiral superconductors, and found that in the
low temperature limit, the thermal Hall coefficient are quantized as, κ=cπT/6, where c is one-halfof the bulk Chern number defined from the BdG Hamiltonian and corresponds to the central chargeof the Majorana edge mode. It reproduces the result of a previous study from edge calculations [2],but can be applied for any superconductors even with nodes including Weyl superconductors, andtherefore useful for discovering broad types of topological materials.
Second, we propose a new topological response, referred to as the torsional chiral magnetic effect(TCME), in Weyl semimetals. Weyl semimetals are materials, which are characterized by one ormore pairs of Weyl cones protected by topological numbers, and many exotic transport phenomena
have been theoretically predicted and some of them are observed in the materials [3]. The TCME is
the generation of a ground state current
along an edge or screw dislocation line
(Fig.1). We investigate it by using thequantum field theory with backgroundgeometry. We also confirm the effectfrom a numerical calculation of realistic
lattice models of Weyl semimetals.

Fig. 1. Torsional chiral magnetic effect due to (a) edge and (b) screw
dislocations. Reproduced from [4].

References[1] Hiroaki Sumiyoshi and Satoshi Fujimoto, J. Phys. Soc. Jpn. 82, 023602 (2013).
[2] N. Read and D. Green: Phys. Rev. B 61, 10267 (2000).
[3] For review: P. Hosur and X. Qi, Comptes Rendus Physique 14, 857 (2013).
[4] Hiroaki Sumiyoshi and Satoshi Fujimoto, Phys. Rev. Lett. 116, 166601 (2016).

Nonlocal interaction of light with weakly confined excitons
Solid-State Spectroscopy Group	  	  Mitsuyoshi Takahata
Abstract We have observed film thickness dependences of the radiative recombination rate of
excitons in Cu2O by micro-photoluminescence measurements. This is the first demonstration ofsize-resonance enhancement of the coupling between weakly confined excitons and light by
nonlocal response of an excited state.
© 2016 Department of Physics, Kyoto University

Interaction between light and matter has been studied in many fields, such as physics, chemistry, andinformation technology. Recent advances in sample fabrication techniques opened up opportunities to explore
excited states with a coherence length extending over the wavelength of light, and light modes enhanced withmetal nanostructure in a molecular scale. In such a situation, the interaction of light and matter is expected to beenhanced due to nonlocal optical response [1] beyond the long wavelength approximation. We demonstrated
size-resonance enhancement of radiative coupling with weakly confined excitons using a high-quality Cu2O thinfilm, where Cu2O is known for the long exciton lifetime [2] and giant Rydberg exciton states [3].
Figure 1(a) shows photoluminescence (PL) spectra of the 2p-4p yellow excitons under continuous-wave laser
excitation of a Cu2O thin film at 2.33 eV. The PL spectral width (FWHM) and the PL intensity of 2p excitons
periodically varied depending on the film thickness. As shown in Fig. 1(b), the peaks of the FWHM (squares)
appeared at thicknesses different from those for the intensity (circles). The FWHM of 2p excitons in bulk Cu2O isknown to be caused by non-radiative relaxation involving phonon emission [4]. Since the non-radiative relaxation
rate is unchanged with respect to the film thickness, the varying FWHM observed in the film is attributed to thechange in the radiative width. The solid lines were calculated using the Green’s function approach to the nonlocal
response [1]. The good agreement between the experimental results and calculations clearly demonstrates that theradiative recombination rate of 2p excitons is enhanced due to the nonlocal response depending on the film
thickness.

(a)

(b)
7
9

Fig. 1: (a) PL spectra of a Cu2O thin film at 4.3 K, measured in steps of about 10 nm thickness. (b) Thickness
dependence of FWHM (squares) and intensity (circles) of 2p exciton PL in comparison with calculations
assuming nonlocal response (solid lines).

References[1] L. Q. Phuong, M. Ichimiya, H. Ishihara, and M. Ashida, Phys. Rev. B, 86, 235449 (2012).
[2] S. Koirala, M. Takahata, Y. Hazama, N. Naka, and K. Tanaka, J. Lumin. 155, 65 (2014).
[3] T. Kazimierczuk, D. Fröhlich, S. Scheel, H. Stolz, and M. Bayer, Nature, 514, 343 (2014).
[4] T. Kitamura, M. Takahata, and N. Naka, submitted to Phys. Rev. B.

Laser-induced Kondo effect and topological phases
in ultracold alkaline-earth atoms
Condensed matter theory group

Masaya Nakagawa

Abstract We propose that intense laser fields can induce a novel Kondo effect in ultracold alkaline-earth
atoms. The Kondo effect overcomes the heating effect caused by the irradiation and shows several
peculiar properties which originate from the laser-spin coupling. We also demonstrate that this systemhosts a symmetry-protected topological phase in one dimension.
© 2017 Department of Physics, Kyoto University

Kondo effect appears ubiquitously in condensed matter systems. This effect arises from the screening of a
localized magnetic moment by a surrounding fermion cloud, and leads to the formation of a many-body entangledstate called Kondo singlet between them, as a consequence of interplay of Fermi statistics and strong interactions.
The Kondo effect is thus an important phenomenon for condensed matter physics not only as a typical example ofquantum many-body effects, but also as a source of many-body quantum entanglement. For example, the Kondo
effect is one of the keys to understand correlation effects in various rare-earth materials, where the Kondo singlet
at every lattice site (“Kondo lattice”) leads to the formation of heavy fermion liquids with highly renormalized
quasiparticles. The Kondo lattice is also a significant platform to realize topological phases with strong
interactions, known as topological Kondo insulators, where the quantum entanglement is indispensable.
In this study, we propose that intense laser application to atoms can coherently induce a novel Kondo effect inultracold atomic gases [1]. In this setup, two long-lived metastable electronic states of alkaline-earth atoms play
the role of localized moments and a fermion cloud respectively, and the Kondo effect is induced by opticaltransitions between these two internal states. One of the problems addressed here is whether the quantum
many-body effect (the Kondo effect) survives or not in nonequilibrium situations under strong irradiation. We
demonstrate that the optically coupled two internal states are dynamically entangled to form the Kondo singlet
state, and they actually overcome the heating effect caused by the irradiation. Furthermore, it is shown that thelaser-induced Kondo effect has several peculiar properties, which cannot be realized in ordinary solid-statesystems. For example, a lack of SU(N) symmetry in the optical coupling gives spin-selective heavy fermion
liquids in which higher spin components have larger effective masses. We further found that the laser-induced
Kondo effect has unusual spin states different from the well-known Kondo singlet. This unusual Kondo state iscertainly distinct from the ordinary Kondo singlet state, if we assume the spin π-rotational symmetry along the xaxis.
In addition to such properties, the cold-atom realization of the Kondo lattice can host a topological phase
protected by symmetries, in one spatial dimension with tightly confined optical lattices. This phase is reminiscentof the celebrated Haldane phase in spin chains, but here the charge degrees of freedom play a key role on the fateof the topological phase. We uncover the role of various symmetries on the phase diagram of the one-dimensional
Kondo lattice, using field-theoretical methods and characterization with matrix product states. The
one-dimensional Kondo lattice provides a typical example of a crossover from fermionic topological phases inweakly interacting regime to bosonic topological phases in the strong coupling limit, and ultracold alkaline-earth
atoms are a promising candidate to realize the symmetry-protected topological phase.

References[1] M. Nakagawa and N. Kawakami, Phys. Rev. Lett. 115, 165303 (2015).

Gap classification of multi-orbital Superconductors andnon-symmorphic Superconductors
Condensed Matter Theory group

Takuya Nomoto

Abstract Motivated by a growing interest in multi-orbital superconductors with spin-orbit coupling, we perform the group-theoretical classification of gap structure in symmorphic/nonsymmorphic space groups. Our study shed light on several possibilities; the E2u pairing state in
UPt3, electron-phonon mediated anisotropic pairing states, and hidden symmetry-protected
nodes in UCoGe and UPd2Al3.
© 2016 Department of Physics, Kyoto University

Classification of superconducting gap functions in single-orbital systems (See a review by M. Sigrist and K.
Ueda [1]) is indispensable for an analysis of nodal structure in various unconventional superconductors. However,attention should be paid in its use, since the actual gap structure does not exactly have such a simple momentum
dependence summarized in tables of Ref. [1]. A remarkable example is an exotic multi-gap structure in aheavy-fermion superconductor UPt3, which we found from a gap analysis based on the first-principles calculations[2]. The obtained E2u state has in-plane twofold vertical line nodes, point and horizontal nodes on each Fermi surface, which is completely different from previous phenomenological models. It is very difficult to understand thenodal structure based on the classification of Ref. [1], and we need an explicit consideration of the multi-orbital
character [2]. In addition, T. Micklitz and M. R. Norman demonstrated in their pioneering work that new types ofsymmetry-protected nodes can appear at the Brillouin zone boundary in non-symmorphic systems [3]. These facts
imply importance of revisiting the classification of gap structure in symmorphic/non-symmorphic space groups by
explicitly considering the multi-orbital character.
In the present study, we provide the group-theoretical classification of gap functions in the multi-orbital superconductors with spin-orbit interactions [4] and in non-symmorphic magnetic space groups [5]. In the former
study, we perform the gap classification by introducing generalized Cooper pairs, which possess spin-orbital coupled (multipole) degrees of freedom, instead of the conventional spin singlet/triplet in the single-orbital systems.
From the classification, we obtain the following consequences: (a) A gap function with 9⊗9 in D6 possesses
nontrivial momentum dependence, which is different from the usual spin 1/2 classification. (b) Unconventional
gap structure can be realized in the BCS approximation for a purely local interaction. This fact implies emergenceof the electron-phonon mediated unconventional superconductivity.
Furthermore, we extend the gap classification to the non-symmorphic magnetic space groups, which havebeen less understood in spite of the growing interest in superconductors coexisting with magnetism. The results
are applied to the analysis of superconductivity in UCoGe and UPd2Al3. Based on the weak-coupling BCS theory,
we show that the UCoGe-type ferromagnetic superconductors must have horizontal line nodes on either kz=0 or
kz=±/c plane. On the other hand, in UPd2Al3-type antiferromagnetic superconductors, Ag symmetry possesses
line nodes in antiferromagnetic Brillouin zone boundary perpendicular to c-axis, namely, the conventional fully-gapped s-wave superconductivity is forbidden irrelevant to the pairing mechanism. Therefore, UCoGe and
UPd2Al3 are candidates of unconventional superconductors possessing hidden symmetry-protected line nodes,
peculiar to non-symmorphic magnetic space groups.
References[1] M. Sigrist and K. Ueda, Rev. Mod. Phys. 63, 239 (1991).
[2] T. Nomoto and H. Ikeda, arXiv:1607.02708 (to be published in Phys. Rev. Lett.).
[3] T. Micklitz and M. R. Norman, Phys. Rev. B 80, 100506(R) (2009).
[4] T. Nomoto, K. Hattori, and H. Ikeda, arXiv:1607.02716 (to be published in Phys. Rev. B).
[5] T. Nomoto and H. Ikeda, arXiv:1610.04679.

Topological Aspects of Cold Atoms in Optical Superlattice
Condensed matter theory group

Fuyuki Matsuda

Abstract We study the topological aspects of cold atoms loaded on an optical lattice with an added
optical superlattice potential. We find that the topological insulator-like behavior in this model can beunderstood clearly when we apply a mapping into higher dimensions, and that a set of weak Chern
number determines the adiabatic pumping in the original 2D system. We also study the correlation effects
in a 1D quasiperiodic superlattice.
© 2016 Department of Physics, Kyoto University

Recently, experiments of cold atoms in an optical lattice are developing rapidly. It is possible to realize
many different kinds of models on lattices and to control the interaction between particles with high flexibility byusing cold atom systems. This technology is important not only for simulating theoretically well-known models,
but also makes it possible to handle exotic models, which have not been realized in metals and alloys, such as new
types of topological insulators [1], quasiperiodic systems [2], and so on.
On the other hand, mapping the hamiltonian between different dimensions plays an important role inunderstanding their physical properties. One of the significant examples of these mappings can be seen in the
problem of the Hofstadter butterfly. Hofstadter mapped a two-dimensional (2D) system of square lattice underuniform magnetic field into a 1D equation, which is called the Harper equation [3]. Interestingly, a recent researchof adiabatic pumping used this mapping in reverse, and showed the topological equivalence over a certain groupof hamiltonians [4]. This kind of mapping into higher dimensions can be applied not only to 1D systems, but also
to two and higher dimensional systems.
Motivated by these backgrounds, we consider a 2D tight-binding model of particles that hop on a squarelattice in the presence of modulation in the on-site potential and the hopping amplitude, where the modulation
amplitude only depends on the distance from an inclined line. We find that this model has a topological nature.
First, we show that edge-localized states appear when the strong confining potential which constrain the particle
around inclined line exists. Also, when we pay attention to energy spectra, these edge states cross the bulk energy
band gap. These topological insulator-like behavior can be understood clearly when we map this 2D system into a
3D system. In the 3D mapped hamiltonian, a set of weak Chern numbers of this system becomes non-zero when
the chemical potential is controlled adequately, which implies that this system is a weak topological insulator. Thisset of weak Chern numbers determine the adiabatic pumping in the original 2D system.
Furthermore, we study correlation effects in the 1D quasiperiodic model. We consider the 1D
quasiperiodic Anderson-lattice model, which has quasiperiodically ordered impurities. The sites with an f-orbital
are ordered as a “Fibonacci word”, one way to form 1D quasiperiodic orderings. To treat the correlation effectprecisely, we use the density matrix renormalization group (DMRG) method. We show that the spin correlation
function in the quasiperiodic system gives a characteristic pattern. Also, by analyzing the f-electron number andits fluctuation, we find that a valence transition, which usually occurs in the periodic Anderson model when theon-site interorbital interaction is large, is not sharp in the quasiperiodic system. Finally, we discuss the propertiesof the quasiperiodic Anderson-lattice model, comparing them against the Anderson-lattice model with randomly
located f-orbitals. We find that the quasiperiodic Anderson-lattice model has a similar property to the periodic
Anderson model for spin correlation, but also has a similar property to the Anderson-lattice model with randomly
located f-orbitals for the valence fluctuation.
References[1] G. Jotzu et al., Nature 515, 237 (2014).
[2] L. Fallani et al., Phys. Rev. Lett. 98, 130404 (2007).
[3] D. R. Hofstadter, Phys. Rev. B 14, 2239 (1976).
[4] Y. E. Kraus et al., Phys. Rev. Lett. 109, 106402 (2012).

Study of superconducting gap structure in prototypicalheavy-fermion CeCu2Si2
Quantum Condensed Matter Group

Takuya Yamashita

Abstract We studied the superconducting gap structure and impurity effect on prototypicalheavy-fermion CeCu2Si2. In contrast to long standing belief, we found full gap superconductivity
with no sign change in this material. This leads to the surprising conclusion that strongly correlated
electrons in heavy-fermion can condense into a fully-gapped s-wave superconducting state.
© 2016 Department of Physics, Kyoto University

Strongly correlated electrons have been one of the most fascinating systems in modern physics. They exhibit arich variety of exotic phenomena such as unconventional superconductivity, non-Fermi liquid, exotic orders andso on. The strongest electron correlation is realized in heavy-fermion (HF) compound, in which the quasiparticle
effective mass is typically two or three orders of magnitude larger than the bare electron mass. In particular, suchheavy quasiparticles often exhibit unconventional superconductivity in the vicinity of the magnetic instability,
so-called quantum critical point (QCP).
CeCu2Si2 is a prototypical HF superconductor with Tc of 0.6 K discovered in 1979 [1]. The large specific heatjump at Tc indicates that the heavy electrons form Cooper pairs. The decrease of the Knight shift below Tc
indicates the spin-singlet superconductivity [2]. Moreover, T3/2 behavior in the resistivity and T1/2 behavior in 
indicate that the superconductivity occurs in the vicinity of an antiferromagnetic (AF) QCP [3]. Unconventional
nature of the superconductivity has been reported in some experiments, including the T-linear behavior of C/T [4],and no coherence peak just below Tc and the T3 behavior in NMR relaxation rate 1/T1 [5]. Based on these results,
the superconducting symmetry in CeCu2Si2 has been suggested to be d-wave with line nodes mediated by AF
spin-fluctuations. On the other hand, recent specific heat measurements [6] suggested fully-gappedsuperconductivity. However, since the specific heat mainly detects the quasiparticle contribution in the heavy
electron band, the possibilities of the existence of nodes in the light hole band cannot be excluded.
Here, to clarify the superconducting gap structure of CeCu2Si2, we performed the thermal conductivity ()measurements which sensitively probe the quasiparticle contribution in the light band. At the lowest temperaturesin zero-field, /T extrapolated to T = 0 goes to zero within our experimental resolution or is at least an order ofmagnitude smaller than that expected for line nodes. Moreover, field dependence of /T shows that the magnetic
field hardly affects the thermal conduction in the low field regime, which is in stark contrast to the nodal
superconductor. Based on these results, we conclude the absence of gap nodes at any point on the Fermi surface.
Furthermore, in order to clarify whether there is sign change in the superconducting gap, we performed theelectron-irradiation experiments. We found a very small pair-breaking effect, which suggests there are no signchanges in the superconducting gap function.
These results imply that, contrary to long-standing belief, heavy electrons with extremely strong Coulomb
repulsions can condense into a fully-gapped s-wave superconducting state, which has an on-site attractive pairing
interaction [7].
References[1] F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979).
[2] K. Ueda et al., J. Phys. Soc. Jpn. 56, 867 (1987).
[3] P. Gegenwart et al., Phys. Rev. Lett. 81, 1501-4 (1998).
[4] C. D. Bredl et al., J. Magn. Magn. Mater. 31–34, 373 (1983).
[5] K. Ishida et al., Phys. Rev. Lett. 82, 5353 (1999).
[6] S. Kittaka et al., Phys. Rev. Lett. 112, 067002 (2014).
[7] T. Yamashita et al., submitted for publication.

Mechanism of excitonic valley coherence inmonolayer transition metal dichalcogenides
Solid State Spectroscopy Group

Naotaka Yoshikawa

Abstract We investigated the polarization dependence of the exciton photoluminescence and Raman
scattering of CVD-grown monolayer MoS2. We found that the photoluminescence become polarizedaccompanied by a drastic change of the polarization selection rule of Raman scattering under resonant
excitation. We concluded that a domination of Raman component in the resonant second order opticalprocess should be the mechanism of valley coherence.
© 2016 Department of Physics, Kyoto University

The investigation of internal quantum degree of freedom is one of the most active research fields of condensed
matter physics. The representative example is electron spin which connects with magnetic information storage,
leading to the vast field of spintronics. The recent emergence of monolayer transition metal dichalcogenides suchas MoS2 and WSe2 monolayers provides a new platform for exploring valley pseudospin and their potential for
new electronics. These two-dimensional materials have a direct bandgap at the corners of the hexagonal Brillouinzone, that is, K and K’ points. Because of the inversion symmetry breaking and strong spin-orbit interaction, theenergetically degenerated exitons in K and K’ valleys have the opposite spins and can be selectively excited using
circularly polarized light. Robust valley polarization was demonstrated by the conservation of helicity in the
exciton photoluminescence [1]. In addition, linearly polarized photoluminescence whose orientation coincides
with that of the linearly polarized excitation from excitons was also observed in monolayer WSe2 [2]. This
indicates that excitons in different valleys may maintain their phase coherence in the recombination process
because excitation with a linearly polarized photon generates a coherent superposition of the exciton states in K
and K’ valleys. The optical generation and readout of valley coherence are attracting much attention for quantum
information applications using a valley pseudospin. However, the physical mechanism of valley coherence
remains unclear, in particular, it has not been revealed how excitonic valley coherence can be kept underrelaxation.
In this study, we investigated the polarization dependence of the exciton photoluminescence and Raman
scattering of CVD-grown monolayer MoS2, with circular and linear polarization and at a resonant andnon-resonant excitation condition. Figure 1(a) shows linear polarization-resolved photoluminescence spectra at 34
K with resonant excitation. The exciton photoluminescence is strongly polarized, indicating the effective
generation of valley coherence. In addition, spectrally narrow Raman peaks by phonons are also linearly polarized.
The detection polarization dependence of the resonant Raman scattering is same as that of exciton
photoluminescence as shown in Fig. 1(b). This indicates that the exciton photoluminescence is polarized just as
the resonant Raman scattering is. Since the photoluminescence and Raman scattering cannot be distinguishedunder resonant excitation, they should be considered as a resonant second-order optical proces [3]. The resonant
second order process should be a key to understanding the mechanism of valley coherence. By comparing thepolarization dependence of photoluminescence andRaman scattering under non-resonant excitation withthat under resonant excitation, it is indicated that theresonant second order optical process should be a key
to understanding the mechanism of valley coherence.
Considering the temperature dependence and theory ofpolarization selection rules, we concluded that themechanism of valley coherence should be adomination of Raman component in the resonant
second order optical process.
References
Fig.
1
(a)
Linearpolarization-resolved[1] K. Mak et al., Nature Nanotech. 7, 494 (2012).
photoluminescence spectra with 1.96 eV excitation at[2] A. Jones et al., Nature Nanotech. 8, 634 (2013).
34 K. (b) Normalized intensity of photoluminescence[3] Y. Toyozawa et al., J. Phys. Soc. Jpn. 42, 1495
and Raman scattering as a function of detection(1977).
polarization angle.

Quasiparticle excitations in FeSe in the vicinity of BCS-BEC
crossover studied by thermal transport measurements
Quantum Condensed Matter Group

Tatsuya Watashige

Abstract To investigate the field-induced superconducting state (B-phase) in FeSe in the vicinity of
BCS-BEC crossover, we performed the thermal Hall conductivity κxy measurements. Our results suggestthat the B-phase is likely to a highly anomalous inhomogeneous superconducting state, rather than aconventional Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state.
© 2016 Department of Physics, Kyoto University

The iron-based superconductor FeSe has recently attracted growing interest because it exhibits several
fascinating properties. The Fermi surface of FeSe consists of one small hole pocket at the Γ point and one or two
small electron pockets at the M point. Remarkably, the Fermi energies of hole and electron pockets are extremely
small, EFh ∼ 15 meV and EFe ∼ 10 meV, respectively [1,2]. FeSe is a multi-gap superconductor with two distinct
superconducting gaps of Δ ≈ 2.5 and 3.5 meV [1,3] which are comparable to EF. This strongly indicates that
FeSe is located in the BCS-BEC crossover regime.
Recently, it has been reported a possible new field-induced superconducting phase (B-phase) in the
low-temperature and high-field region (Fig. 1) [1]. In the B-phase, the Zeeman effect gives rise to highly
spin-imbalanced superconducting state with the magnitude of the spin imbalance P = (N↑ - N↓) / (N↑ + N↓) ∼ 0.1,
where N↑ and N↓ are the numbers of up and down spins respectively. However, very little is known about thenature of the B-phase.
To investigate the properties of the B-phase, we performed thermal conductivity κxx and thermal Hall
conductivity κxy measurements on FeSe single crystals up to 20 T. Both quantities are very sensitive probes of thelow-energy quasiparticle excitations in the superconducting state.
The field-dependence of the thermal Hall angle tanΘH/B ≡ κxy/κxxqpB, which is proportional to thequasiparticle scattering time τqp (Fig. 2), shows a distinct kink at the boundary of the B-phase at H*. Here κxxqp is
the quasiparticle thermal conductivity obtained by subtracting the phonon contribution κxxph from κxx. This
indicates that the quasiparticle scattering mechanism is dramatically modified on entering the B-phase. Based onthese results, we suggest that the B-phase is likely to a highly anomalous inhomogeneous superconducting state
which has not been addressed before, rather than a conventional FFLO state [4].

Fig. 1: H-T phase diagram of FeSe. The highly
spin-imbalanced superconducting phase (B-phase)appears above H* [1,4].

Fig. 2: Field dependence of the thermal Hall angle
tanΘH/B at T = 0.59 and 0.95 K. The clear kinkanomalies appear on entering the B-phase at H*.

References[1] S. Kasahara and T. Watashige et al., Proc. Nat. Aca. Sci. 111, 16309-16313 (2014).
[2] T. Terashima et al., Phys. Rev. B 90, 144517 (2014).
[3] T. Watashige et al., Phys. Rev. X 5, 031022 (2015).
[4] T. Watashige et al., submitted for publication.

Geometric Fluctuation Theorem
Advanced Statistical Dynamics Group

Kota Watanabe

Abstract We derive the current distribution and the fluctuation theorem for a geometric pumping under aperiodic parameters change by using quantum Markovian master equation. As a result, we find absence ofthe reciprocal relation which is held for fluctuation in a steady current. We also verify the validity of ourtheory by Monte Carlo simulation in a spin boson system.
© 2016 Department of Physics, Kyoto University

Steady current such as an electrical current or a heat current is a fundamental quantity tocharacterize the corresponding nonequilibrium phenomenon. There exists a steady current in thesimplest setup where the bias such as the chemical potential difference or the temperature
difference between reservoirs is constant in time. Such a steady current and its fluctuation can bederived by the current distribution. For example, a current distribution P (J ) during the timeinterval  by applying a bias voltage V satisfies the steady fluctuation theorem

lim ln{ P ( J ) / P (  J )} /    eVJ , 

where  1 is the temperature of reservoirs and e is the electrical charge. This fluctuation
theorem give several kind relations such as the fluctuation dissipation between the conductance( dissipation ) and noise power ( fluctuation ) or the reciprocal relation.
There exists different type current from a steady current, so called the geometric pumping current
in a mesoscopic system. The geometric pumping whose origin is related to Berry-like phase [1] isrealized by modulation of control parameters such as chemical potentials, gate voltages, andtunneling barriers. It is remarkable that there exists a net current without dc bias if there exists
Berry-like phase.
In previous studies of the geometric pumping current, fluctuation theorems have not beensystematically discussed, though e. g. we have discussed the non-adiabatic current in the geometricpumping process [2]. In this study, I have derived the current fluctuation theorem in a geometricpumping. My basic equation is the Markovian quantum master equation modified by the fullcounting statistics [3]. The current distribution is derived in the sufficiently small angularfrequency  of parameters change. Then, the fluctuation theorem satisfies

lim ln{ P p ( J ) / P (  J )} /  p  AJ  BJ 3   C 2 J    ( 3 ) , p 

where A, B and C depend on the path of modulation of parameters. In the case of sufficiently
small  and J , this fluctuation theorem seems to be reduced to the similar equation in a steadycurrent. Nevertheless, there exist some differences between our fluctuation theorem and theconventional one. In particular, there is no reciprocal relation in our case, though thefluctuation-dissipation relation is satisfied. I also verify the validity of our theoretical current
distribution and the fluctuation theorem by the Monte Carlo simulation in a spin boson system.
References[1] J. Ren, P. Hänggi and B. Li, Phys. Rev. Lett. 104, 170601 (2010).
[2] K. L. Watanabe and H. Hayakawa, Prog. Theor. Exp. Phys. 2014, 113A01 (2014).
[3] M. Esposito, U. Harbola and S. Mukamel, Rev. Mod. Phys. 81, 1665 (2009).