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京 都 大 学 大 学 院 理 学 研 究 科

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要旨集

２０１８年１１月２６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：２０分（別に質問１０分程度）
２０１８年１１月２６日（月）９：００～

目

開始

次

１．Topological quantum phase transition in an SU(N)-invariant spin chain
Kazuhiko Tanimoto（ ９：００）・・・・・

１

２．Photoluminescence dynamics of lead-halide perovskite single crystals
Takumi Yamada（ ９：３０）・・・・・

２

３．Superconductivity in Sr2RuO4 micro-rings
Yuuki Yasui（１０：００）・・・・・

３

４．Enhancement of Spin-Triplet Superconductivity by PressureInduced Critical Ferromagnetic Fluctuations in UCoGe
Masahiro Manago（１０：３０）・・・・・

４

１１：００～１１：１０

休憩

５．The interface motion in the Ising model with heat transfer
Yusuke Masumoto（１１：１０）・・・・・

５

６．Anomalous diffusion of microspheres in a cell-sized actomyosin droplets
Masahiro Makuta（１１：４０）・・・・・

６

７．Emergence of spatially-localized nonlinear agents in a simple 2D Navier-Stokes model
Yoshiki Hiruta（１２：１０）・・・・・

７

１２：４０～１３：４０

昼休み

８．Inelastic X-ray Scattering Study of Plasmons in liquid Rb at elevated temperatures
Toru Hagiya（１３：４０）・・・・・

８

９．Statistical mechanical expressions of slip length
Hiroyoshi Nakano（１４：１０）・・・・・

９

１０．Semiclassical non-equilibrium dynamics after a quench of ultracold Bose gases in optical lattices
Kazuma Nagao（１４：４０）・・・・・１０
１１．Quantum many-body dynamics of the Bose-Hubbard systemwith artificial and intrinsic dissipation
Takafumi Tomita（１５：１０）・・・・・１１

１２．Nonequilibrium phenomena and controlsin strongly correlated
quantum systems driven by AC and DC electric fields
Kazuaki Takasan（１５：４０）・・・・・１２

１６：１０～１６：２０

休憩

１３．Dark exciton in monolayer transition metal dichalcogenides
Satoshi Kusaba（１６：２０）・・・・・１３
１４．Generalization of Jordan-Wigner transformation andexact solution of Majorana-spin coupled model
Yukihisa Imamura（１６：５０）
・・・・・１４
１５．Time-resolved exitonic Lyman spectroscopy in diamondTomoaki Ichii（１７：２０）・・・・・１５
１６．Magnetic-field and dimensionality effects on superconductors
with strong attractive interaction between electrons
Kyosuke Adachi（１７：５０）・・・・・１６

Topological quantum phase transition in
an SU(N)-invariant spin chain
Physics of Matter: Condensed Matter Physics (YITP)

Kazuhiko Tanimoto

Abstract We study the topological and entanglement aspect of SU(N)-invariant spin chain. We calculate
numerically the ground state wave function, entanglement spectrum and string order parameter. As aresult, we classify the ground states as topologically trivial or nontrivial and also determine phaseboundary between them.
© 2018 Department of Physics, Kyoto University

In traditional condensed-matter physics, high symmetries like SU(N) are realized in rather idealized situations
and have been used mainly as mathematical convenience. With the development of experimental techniques,
however, SU(N)-symmetric fermion systems were recently realized using ultracold alkaline-earth atoms and their
cousins, such as ytterbium (173Yb) and strontium (87Sr), trapped by optical lattices [1,2]. The SU(N) symmetry
comes from the nuclear spin I (N = 2I + 1) decoupled from the electronic angular momentum J. Furthermore, ametastable excited state 3P0 which is coupled to the ground state 1S0 through an ultranarrow doubly forbidden
transition serves as an additional SU(N) degrees of freedom thereby enriching SU(N)-symmetric systems. When
we take the strong-coupling limit, the low-energy physics of an SU(N)-symmetric fermion system is described by
an SU(N)-symmetric spin model.
Since a variety of novel quantum phases related to SU(N) symmetry are expected [3,4], theoretical understandingof quantum phases in the SU(N)-symmetric fermion systems is urgent. Symmetry-protected topological (SPT)phases are among such novel quantum phases. For example, the Haldane phase in one-dimensional spin systems
and topological insulators belong to the SPT phases. SPT phases are characterized by short-range entanglement
and the necessity of protecting symmetries. It is known that Gapped (bosonic) phases with SU(N)-symmetry areclassified into one trivial and N - 1 nontrivial topological classes [5]. Topological edge mode emerges reflecting
nontrivial bulk state and can be probed by from the entanglement property [6].
In this study, to classify the nature of the bosonic SPT phases predicted for an SU(4) cold-fermion system [3,4],
we consider a variant of the SU(4) Heisenberg spin chain with quadratic and cubic terms and investigate aquantum phase transition out of the topological phase. First, we numerically obtained the infinite-size ground state
wave function. Then, we calculate the string order parameter and the entanglement spectrum to show that the
ground state of the SU(4) Heisenberg model, which is the low-energy effective Hamiltonian of the originalfermion system, is adiabatically connected to that of the solvable model. Using them, we also identify the groundstate as one of the three topological classes
predicted for SU(4) spin systems [6]. Finally, tostudy the quantum phase transition between the
SPT phase and the dimer phase, we control aparameter of the Hamiltonian and determine thephase boundary between them.
Fig. 1. The exact ground state of the solvable
Referencesmodel. The boxes denote Young tableau.
[1] A. V. Gorshkov, et al., Nature Phys, 6, 289 (2010).
[2] S. Taie, R. Yamazaki, S. Sugawa, and Y. Takahashi, Nature Phys. 8, 825 (2012).
[3] H. Nonne, et al., Europhys. Lett. 102, 37008 (2013).
[4] V. Bois, et al., Phys. Rev. B 91, 075121 (2015).
[5] K. Duivenvoorden and T. Quella, Phys. Rev. B 87, 125145 (2013).
[6] K. Tanimoto, K. Totsuka, arXiv:1508.07601.

Photoluminescence dynamics of lead-halide perovskite single
crystals
Nanophotonics Group

Takumi Yamada

Abstract Photoluminescence (PL) properties of lead-halide perovskite MAPbX3 (MA = CH3NH3,
X = I, Br, Cl) single crystals have been investigated. Time-resolved two-photon excitation PL
microscopy revealed that photon recycling plays an important role in the PL dynamics of thicksingle crystals. One- and two-photon absorption spectra of the perovskite single crystals were
determined in consideration of the photon recycling effect. Furthermore, we found the efficientphonon-assisted anti-Stokes PL at room temperature.
© 2018 Department of Physics, Kyoto University

Organic-inorganic hybrid perovskites MAPbX3 (MA = CH3NH3, X = I, Br, Cl) are drawing considerable
attention as a new class of optical device materials. Despite its easy fabrication by simple solution methods, theyshow the excellent optical and electronic properties such as highly efficient photoluminescence (PL), sharp
absorption spectra resulting from low defect densities, and the long carrier lifetime. However, the exciton andcarrier recombination processes determining optical and transport properties have not been fully understood yet.
Therefore, it is inevitable to clarify fundamental optical responses of these perovskite single crystals in order tounderstand their superior properties.
In this work, we found that the PL dynamics of the perovskites are quite different between the polycrystalline
thin film and single crystal. The high-efficiency PL was observed in both forms but the redshift of PL appeared
only in thick single crystals [1]. Considering the spatial carrier distribution and efficient PL with no Stokes shift,
we conclude that this difference is caused by photon recycling effect (repeated light emission and reabsorption).
Using time-resolved two-photon excitation microscopy, we measured the excitation depth dependence of PL
spectra to confirm the influence of photon recycling on the PL dynamics of the perovskite single crystals [2]. As
the excitation depth increased, the shape of the PL spectrum changed from symmetrical to asymmetrical and the
PL lifetime increased. These changes can be quantitatively explained by taking into account the excited carrier
profile and the photon recycling effect.
In MAPbI3 and MAPbBr3 single crystals, where free carries govern their optical properties, and the dynamics ofthese free carriers are strongly affected by photon recycling [1,2]. Interestingly, photon recycling was clearlyobserved even in wide gap perovskite MAPbCl3 where excitons dominate optical properties at room temperature[3]. This means that photon recycling is a common feature of halide perovskites. In consideration of the photon
recycling effect, we determined the accurate band gap and exciton binding energy, which are fundamental opticalproperties of materials, based on one- and two-photon PL excitation spectra.
In addition, when focused on one-photon excitation energy below the band gap, phonon-assisted anti-Stokes PL(AS-PL) was clearly observed in thick single crystals. From the excitation energy dependence of AS-PL andStokes PL (S-PL) spectra, we obtained the up-conversion gain spectrum [4]. The broad shape of the up-conversion
gain spectrum suggests that unique phonon dynamics, such as large anharmonicity and polaron formation, play anessential role in the efficient up-converted PL.
References[1] T. Yamada et al., Adv. Electron Mater. 2, 1500290 (2016).
[2] T. Yamada et al., Phys. Rev. Applied 7, 014001 (2017).
[3] T. Yamada, T. Aharen, and Y. Kanemitsu, Phys. Rev. Lett. 120, 057404 (2018).
[4] T. Yamada, T. Aharen, and Y. Kanemitsu, submitted for publication.

Superconductivity in Sr2RuO4 micro-rings
Quantum Materials Laboratory

Yuuki Yasui

Abstract We report transport properties of Sr2RuO4 micro-rings fabricated by using the focusedion beam technique. In the magnetoresistance of some micro-rings, we found features ofhalf-quantum fluxoid, which is unique in spin-triplet superconductivity. Besides, in other rings,
unexpected quantum interference is observed, suggesting interference across chiral domain walls.
(48/50 words)© 2018 Department of Physics, Kyoto University

A number of experiments and theories support that the superconductivity of Sr2RuO4 is in a chiralspin-triplet state with an equal-spin pairing, while some experiments cannot be explained within this scenario [1].
In a spin-triplet equal-spin pairing state, due to the spin degree of freedom of the Cooper pair, the fluxoid stateaccompanying a half of the flux quantum can be realized. This is called the half-quantum fluxoid (HQF).
Although the observation of HQF in Sr2RuO4 micro-rings using magnetic torque has been reported [2], detection
with other experimental technique is still desired. For a chiral state, the time-reversal symmetry is broken in theorbital section of the wave function, and the ground states are two-fold degenerate between states with clockwise
and anti-clockwise orbital angular momenta. With an analogy to ferromagnets, such chiral superconductors are
expected to form domains of the degenerate ground states. Therefore, detection of the HQF and chiral domains
strongly progress the understanding of spin and orbital information of the superconductivity in Sr2RuO4.
To achieve this goal, we micro-structured Sr2RuO4 single crystals using the focused ion beam techniqueand measured their transport properties down to 300 mK under magnetic fields applied using a tri-axial vector
magnet. We observed magnetoresistance oscillations that is in good agreement with the Little-Parks oscillations
for the conventional fluxoid state. To our knowledge, this is the first report of the Little-Parks oscillations using
any bulk single crystals. We further investigated their transport properties with additional in-plane magnetic fields
and observed splitting of magnetoresistance oscillation peaks. This suggest the unconventional fluxoid state,
namely the HQF, is stabilized by the in-plane magnetic field [3].
In contrast, some other Sr2RuO4 micro-rings show magnetoresistance oscillations with unexpectedly large
amplitudes. Such behavior was previously reported in the same system [4]. Hence, to search for its origin, we
further investigated their current-voltage characteristics. Then, we found that the critical current also oscillateswith magnetic field with the same period as the magnetoresistance oscillations. Such critical current oscillations is
known to occur in the superconducting quantum interference device (SQUID). A SQUID requires a pair of “weaklinks” in the ring. However, artificial junction structures are not visible in scanning electron microscope images.
In addition, other transport properties do not show behavior of artificial junction structures. Therefore, theformation of “weak-links” should originate from its superconducting state and most naturally attributable todomain walls separating different ground states [5].

References[1] A. P. Mackenzie et al., npj Quantum Materials 2, 40(2017).
[2] J. Jang et al., Science 331, 186 (2011).
[3] Y. Yasui et al., Rhys. Rev. B 96, 180507(R) (2017).
[4] X. Cai et al., Phys. Rev. B 87, 081104(R) (2013).
[5] Y. Yasui et al., submitted to npj Quantum Materials.

Fig. 1. False-color scanning ion microscope
image of a Sr2RuO4 micro-ring. The blue region
is a Sr2RuO4 single crystal, and the yellow
regions are the electrodes made with silver paint.

Enhancement of Spin-Triplet Superconductivity by PressureInduced Critical Ferromagnetic Fluctuations in UCoGe
Quantum Materials Laboratory

Masahiro Manago

Abstract 59Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)
were performed on a ferromagnetic superconductor UCoGe to investigate the relationshipbetween the pressure-induced ferromagnetic quantum criticality and superconductivity.
Superconductivity remains robust as ferromagnetic fluctuations are enhanced by pressure,leading to the reinforcement of spin-triplet superconductivity.
© 2018 Department of Physics, Kyoto University

Superconductivity is often induced around a quantum critical point (QCP) of an ordered state, namely, thesecond-order phase transition at absolute zero. This phase diagram is widely seen in antiferromagnets including
heavy-fermion and iron-based superconductors. The ferromagnetic QCP has been also expected to induce thesuperconductivity [1]; however, no clear example of this was reported so far. This is because the spontaneous
magnetic field destroys the superconductivity, and the metallic ferromagnetic phase transition generally exhibits
first-order behavior at low temperatures as Curie temperature TCurie is nearly zero [2]. Thus, the superconductivity
by ferromagnetic QCP is rare despite the fact that these phases are familiar in condensed matter physics.
Coexistence of these is realized in the family of the uranium-based systems UGe2, URhGe, and UCoGe [3].
Among them, UCoGe has the lowest TCurie and relatively high superconducting transition temperature TSC. The
ferromagnetism is suppressed by the hydrostatic pressure, while the superconductivity survives in theparamagnetic phase without discontinuity [4]. This feature differs from the other systems, and thesuperconductivity has been believed to have relationship with ferromagnetic quantum criticality.
59Co NQR measurement was performed to reveal the relationshipbetween the ferromagnetic quantum criticality and superconductivity
in UCoGe. The nuclear spin-lattice relaxation rate divided bytemperature 1/T1T enhanced where the ferromagnetism is suppressedby pressure (~ 1 GPa). The slight increase of TSC suggests the
scenario that the ferromagnetic fluctuations are pairing glue. The
NQR spectrum at intermediate pressure indicates a weak first-orderferromagnetic transition, but the behavior of TCurie is similar to thecase of the second-order one. Thus, UCoGe is a clear example ofsuperconductivity induced by the ferromagnetic quantum criticality.
The 59Co NMR measurement was performed to investigate thesuperconducting properties at the paramagnetic side at 1.09 GPa. The
Knight shift, corresponding the spin susceptibility, did not decrease
drastically below TSC, consistent with the spin-triplet pairing. The
small decrease is ascribed to the pinning of the Cooper-pair spin andis suppressed as the field increases. This suggests the rotation of the59
Cooper-pair spin by the field to avoid the Pauli-paramagnetic effect, Fig. 1. Temperature dependence of Co
nuclear spin-lattice relaxation rate divided
as expected from the large upper critical field [4].
by temperature (1/T1T) at some pressures
on UCoGe [5].

References[1] D. Fay and J. Appel, Phys. Rev. B 22, 3173 (1980).
[2] M. Brando et al., Rev. Mod. Phys. 88, 025006 (2016).
[3] D. Aoki and J. Flouquet, J. Phys. Soc. Jpn. 83, 061011 (2014).
[4] E. Slooten et al., Phys. Rev. Lett. 103, 097003 (2009).
[5] MM et al., submitted.

The interface motion in the Ising modelwith heat transfer
Nonequilibrium Dynamics Group

Yusuke Masumoto

Abstract It is clarified that heat conduction generates a drift force toward the higher-temperature side on
the interface motion in the two dimensional Ising model. For the three dimensional case, changes ofdynamical properties of the interface near the roughening transition temperature are discussed.
© 2018 Department of Physics, Kyoto University

Recently heat transport in spin systems is intensively investigated. Besides
experiments on magnetic materials, there are a lot of theoretical studies on quantum
spin chains and on the origin of diffusive behavior. However, the role of interface
motion in heat transport has not yet been well studied. The existence of an interface
may enhance or reduce heat transport. Conversely, heat transfer may affect the
dynamics of an interface. In fact, the domain wall motion towards the higher
temperature side is reported in a nanowire [1]. We examine such interrelationshipbetween the interface and heat transport using the Ising model, because it is theFig.1 Snapshot of an interface
simplest example.
Because the Ising model does not have its own dynamics, various time evolution rules have been proposedin the literature to simulate nonequilibrium behavior. One example is Creutz cellular automaton, where kinetic
energy is added to each site and a spin is flipped if it can be done without changing total energy. However, it has aproblem that the spins freeze in low temperature. To circumvent the problem,
we have devised a new dynamical rule (KSC dynamics) for bulk spins. Spinsnear one end of x direction are in contact with a hot reservoir and those near
the other end are in contact with a cold reservoir. Those spins are evolved by
Glauber dynamics. Furthermore, spins at the both ends of x or z direction are
fixed in opposite directions to each other. Thus, the moving interface underheat-conduction conditions is realized if temperatures of the both reservoirs
are below the critical temperature (Fig.1).
Fig.2 Power spectrum
As a result, it is clarified that the interface motion in the Ising model is arandom walk with a drift force toward the higher-temperature side. It is well known in one-dimensional systems
that the power spectrum of a time sequence of spin values at a site shows the power law with exponent −1.5 due tothe diffusion of the interface [2]. We have extended it to two dimensions and analytically calculated the power
spectrum for the case where the interface has a finite width and is subjected to a drift force. It is numericallyverified that the power spectrum for the column-averaged magnetizations
agrees with the analytical one (Fig.2) and that the drift force is proportional to
the temperature difference between the heat reservoirs [3].
In the three-dimensional Ising model, the roughness of the interfacechanges at the roughening transition temperature 𝑇𝑅 . We have observed thatthe diffusion constant of the interface rapidly decreases below 𝑇𝑅 as shown in
Fig. 3. We also discuss the possibility of change of 𝑇𝑅 in nonequilibrium
conditions.
Fig.3 Diffusion constant in
References
3d Ising model[1] D. Hinzke and U. Nowak, Phys. Rev. Lett. 107, 027205 (2011).
[2] S. Takesue, T. Mitsudo and H. Hayakawa, Phys. Rev. E 68, 015103(R) (2003).
[3] Y. Masumoto and S. Takesue, Phys. Rev. E 97, 052141 (2018).

Anomalous diffusion of microspheres in a cell-sized actomyosin
droplets
Dissipative structure and Biological physics laboratory

Masahiro Makuta

Abstract We studied active fluctuation induced by actomyosin encapsulated in a micrometer-sized
sphere through the motions of the interface and the microbeads. Fluctuation properties of the microbeads
correlate with phases of the interface motion. Diffusion coefficients along the radial and the tangential
directions suggest anisotropic structural forming of actomyosin bundles and their dynamics.
© 2018 Department of Physics, Kyoto University

Actin filament is one of the cytoskeletons, which plays a major role in force generation of living cells. For
example, muscle actin generates contraction force in cooperation with the motor protein myosin. Actomyosin, which
is a composite material of actin and myosin also works in deformation and maintenance of the cell shapes. We have
performed in vitro experiments by using artificial cells to clarify the primitive mechanism of the deformationinduced by actomyosin [1,2]. These works figured out the dynamic interface deformation and analyzed theirstatistical features, however, it has not yet examined internal dynamics inside the artificial cells correlated with theinterfacial deformations.
We here studied active fluctuation induced by actomyosin through the motion of probe beads encapsulated in theactomyosin droplet [3]. The basic behaviors of the interface were consistent with the results of previous studies, i.e.,
induction, nonperiodic oscillation, and winkling phases were emerged in order. Through the experiments,
fluctuation of microbeads accompanied with motion transition of the interface exhibited correlations as shown in
Figs. 1. (a) and (b). In induction phase (0-300 s), microbeads fluctuated around the initial position, and the motion
became larger in the nonperiodic oscillation phase (400 – 500 s). In the early winkling phase (600 – 700 s), themicrobead was fallen into the interface where the actomyosin cortex formed. The trapped bead at the interface wasslightly moved with the winkling of the interface. In the former phases, anisotropic fluctuation was observed asshown in Fig 1(c), e.g., diffusion coefficient in the tangential direction was approximately 1.5 times larger than thatof radial direction. The result suggests anisotropic formation of actomyosin bundles and their force network due tothe small spherical boundary condition.

Fig. 1. (a)Trajectory of microsphere in actomyosin droplet. (b)Displacement of microsphere from initial
position. R represents the displacement in the radial direction, and R ⊥ represent the displacement in thetangential direction. (c)Distribution of diffusion coefficient and velocity of microsphere.

References[1] H. Ito, Y. Nishigami, S. Sonobe, M. Ichikawa, Phys. Rev. E 92, 062711 (2015).
[2] Y. Nishigami, H. Ito, S. Sonobe, M. Ichikawa, Sci. Rep. 6, 18964 (2016).
[3] H. Ito, M. Makuta, Y. Nishigami, M. Ichikawa, J. Phys. Soc. Jpn 86 (2017).

Emergence of spatially-localized nonlinear agents in a simple
2D Navier-Stokes model
Fluid Physics Group Yoshiki Hiruta
Abstract Spatially-localized turbulent states, widely observed in wall-bounded flows, are reproduced
in a simple 2D Navier-Stokes model in a periodic box. Moreover, it is confirmed that laminar-turbulencetransition which is regarded as a non-equilibrium phase transition, occurs for some suitable parameters.
The origin of the locality is discussed for Kolmogorov flow. c 2018 Department of Physics, Kyoto
University

Near the onset of the subcritical laminar-turbulence transition in wall-bounded flows such as pipe
flow[1,2] and Couette flow[3], a whole flow state is composed of a calm laminar state and spatially-localized
turbulent states[2,3].
On such wall-bounded systems, the transition from the laminar state observed in experiment occurs
at a finite non-dimensionalized flow rate, called Reynolds number, that is much less than critical values
estimated by linear instability theory. In that case, because it is usually difficult to identify dominantmodes governing transition, the flow often is modeled as complex excited agents embedded in a calm
background[4]. Such localized excited agents should play a key role to understand statistical behavior for
subcritical Reynolds number[2,4].
From the point of view of the governing equations, though Navier-Stokes (NS) equations
∂ui
∂ui
∂P
∂ 2 ui+ uj=−+ ν 2 + fi ,
∂t
∂xj
∂xi
∂xj

(1)

∂ui
= 0,
(2)
∂xi
have been established, a spatial coexistence of the laminar state and turbulent states is still shrouded inmystery. Characteristics of solutions of NS equations depend strongly on the way to be driven and boundary
condition as well as spatial dimension.
We try to answer the questions: ”Why are spatially-localized turbulent states widely observed in wallbounded flow?” or ”What are the condition for spatially-localized turbulent states to exist?”. In generally,
as a solution of NS equations, a realized laminar state is not unique, and hetero-clinic kink easily breaks
the locality of the dynamics. Theoretically the boundary conditions that restrict the velocity field are shown
to be significant, that is, imposing stick (non-slip) boundary condition or adding a linear drag (frictional)
force is sufficient condition for the unique local laminar state to exist.
In this talk, the stability of the laminar state and the locality of the dynamics are discussed for a classof Kolmogorov flow where fi = sin(ny)δix − ν0 ui [5,6]. Numerical experiments show a linear drag force
with the periodic boundary condition is sufficient to observe spatially-localized agents which realizes the
non-equilibrium phase transition.

References[1] O.Reynolds, Phil. Trans. R. Soc. Lond. 174, 935 (1883)[2] K.Avila et.al, Science 333, 192 (2011)[3] D.Coles, Journal of Fluid Mech. 21, 385 (1965)[4] D.Barkley, Phys.Rev.E 84, 016309 (2011)[5] V.I.Arnol’d and L.D.Meshchalkin, Uspekhi Mat. Nauk 15, 247 (1960)[6] Y.Hiruta and S.Toh, Phys.Rev.E 92, 063025 (2015)

Inelastic X-ray Scattering Study of Plasmons in liquid Rb
at elevated temperatures
Physics of Disordered Systems GroupToru Hagiya

Abstract Inelastic X-ray scattering measurements have been performed to investigate the plasmons inliquid Rb at elevated temperatures. We have experimentally determined the density dependence of theplasmon energy and linewidth and found that the electron-ion interaction is essential for describing theplasmon at high temperatures.
© 2018 Department of Physics, Kyoto University

The electronic state in liquid metals are deeply connected with the ionic structure and the electron density, which
varies with the thermodynamic state [1]. The electronic state with a variety of the thermodynamic state has been
investigated by ab initio MD simulation studies. They provide information about the ground state property, however,
little is known about the electronic excited state.
In order to reveal the relation between the excited state and ionic structure, we focus on the plasmon, which is afundamental electronic excitation in the electron gas. It has been pointed out that plasmons in liquid metals areaffected by the electron-ion interaction, which is closely related to ionic structure [2]. However, plasmons in liquid
metals were investigated only near the melting point and the relation between the plasmon and ionic structure has
not been sufficiently clarified yet.
In this study, we have performed inelastic X-ray scattering (IXS) experiments on liquid Rb at higher temperatures
than the melting point [3]. The temperature dependence of the plasmon behavior gives information on the effect ofstructural change on the plasmon behavior in liquid metals. From the IXS spectra, we determined the momentum
transfer
dependence of the plasmon energy and linewidth.
Fig. 1 shows the experimentally obtained plasmon energy at
0 in liquid Rb. We have experimentally
determined the density dependence of the plasmon energy by controlling the thermodynamic condition. This density
dependence is qualitatively consistent with the prediction of
0 , but the absolute value is lower
the electron gas
than this prediction. We demonstrate that this discrepancy is
resolved by calculating the plasmon energy
0 ,which includes the effect of the ionic structure. Furthermore,
we found that the effect of ionic structure on the plasmonenergy is correlated with the shape of the structure factor,
which describes the ionic structure of liquid. We also
calculated the plasmon linewidth with the effect of ionic
structure, and compared them with the experimental values.
The inclusion of the effect of ionic structure greatly
improves the agreement with the experimental values. Thisshows that the effect of ionic structure is essential for
describing plasmons in high-temperature liquid Rb.
Fig. 1. Density dependence of plasmon energy at
0 in liquid Rb. The experimental results
References
0 are compared with the calculated results.
[1] F. Hensel, W.W. Warren, Jr., “Fluid Metals: Liquid-Vapor
0 is the calculated result of the electron gas
Transitions of Metals”, (Princeton Univ. Press, 1999).
[2] K. Kimura et al., Phys. Rev. B 89, 014206 (2014).
and
0 is that with the effect of ionic[3] T. Hagiya et al., J. Phys. Soc. Jpn. 87, 084703 (2018).
structure.
0 is represented by different
symbols depending on the references data for the
calculation.

Statistical mechanical expressions of slip length
Nonlinear Dynamics Group

Hiroyoshi Nakano

Abstract We study the microscopic foundation of partial slip boundary conditions on the basis of
microscopic dynamics and linearized fluctuating hydrodynamics. By developing two linear response
theories from the microscopic dynamics, we obtain two statistical mechanical expressions of slip length.
Then, by exactly calculating these expressions from the linearized fluctuating hydrodynamics, we
confirm that these expressions provide the different expressions for the same quantity.
© 2018 Department of Physics, Kyoto University

Behaviors of a fluid near a solid surface are different from those in a bulk. In hydrodynamics, these behaviors
are described as boundary conditions (BCs) on the hydrodynamic equations. Since the 19th century, there havebeen wide discussions about the nature of BCs. These studies focused on whether or not the fluids may slip on thesolid surface [1]. In the 19th century, by employing a kinetic theory, it has been revealed that rarefied gases havefinite slip velocities. Recently, there were remarkable developments of experimental techniques in the field of
microfluidics. Thanks to these developments, the slips were experimentally observed for Newtonian liquids
confined in the small-scale systems [2].
The purpose of our study is to reveal the behavior of the fluid near the solid surface from the microscopic
viewpoint. In particular, this presentation focuses on a partial slip BC, which was introduced as a special BC so as
to describe slip phenomena from the phenomenological viewpoint. Under the partial slip BC, slip velocity of thefluid is linearly proportional to a shear rate at the solid surface (Figure 1). The proportional constant is called as aslip length. Some of experimental results reported in the past two decades are accurately described by the partial
slip BC.
In this presentation, we derive the partial slip BCs based on the underlying microscopic theories and thelinearized fluctuating hydrodynamics (Figure 2). Concretely, we consider the fluid enclosed between two parallelwalls. First, we develop two linear response theories for the microscopic systems under the assumption of
separation of scales between the microscopic behavior, such as collision of particles, and macroscopic behavior,such as relaxation to global equilibrium. The first linear response theory describes behaviors in an early stage ofthe macroscopic motion of fluid, specifically, the relaxation of fluid. The second one describes the steady state of
the fluid. Next, we demonstrate that these two linear response theories lead to the partial slip BCs. Then, we
obtain two statistical mechanical expressions of the slip length. Finally, by exactly calculating these expressions
from the linearized fluctuating hydrodynamics with the partial slip BC, we confirm the validity of these
expressions. Furthermore, we show that these two expressions provide different expressions for the same quantity.

Figure 1 Schematic illustration ofpartial slip BC

Figure 2 Diagram illustrating the relationship between two
linear response theories

References[1] C. L. M. H. Navier, Mem. Acad. Sci. Inst. Fr. VI, 389-440 (1823)[2] Y. Zhu and S. Granick, Phys. Rev. Lett. 88(10), 106102 (2002)

Semiclassical non-equilibrium dynamics after a quenchof ultracold Bose gases in optical lattices
Physics of Matter: Condensed Matter Physics (YITP) Kazuma Nagao
Abstract We analyze redistribution dynamics of the kinetic and on-site energies of the three-dimensional
Bose-Hubbard model using the truncated-Wigner approximation. Our numerical result agrees well withthe experimental one. We further study density-density correlation spreading after a quench in two
dimensions and reveal qualitatively different dynamics depending on the initial state.
© 2018 Department of Physics, Kyoto University

The high controllability and cleanness of ultracold atoms trapped in optical lattices make it possible to realize
an analog quantum simulator for exploring various quantum many-body phenomena [1]. An interesting target ofsuch quantum simulators is non-equilibrium quantum dynamics of macroscopic many-body systems, which is
hard to treat using currently available numerical tools on classical computers due to the exponential growth of theHilbert space with system size and the minus-sign problem in quantum Monte-Carlo simulations. In recent years,
the quantum optics group in Kyoto University has explored non-equilibrium dynamics of Bose-Hubbard systems
arising after a sudden quantum quench from singly-occupied Mott insulator states [2]. In the experiment, thegroup observed redistribution of the kinetic and onsite-interaction energies of Bose gases in a three-dimensional
optical lattice at an early time after a quench. An immediate usage of such quantum simulation results is to
examine or develop numerical methods for computing quantum many-body dynamics by taking them as an
accurate reference. Nevertheless, any quantitative approach that can recover the experimental results at three
dimensions has not been established thus far.
In this study, aiming to simulate the energy redistribution dynamics quantitatively, we perform a numericalcalculation utilizing the truncated-Wigner approximation (TWA), which is a semiclassical approximation
formulated in a phase space representation of quantum dynamics [3]. This method allows one to obtain
quantitative descriptions of short-time dynamics of the quantum averages even for macroscopic quantum systems,
to which exact diagonalization methods are inaccessible. Applying the TWA to the Bose-Hubbard model on a
cubic lattice, we simulate a short-time evolution after a rapid ramping down of the lattice depth from asingly-occupied Mott insulator state into a weakly-interacting regime. It is shown that our numerical result of thesemiclassical analysis agrees well with the experimental one with no fitting parameter [2, 4].
As a further application of our method, we also study non-local spreading of density-density correlations at
equal time after a sudden quench from several initial states in the two-dimensional Bose-Hubbard model. We
focus on dynamics inside a semiclassical regime where the interaction is relatively weak and the filling factor issufficiently large. Our analysis reveals that when the system is initially prepared in a coherent state, then apropagation velocity of the correlation wave packet in the correlation function strongly depends on the final
interaction strength, and it is bounded by twice the maximum group velocity of the elementary excitations, i.e., theBogoliubov excitations [4]. In contrast, when the system is initially in a Mott-insulator state, the propagation
velocity of the wave packet is approximately independent of the final interaction strength [4].
References[1] I. Bloch, J. Dalibard, and S. Nascimabéne, Nat. Phys. 8, 267 (2012).
[2] Y. Takasu, T. Yagami, H. Asaka, Y. Fukushima, Y. Takahashi, K. Nagao, S. Goto, and I. Danshita, in
preparation.
[3] A. Polkovnikov, Ann. Phys. 325, 1790 (2010).
[4] K. Nagao, M. Kunimi, Y. Takasu, Y. Takahashi, and I. Danshita, arXiv:1810.04347 (submitted).

Quantum many-body dynamics of the Bose-Hubbard systemwith artificial and intrinsic dissipation
Quantum Optics Group

Takafumi Tomita

Abstract We report on an experimental investigation of the Bose-Hubbard system with dissipation using
ultracold ytterbium (Yb) atoms in an optical lattice. We implement two-body particle loss in two different
manners and reveal the effect of the dissipation on the atom loss behavior and the quantum phasetransition.
© 2018 Department of Physics, Kyoto University

Thanks to their exquisite controllability, cold-atom quantum simulations have been successfully used forengineering various Hamiltonians of intensive theoretical interest [1]. For these well isolated and closed quantum
many-body systems various quantum phases are realized and the transitions among those phases are successfully
observed. In addition, several recent experiments have extended the applicability of the quantum simulators to
Liouvillian dynamics of open quantum systems by introducing coupling to environment, namely, dissipation [2,3].
We experimentally study a Bose-Hubbard system with thedissipation of particle losses using ultracold Yb atoms in thethree dimensional optical lattice. Firstly, we investigate thedissipative Bose-Hubbard system with two-body inelastic atom
loss with controllable strength, which is implemented byintroducing a single-photon photo-association (PA) process [4].
The inelastic collision rate, which characterizes the strength ofthe dissipation, can be controlled by varying the intensity of the
PA beam. In the dynamics subjected to a slow ramp-down of the
optical lattice, we find that strong on-site dissipation favors theFig. 1. Absorption images of the atoms. γ
Mott insulating state: the melting of the Mott insulator ischaracterizes the dissipation strength. As γ
increases, the interference pattern becomesdelayed and the growth of the phase coherence is suppressed byunclear in the shallow lattice regime, which
the strong dissipation (Fig. 1). This can be understood as theindicates the suppression of the growth of thecoherence.
quantum Zeno effect, that is, the strong dissipation suppresses
the unitary dynamics of the system.
Secondly, we realize the dissipative Bose-Hubbard system with the Yb atoms
in the metastable 3P2 state [5]. Because the collision between two metastableatoms induces the change of the internal degree of freedom, the Yb atoms inthe 3P2 state has intrinsic large inelastic collision rate, which induces thedissipation. In this study, we investigate the atom loss behavior with theunit-filling Mott insulator as the initial state. We find that the atom loss is
suppressed by the strong correlation between atoms (Fig. 2) and reveal that theinelastic interaction contributes to the formation of the correlation in addition
to the elastic interaction.
References[1] I. Bloch et al., Rev. Mod. Phys. 80, 885 (2008).
[2] A. J. Daley, Adv. Phys. 63, 77 (2014).
[3] M. Müller et al., Adv. At. Mol. Opt. Phys. 61, 1 (2012).
[4] T. Tomita et al., Sci. Adv. 3, e1701513 (2017).
[5] T. Tomita et al., arXiv: 1809. 09989 (2018).

Fig. 2. Two-body loss rate of
the atoms in the 3P2 state as afunction of the lattice depth.
Because of the strong
correlation, the measured
loss rate is much smaller
than the tunneling rate
(dashed line) which naively
characterizes the loss rate.

Nonequilibrium phenomena and controls in strongly correlated
quantum systems driven by AC and DC electric fields
Condensed Matter Theory Group

Kazuaki Takasan

Abstract We investigated nonequilibrium phenomena in strongly correlated systems irradiated by laser light (ACelectric fields) or DC electric fields and proposed new control scheme utilizing these nonequilibrium phenomena.
Our study paves a new way to control the quantum states of strongly correlated materials.
© 2018 Department of Physics, Kyoto University

In recent years, various interesting nonequilibrium phenomena are observed in solids and gathering great attention.
For example, it has been reported that applying strong laser fields induces transient superconductivity above theequilibrium critical temperature Tc [1]. Topological states of matter can also be changed with laser light. This has
been theoretically proposed [2] and already experimentally observed [3]. It has been revealed that not only thelaser light（AC field), but also the DC-field also induces quite interesting phenomena recently. For instance, it has
been reported that DC electric field induces a finite current in Mott insulators, giving rise to very strong
diamagnetism in the nonequilibrium steady states [4]. Furthermore, based on the study of these nonequilibrium
phenomena, the possibility of controlling states of matter with nonequilibrium phenomena has also been
investigated. It is expected that some important controls which are impossible or difficult in equilibrium become
available in nonequilibrium. In fact, a control using periodic driving with external fields is called “Floquet
engineering” and this has been established as one of the useful experimental techniques to realize desired quantum
states in the field of cold atomic systems [5]. As shown above, nonequilibrium phenomena and their controls incondensed matter systems are forming a rapidly growing research field in recent years.
On the other hand, theoretical understanding of these nonequilibrium phenomena still has been a challenging
problem. In particular, nonequilibrium phenomena in strongly correlated systems are difficult to treat theoreticallyand proposing nonequilibrium control in strongly correlated systems is more tough problem because we have totreat the effect of interaction and nonequilibrium dynamics at the same time. However, the various interesting andexotic phases of matter (e.g. unconventional superconductivity and quantum spin liquid) mostly appear in strongly
correlated materials and thus it is an important task to investigate strongly correlated systems.
Motivated by this situation, we have studied the nonequilibrium phenomena in strongly correlated systems andproposed possible schemes to control the states of matter using nonequilibrium phenomena. In this talk, after
explaining my motivation and backgrounds, I would like to introduce several studies by us in this direction [6-8].
Brief summaries of these studies are shown below.
i.) Laser-induced topological superconductivity in d-wave superconductors [6]Topological superconductivity (TSC) has attracted much attention because it can host Majorana fermions that are
expected to be applied to quantum computation. However, the experimental realization of TSC is very limited andstrongly desired. In this study, we proposed a possible scheme to realize TSC in laser-irradiated d-wave
superconductors such as cuprate superconductors. Our calculation based on Floquet theory revealed that thelaser-induced effective Zeeman-field plays an important role to realize TSC.
ii.) Control of Kondo effect and topological phases in heavy fermion systems with laser light [7]In heavy fermion systems, interplay between Kondo effect and RKKY interaction plays an important role andrealizes various quantum phase transitions. In this study, we investigated how this interplay is affected by theapplication of laser fields and how we can control the quantum phase transitions. We found that either ofenhancement and suppression of Kondo effect can occur depending crystalline structure and systematicallystudied the effect on the phases of matter including topological phases (e.g. topological Kondo insulators).
iii.) Control of insulating magnets with DC electric fields or THz laser fields [8]In this study, we considered the application of DC electric fields or slow AC fields (such as THz fields) toinsulating magnets and investigated how the exchange interaction is modified with electric fields. We found thatthe coupling in the direction of electric fields is enhanced and this is very useful to control the anisotropy ofmagnetic interactions. Based on this idea, we proposed exotic phases of matter induced by electric fields, e.g.
electric-field-induced quantum spin liquids and electric-field-induced topological phases.
[1] M. Mitrano, et al. Nature 530, 461–464 (2016).
[2] T. Oka and H. Aoki. Phys. Rev. B 79, 081406 (2009).
[3] Y. H. Wang et al. Science 342, 453-457 (2013).
[4] C. Sow, et al. Science 358, 1084-1087 (2017).
[5] G. Jotzu et al. Nature 515, 237-240 (2014).
[6] KT, A. Daido, N. Kawakami, and Y. Yanase, Phys. Rev. B 95, 134508 (2017).
[7] KT, M. Nakagawa, and N. Kawakami, Phys. Rev. B 96, 115120 (2017).
[8] KT and M. Sato, arXiv:1802.04311.

Dark exciton in monolayer transition metal dichalcogenides
Solid State Spectroscopy Group

Satoshi Kusaba

Abstract We investigated dark exciton states by prism-coupled photoluminescence (PL)measurements and observed strong nonlinear saturation behavior of dark exciton PL. We
revealed that exciton-exciton annihilation is an influential dark exciton relaxation mechanism.
© 2018 Department of Physics, Kyoto University

Monolayer transition metal dichalcogenides (TMDs) are promising materials for “valleytronics.” They havebandgaps at energy-degenerate K and K’ valleys with opposite spins. Due to the lack of inversion symmetry,
excitons or electron-hole pairs can be selectively created at K or K’ valleys by circularly polarized light. Exciton
plays an important role in photoluminescence (PL) in TMDs because of its large binding energy. Some of theexciton states in monolayer TMDs have a zero in-plane (Fig.1(a)) transition dipole moment, so this is often called
dark excitons. Very recently, one of the dark exciton states with the electric dipole moment perpendicular to the
crystal layer have been confirmed in WSe2 through the observation of perpendicularly polarized PL by a unique
optical configuration [1] or using designed coupling with surface plasmon polaritons [2]. Since this dark exciton islower excited state, it should play a key role in the relaxation dynamics. However, although bright excitons havebeen much studied, the detailed dark exciton dynamics has not been clarified.
In this study, we performed polarization-resolved PL spectroscopy of monolayer WSe2 using prism coupling
method as shown in Figure 1(a). The mechanically exfoliated monolayer WSe2 was transferred on the slant of aright-angle glass prism. PL was collected from the direction of an angle of 45 degrees. Prism coupling method
enables us to collect PL with large in-plane wavevector efficiently. Figure 1(b) shows polarization-resolved PL
spectra (TM and TE) at 7 K normalized at 1.67 eV. We observed four pronounced peaks: neutral A exciton at 1.76
eV, trion at 1.73 eV, and other two peaks at 1.69 eV (L1) and 1.67 eV (L2) which were attributed to localized
excitons in the previous report [3]. It is clear that a large difference
exists around L1 peak between TE and TM polarizations. Since
TM spectrum includes PL from both in-plane and out-of-plane
dipole components and TE spectrum includes only in-plain dipole
moment, one can extract an out-of-plane polarized PL bysubtracting TE spectrum from TM spectrum. Figure 1(c) shows thedifference spectrum. Two peaks exist at 50meV and 65meV below
neutral A exciton peak. We assign these peaks to dark exciton anddark trion by comparing the energy differences between each peakand neutral A exciton to the previous reports [1,2,4].
Then we investigated excitation-photon-energy dependence
andexcitation-intensity
dependence.
Excitation-intensity
dependence shows strong nonlinear saturation behavior under low
excitation power regimes caused by exciton-exciton annihilation[5] due to their higher density and longer lifetime than the bright
excitons. Our results reveal that exciton-exciton annihilation is an
influential dark exciton relaxation mechanism.
References
Fig.1 (a) Schematic diagram of theexperimental setup. We also show the[1] G. Wang et al., Phys. Rev. Lett. 119, 047401 (2017).
direction of the PL polarization (TE and[2] Y. Zhou et al., Nat. Nanotech. 12, 856 (2017).
TM) and each dipole moment. (b)Polarization-resolved
PL spectraof[3] J. Huang et al., Sci. Rep. 6, 22414 (2016).
monolayerWSenormalized
at
L2
peak.
2
[4] X. X. Zhang et al., Nat. Nanotech. 12, 883-889 (2017).
The excitation photon energy is 2.33eV. (c)[5] S. Mouri et al., Phys. Rev. B 90, 155449 (2014).
Their difference spectrum.

Generalization of Jordan-Wigner transformation andexact solution of Majorana-spin coupled model
Physics of Matter: Condensed Matter Physics (YITP)

Yukihisa Imamura

Abstract We study the bond-algebraic generalization of Jordan-Wigner transformation. We apply its
method to Majorana-spin coupled model on one dimensional lattice and find that it can be exactly
solved by the transformation.
© 2018 Department of Physics, Kyoto University
Duality is one of the most important concepts in theoretical physics. When we have several different
theoretical descriptions for the same physical phenomenon, there are dualities among them. Dualities canprovide reliable quantitative information about system. For example, it is well known that the strong
coupling region of a model is often transformed into the weak-coupling region of another model (or viceversa) by duality and we can use perturbation theory for investigating the original system in the strongcoupling region. One of the simplest examples which has that kind of duality would be the transverse-field
Ising model on a one-dimensional lattice (also known as “Kitaev chain”). Its boson-to-boson (spin-to-spin)
duality is self-duality and so we can find the exact critical point of the ferromagnetic-to-paramagnetic
quantum phase transition.
The transverse-field Ising model has another duality, boson-to-fermion duality. It is widely known thatspin-1/2 variables on a one-dimensional lattice can be transformed into Majorana fermions by a non-trivial
and non-local transformation, the Jordan-Wigner transformation. Its transformation changes the transverse field Ising model into the spinless p-wave superconductor[1], and so it is boson-fermion duality. It is thepoint to notice that the Hamiltonian of the superconductor becomes a sum of fermion-bilinears and that we
can exactly find the energy eigenvalues. In this case, the duality transformation enables us to solve thesystem exactly.
In this study, we demonstrate that it is possible to generalize the standard Jordan-Wigner transformation
and we can apply it for a Majorana-spin coupled model. We first reconsider the Jordan-Wignertransformation for the transverse-field Ising model[2] and introduce the method of bond algebra[3]. When
Hamiltonian is described as a sum of local Hamiltonians, bond algebra is an algebra generated from the local
Hamiltonians. It enables us to generalize the usual Jordan-Wigner transformation and to wider its range ofapplicability, beyond the boson-fermion duality. After introducing the foundations of the method, we thenintroduce another example, Majorana-spin coupled model[4]. This model is fermion-boson mixture model
which is constructed from Majorana fermions and spin-1/2 variables on a one-dimensional lattice. It has
different algebraic structure from the bond algebra of the transverse Ising model. However, we can also
construct Majorana fermionic operators from the Majorana-spin bond algebra and transform the fermionboson mixture system into the system which has
only fermions. It is a generalization of the JordanWigner transformation. Moreover, we find that thetransformed Hamiltonian is described as a sum of
fermion-bilinears and it can be exactly solved by
the Bogolyubov transformation. In addition to it,
we finally show that the Majorana-spin coupledmodel has a lot of conserved charges anddegeneracy of energy spectrum, and check theconsistency of the bond-algebraic dualitytransformation found there.
References[1] A. Y. Kitaev, Physics-Usp. 44, 131 (2001).
[2] K. Minami, J. Phys. Soc. Jpn. 85, 024003 (2016).
[3] Z. Nussinov and G. Ortiz, Phys. Rev. B 79, 214440(2009).
[4] Y. Imamura and H. Katsura, in preparation.

Fig.1. Duality web. The type I and II of the transversefield Ising model (TFIM) are self-dual each other. Bothare dual with p-wave superconductor by the JordanWigner transformation (JW). The Majorana-spin coupledmodel can be transformed into the Majorana chain by thebond-algebraic JW.

Time-resolved exitonic Lyman spectroscopy in diamond
Solid State Spectroscopy Group  Tomoaki Ichii
Abstract We studied exciton formation dynamics in diamond by time-resolved Lyman spectroscopy. We
found that formation dynamics is dominated by thermalization of carrier system via phonon scatteringand excitons are randomly distributed in splitting 1s exciton states.
© 2018 Department of Physics, Kyoto University

Excitons in semiconductors have been attracting many interests over decades in the viewpoint of fundamental
quantum many-body physics. Excitonic Lyman spectroscopy, which directly observes internal transitions of
excitons, is known as a powerful method to study exciton-exciton interactions because of the excellent ability toevaluate the density of the exciton. The recent remarkable progress of the terahertz (THz) spectroscopy based on
femtosecond laser technologies has made it possible use Lyman spectroscopy for excitons in group IV
semiconductors (ex. Si and Ge) and revealed exciton formation dynamics. Diamond, which also belongs to group
IV semiconductors, has characteristic properties in semiconductors, such as wide band gap, large phonon
frequency, and small spin-orbit interaction. These features cause the fine structure states of 1s exciton [1] andextremely large exciton diffusivity at low temperature [2]. Lyman spectroscopy will enable us to reveal various
fundamental properties of excitons in diamond, however, it remains undeveloped because of the difficulties ofbroad band generation of the THz pulse that covers the inner transition of excitons (~15 THz).
In this work, we overcome this problem with the THz generation method from air-based plasma [3] andconstructed the experimental system of deep ultraviolet (DUV) pump – terahertz (THz) probe spectroscopy.
Figure 1a shows DUV pump-induced changes in the imaginary part of the dielectric function at 100 K for two
pump-probe delays (t). At t = 12 ps, we observed the drude response, indicating the creation of free e-h pairs.
The solid curves represent analysis by a Drude model, which yields the e-h pair density, Neh. At t = 200 ps, weobserved the decrease of Drude response and emergency of a peak, which should be attributed to internaltransitions of the exction. This shows formation of excitons from free e-h pairs. On the other hands, the peak due
to inner transition of excitons is divided into several peaks at 13 K (arrows in Figure 1b), suggesting a splitting of
the 1s exciton state. Temperature dependence and time-development of the dielectric function allows us to clarify
the fundamental properties of free carriers and excitons as follows. First, we studied thermal ionization of excitonsat equilibrium state. We found that the ionization ratio is quantitatively well reproduced by taking account of finestructure of the 1s exciton. Second, we investigated non-equilibrium formation dynamics of excitons fromphoto-excited carriers. We quantitatively found that formation dynamics is dominated by the thermalization ofcarrier system via the interband and intraband scattering of carriers with phonons. Further, we revealed that
excitons are randomly distributed in splitting 1s exciton states.

Figure 1. Photoinduced changes in the imaginary part of the dielectric function (a) at 100 K and (b) at 13 K. Δt isa delay time after DUV-pump. Solid lines are best functions according to the Drude model.

References[1]
Y. Hazama, N. Naka, and H. Stolz, Phys. Rev. B 90, 045209 (2014).
[2]
H. Morimoto, Y. Hazama, K. Tanaka, and N. Naka, Phys. Rev. B 92, 201202 (2015).
[3]
V. A. Andreeva et al., Phys. Rev. Lett. 116, 0639s02 (2016).

Magnetic-field and dimensionality effects on superconductors
with strong attractive interaction between electrons
Condensed Matter Theory GroupKyosuke Adachi

Abstract We theoretically study magnetic-field and dimensionality effects on superconductors withstrong attractive interaction between electrons. We propose that superconducting-fluctuation effects canbe enhanced under magnetic field, the vortex-liquid state can be stabilized, and change in dimensionality
can induce the BCS-BEC crossover.
© 2018 Department of Physics, Kyoto University

The BCS-BEC crossover [1] is an exciting phenomenon in Fermionic many-body systems, which connects theseemingly distinct two concepts: the condensation of weakly bound Fermion pairs described within the
Bardeen-Cooper-Schrieffer (BCS) framework and the Bose-Einstein condensation (BEC) of tightly bound pairs.
The BCS-BEC crossover has been investigated in ultracold atomic gases by utilizing tunability of the strength of
attractive interaction between atoms; on the other hand, the BCS framework has been applied to mostsuperconductors since the attractive interaction between electrons is typically very weak.
Surprisingly, recent experiments [2, 3] have suggested that FeSe and related materials may involve strong
attractive interaction between electrons, which can open up opportunities to explore physical properties, forexample, transport phenomena, magnetic-field effects, and crystal-lattice effects, specific to superconductors withstrong attractive interaction. However, a theoretical understanding of such effects is still incomplete, and we study
magnetic-field and crystal-lattice-structure effects on superconductors with strong attractive interaction.
First, we investigate how thermodynamic quantities related to superconducting fluctuation, which is a precursor
phenomenon of superconductivity, is influenced by strong attractive interaction especially in magnetic field [4].
We find that the fluctuation-induced specific heat and diamagnetic susceptibility can be enhanced, andmagnetization curves can show a characteristic behavior called crossing, which are qualitatively consistent withexperimental results on FeSe.
Second, we explore features of the magnetic-field-temperature phase diagram of superconductors with strong
attractive interaction. We find that the vortex-lattice state, which involves a periodic array of quantized vortices
and ubiquitously appears in type-II superconductors under magnetic field, can melt into the vortex-liquid state dueto the enhanced superconducting fluctuation. We also point out that the particle density is an important factor in
determining the resultant phase diagram.
Lastly, we study how dimensionality can manifest itself in superconductors with strong attractive interaction [5].
We find that change in dimensionality from three-dimensional to two-dimensional lattice structure can induce acrossover from the weak-coupling BCS side to the strong-coupling BEC side, or the BCS-BEC crossover, without
tuning interaction strength. We propose that inserting insulating layers or applying anisotropic pressure in layered
superconductors can effectively change dimensionality and thus induce the BCS-BEC crossover.

References[1] M. Randeria and E. Taylor, Annu. Rev. Condens. Matter Phys. 5, 209 (2014).
[2] S. Kasahara et al., Proc. Natl. Acad. Sci. (USA) 111, 16309 (2014).
[3] S. Kasahara et al., Nat. Commun. 7, 12843 (2016).
[4] K. Adachi and R. Ikeda, Phys. Rev. B 96, 184507 (2017).
[5] K. Adachi and R. Ikeda, arXiv:1809.01837 (to be published in Phys. Rev. B).