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京 都 大 学 大 学 院 理 学 研 究 科

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２０１９年１１月２５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）
２０１９年１１月２５日（月）９：００～

開始

目

次

１． Spectroscopic Measurements on the Antiperovskite Oxide Superconductor Sr3-xSnO
Atsutoshi Ikeda（ ９：００）
・・・・・ １
２．Modern classification theory of superconducting gap nodes
Shuntaro Sumita（ ９：３０）・・・・・ ２
３．Study of novel topological superconductivity and bulk-boundary correspondence
Akito Daido（１０：００）・・・・・ ３
４．Controlling the unconventional superconductivity in artificiallyengineered heavy-fermion superlattices
Masahiro Naritsuka（１０：３０）
・・・・・ ４
５．Thermo-optic effects of metal halide perovskites
Taketo Handa（１１：００）・・・・・ ５

Spectroscopic Measurements on the Antiperovskite
Oxide Superconductor Sr3−xSnO
Quantum Materials Laboratory

Atsutoshi IKEDA

Abstract We report properties of the first antiperovskite oxide superconductor Sr3−xSnO. By the
Mössbauer spectroscopy, we experimentally observed the unusual metallic anion Sn4−. By themuon spin rotation experiment, we observed a long penetration depth possibly suggesting an
unconventional superconductivity.
© 2019 Department of Physics, Kyoto University

Recently classification of the materials in terms of the topology of
their electronic band structure is developing rapidly. One candidate of
the topologically non-trivial materials is the antiperovskite (inverse
perovskite) oxide. Antiperovskite oxides A3BO (A: alkaline-earth
element, B: group-14 element) are the materials crystallizing in thesame structure as the ordinary perovskite oxides ABO3 but with theinverted positions of the metal element and oxygen (Fig. 1). Assuming
Figure 1: Crystalline structures of thethe ionic states of A2+ and O2−, unusual metallic anion B4− such as Sn4−
ordinary perovskite oxide SrSnO3 andor Pb4− is expected. Such an ionic picture is indeed supported by the
antiperovskite oxide Sr3SnO [1].
first-principles calculations: the calculations show that the valence andconduction bands originate from the filled B-p and unoccupied A-d orbitals, respectively [2]. Theoretical analyses
further revealed that some of these oxides are topological crystalline insulators [3]. Since the topological nature
originates from the band inversion of the valence and conduction bands, the filled B-p or metallic anion B4− is the
key to understanding of this group of oxides.
We discovered superconductivity in Sr3−xSnO, the first superconductor among antiperovskite oxides, and havebeen studying its properties [1, 4]. Reflecting the topology of the electronic band structure in the normal state,possibility of the topological superconductivity in this material is proposed [1, 5]. In order to investigate the ionicstate of Sn, we measured the Mössbauer spectra (transmission of the γ ray with various energies) at Institute forIntegrated Radiation and Nuclear Science, Kyoto University (Kumatori). As a result, we obtained the firstexperimental evidence for the Sn4− state, which is the origin of the non-trivial topological nature in this material.
In addition, we observed another ionic state of Sn. From the comparison with the first-principles calculation, we
confirmed that this additional ionic state of Sn is related to the deficiency of Sr [6]. For the superconducting state,
we performed the muon spin rotation experiment at Paul Scherrer Institute (Switzerland), which is sensitive to the
distribution of the magnetic field inside the sample. We successfully detected the modulation of the field bysuperconductivity. This is the first evidence of the bulk superconductivity in this compound. The calculated
London penetration depth is long compared to those of the conventional superconductors and similar to those of
the unconventional superconductors [7].
References[1] M. Oudah, A. Ikeda et al., Nat. Commun. 7, 13617 (2016).
[2] T. Kariyado and M. Ogata, J. Phys. Soc. Jpn. 80, 083704, (2011).
[3] T. H. Hsieh, J. Liu, and L. Fu, Phys. Rev. B 90, 081112(R) (2014).
[4] J. N. Hausmann, A. Ikeda et al., Supercond. Sci. Technol. 31, 055012 (2018); A. Ikeda et al., Physica B 536,
752 (2018); S. Kitagawa, A. Ikeda et al., Phys. Rev. B 98, 100503(R) (2018); M. Oudah, A. Ikeda et al., Sci.
Rep. 9, 1831 (2019).
[5] T. Kawakami et al., Phys. Rev. X 8, 041026 (2018).
[6] A. Ikeda, S. Koibuchi, S. Kitao, M. Oudah, S. Yonezawa, M. Seto, and Y. Maeno, submitted to Phys. Rev. B.
[7] A. Ikeda, Z. Guguchia, S. Koibuchi, M. Oudah, S. Yonezawa, H. Luetkens, and Y. Maeno, in preparation.

Modern classification theory of superconducting gap nodes
Condensed Matter Theory Group Shuntaro Sumita
Abstract We classify symmetry-protected superconducting gap nodes on high-symmetry points
in the Brillouin zone by using group theory and topology. Our rigorous classification elucidates
unusual gap structures, which have not been predicted by previous order parameter analyses.
© 2019 Department of Physics, Kyoto University

Classification of a superconducting gap is one of the central subjects in the research field of unconventional
superconductivity. The momentum dependence of the superconducting gap is closely related to the symmetry ofsuperconductivity and the pairing mechanism. Since the superconducting gap structure can be identified byvarious experiments, combined studies of the superconducting gap by theory and experiment may clarify thecharacteristics of superconductivity. Most of the previous studies have been based on the classification of an orderparameter by the crystal point group [1-3], which was summarized by Sigrist and Ueda [4]. Although theirclassification has been used for analyses of excitation spectrum, it may not provide a precise result of thesuperconducting gap, namely excitation energy in the Bogoliubov quasiparticle spectrum. Indeed, several studies
have shown that nonsymmorphic crystalline symmetry induces unconventional gap structures which are notpredicted by the classification of order parameter [5]. That is because nonsymmorphic symmetry is neglected by
the above classification method based on the point group. Furthermore, the method does not take into accountelectrons beyond spin-1/2, which may appear in multi-orbital or multi-sublattice superconductors.
Stimulated by the above background, we developed the classification theory of superconducting gap nodes, byusing the combination of group theory and topology. Summaries of the study are shown below.
(i) Complete gap classification on high-symmetry planes [6-8]In this study, mirror- or glide-symmetric superconductors are considered. By using the group-theoretical
approach, we completely clarify the condition for nontrivial line nodes or gap opening on the Brillouin zone
boundary, which are protected by nonsymmorphic symmetry. Next, we show that such nonsymmorphicsymmetry-characterized gap structures appear only for a primitive or orthorhombic base-centered Bravais lattice;
all space groups under the additional constraint are systematically classified. As an example, we demonstrateunusual gap structures in the model of Sr2IrO4. Furthermore, we unify the topology of symmetry-protected line
nodes and Majorana flat bands, using the knowledge of Clifford algebra extension method.
(ii) Novel jz-dependent gap structures by classification on high-symmetry lines [7, 9]
Superconductors with crystalline rotation symmetry have potential to provide effectively higher-spin states. In
this study, we classify all crystal symmetry-protected nodes on n-fold (n = 2, 3, 4, and 6) axes in the Brillouinzone, by using the combination of group theory and K theory. Especially, we elucidate gap structures dependingon the total angular momentum jz of normal Bloch states on threefold and sixfold rotational-symmetric lines.
Based on the obtained results, we also discuss gap structures in several candidate superconductors.
References[1] G. E. Volovik and L. P. Gor’kov, Pis’ma Zh. Eksp. Teor. Fiz. 39, 550 (1984).
[2] G. E. Volovik and L. P. Gor’kov, Zh. Eksp. Teor. Fiz. 88, 1412 (1985).
[3] P. W. Anderson, Phys. Rev. B 30, 4000 (1984).
[4] M. Sigrist and K. Ueda, Rev. Mod. Phys. 63, 239 (1991).
[5] For example, T. Micklitz and M. R. Norman, Phys. Rev. B 80, 100506 (2009).
[6] S. Sumita, T. Nomoto, and Y. Yanase, Phys. Rev. Lett. 119, 027001 (2017).
[7] S. Sumita and Y. Yanase, Phys. Rev. B 97, 134512 (2018).
[8] S. Kobayashi, S. Sumita, Y. Yanase, and M. Sato, Phys. Rev. B 97, 180504(R) (2018).
[9] S. Sumita, T. Nomoto, K. Shiozaki, and Y. Yanase, Phys. Rev. B 99, 134513 (2019).

Study of novel topological superconductivity and bulk-boundary
correspondence
Condensed Matter Theory Group, Akito Daido
Abstract We study two novel aspects of topology in superconductors. First, we derive Fermi-surface
formulas for 𝒁𝟒 topological superconductivity and propose UCoGe as the first material candidate. Next,
we prove novel bulk-boundary correspondence in one-dimensional chiral symmetric systems. Thereby,
we contribute to deeper understanding of topology in superconductors. (less than 50 words)© 2019 Department of Physics, Kyoto University

Topological superconductivity and bulk-boundary correspondence are among the central topics in modern
condensed matter physics. Stimulated by the proposal to use Majorana fermions as qubits of topological quantum
computation, about two decades of research has revealed that topological superconductivity may be realized insystems such as nanowires and doped topological insulators [1]. Now, topological superconductivity, as well as
bulk-boundary correspondence in superconductors, begins to be accepted as a material phase in nature. Therefore,
one of the next steps of theoretical research is to investigate the variety of topological superconductivity and bulkboundary correspondence. In this work, we focus on two novel aspects of topology in superconductors [2,3].
First, we consider the interplay of topological superconductivity and crystalline symmetry. In particular, we
focus on non-symmorphic symmetries, which are defined as the combination of point-group operation withfractional-lattice translation. It has been pointed out that the non-symmorphic glide symmetry gives rise to an
intriguing topological (crystalline) superconductivity classified into 𝒁𝟒 topological phases [4], which do notappear in conventional topological periodic table [5]. However, material candidate for the 𝒁𝟒 topologicalsuperconductivity was not known. We reveal that the heavy-fermion superconductor UCoGe is the first promisingmaterial candidate of 𝒁𝟒 topological superconductivity [2]. We derive Fermi-surface formulas to predict 𝒁𝟒
topological invariants in the presence of coexisting screw symmetry, and thereby identify UCoGe as 𝒁𝟒
topological superconductivity.
Second, we consider a novel bulk-boundary correspondence in chiral symmetric systems. Examples include
Kitaev chain, Majorana nanowires, and time-reversal symmetric superconductors. In usual bulk-boundary
correspondence, the integer-valued topological invariants predict the number of boundary states. Thus, thecorrespondence is between two integers defined in the bulk and the boundary. Recently, it has been proposed that
this relation can be generalized to the correspondence between two continuous functions defined in the bulk and theboundary, in one-dimensional chiral symmetric systems [6]. In the context of superconductivity, this relation, named
spectral bulk-boundary correspondence (SBBC), means that surface accumulation of odd-frequency Cooper pairs
are given by a bulk quantity named generalized winding number (both of them are functions of frequency). However,original proposal was the conjecture based on the observation of a number of numerical calculations, and formal
proof of SBBC has been lacking. We point out that an analogy holds between SBBC and electric polarization: SBBC
can be understood as the polarization of chirality [3]. We give proofs of SBBC based on this idea.
References[1] M. Sato and Y. Ando, Rep. Prog. Phys. 80, 076501 (2017).
[2] A. Daido, T. Yoshida, and Y. Yanase, Phys. Rev. Lett. 122, 227001 (2019).
[3] A. Daido and Y. Yanase, to appear in Phys. Rev. B.
[4] K. Shiozaki, M. Sato, and K. Gomi, Phys. Rev. B 93, 195413 (2016).
[5] A. P. Schnyder, S. Ryu, A. Furusaki, and A. W. W. Ludwig, Phys. Rev. B 78, 195125 (2009).
[6] S. Tamura, S. Hoshino, and Y. Tanaka, Phys. Rev. B 99, 184512 (2019).

Controlling the unconventional superconductivity in artificiallyengineered heavy-fermion superlattices
Quantum Condensed Matter Group

Masahiro Naritsuka

Abstract To study the effect of interfaces on d-wave superconductivity, we fabricated two types ofheavy-fermion superlattices. In hybrid superlattices, we demonstrate that superconductivity can be
modified by magnetic fluctuations injected through interfaces. In tricolor superlattices, we found a
possible appearance of exotic superconducting phase induced by global inversion symmetry breaking.
© 2019 Department of Physics, Kyoto University

In the study of superconductivity in strongly correlated electron systems, the active use of heterointerface has
attracted attention. For example, two-dimensional superconductivity at the LaAlO3/SrTiO3 interface [1], high-Tc
superconductivity in monolayer FeSe on SrTiO3 substrate [2], the emergence of superconductivity in twisted
bilayer graphene [3], and the like can be mentioned. However, it is difficult to discuss the effect of the interface
because the underlying mechanism of superconductivity has not been clarified.
Heavy fermion systems are known to be unconventional superconductors for a long time. Among them,
CeCoIn5 and its family are well confirmed to be d-wave superconductivity mediated by antiferromagnetic (AFM)
fluctuations. In addition, two-dimensional superconductivity can be obtained by artificially engineered Kondo
superlattices consisting of alternating atomic layers of CeCoIn5 and other materials using a state-of-art molecularbeam epitaxy technique recently developed in our group [4].
Against such background, we fabricated two types of heavy-fermion superlattices in order to study the effect ofinterfaces on d-wave superconducting state.
i.) Tuning pairing interaction in d-wave superconductor by paramagnons injected through interfaces [5, 6]
Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class ofmaterials. A new approach to this issue focuses on the role of interactions between superconducting electronsand bosonic fluctuations at the interface between adjacent layers in heterostructures. In this study, we fabricatedhybrid Kondo superlattices consisting of alternating layers of d-wave superconductor CeCoIn5 and AFM metal
CeRhIn5 or CeIn3, in which the AFM order can be suppressed by applying pressure.We demonstrated that thesuperconducting and AFM states coexist in spatially separated layers, but their mutual coupling via the interface
significantly modifies the superconducting properties. This is the first realization that the superconducting
pairing can be tuned nontrivially by magnetic fluctuations (paramagnons) injected through the interface.
ii.) Emergent exotic superconductivity in artificially engineered tricolor superlattices [7]In the quest for exotic superconducting pairing state, the Rashba effect, which lifts the electron-spin
degeneracy as a consequence of strong spin-orbit interaction (SOI) under broken inversion symmetry, has
attracted considerable interest. In this study, to introduce the Rashba effect in a two-dimensional d-wave
superconductor, we fabricated tricolor Kondo superlattices in which CeCoIn5 is sandwiched by two different
normal metals of YbCoIn5 and YbRhIn5. We found that the Rashba SOI-induced global inversion symmetry
breaking in these tricolor Kondo superlattices leads to profound changes in the superconducting properties of
CeCoIn5. Moreover, the temperature dependence of in-plane upper critical field exhibits an anomalous upturn at
low temperatures, which is attributed to a possible emergence of a helical or stripe superconducting state.
References[1] N. Reyren et al., Science 317, 1196 (2007)[2] Q.-Y. Wang et al., Chinese Phys. Lett. 29, 037402 (2012).
[3] Y. Cao et al., Nature 556, 43 (2018).
[4] Y. Mizukami et al., Nat. Phys. 7, 849 (2011).
[5] MN et al., Phys. Rev. Lett. 120, 187002 (2018).
[6] MN et al., Phys. Rev. B 100, 024507 (2019).
[7] MN et al., Phys. Rev. B 96, 174512 (2017).

Thermo-optic effects of metal halide perovskites
Nanophotonics Group

Taketo Handa

Abstract The thermo-optic properties of metal halide perovskites were investigated. We foundthat metal halide perovskites have large negative thermo-optic coefficients, which originate froman unusual temperature shift in exciton energy and large thermal expansion. Interferometricmeasurements show that light illumination results in a large change in the refractive index via thethermo-optic effect.
© 2019 Department of Physics, Kyoto University

Metal halide perovskites are promising semiconductor materials for photonic applications. For the best
possible utilization and exploring novel optical functions of these emerging materials, the detailed understandingof the fundamental photophysics is indispensable [1-5]. The refractive index and its temperature dependence arevery important for the design and performance of photonic devices and understanding of the electronic band
structure of a material. Halide perovskites exhibit unique electronic band structures with unusual orbital characters
due to the heavy metal (Pb or Sn). It is highly interesting question whether the unique electronic structures affectthe optical responses of halide perovskites. In this work, we found that halide perovskites have a negative
thermo-optic coefficient and clarified its origin.
We investigated the temperature dependence of the refractive index of halide perovskite CH3NH3PbCl3 in thetransparent wavelength region. We found that CH3NH3PbCl3 shows a distinct decrease in the refractive index withincreasing temperature, i.e., a large negative thermo-optic coefficient [6]. This negative thermo-optic coefficient is
opposite to the positive coefficients in conventional inorganic semiconductors, indicating the unusual thermallyinduced change in the electronic structure of CH3NH3PbCl3. To clarify the origin of the negative coefficient, we
performed the temperature dependent absorption measurements. We observed a slight blueshift of the exciton
energy with increasing temperature [7], which can be ascribed to its orbital character. A model based on a Lorentz
oscillator suggested that both the blueshift of the exciton energy and the large thermal expansion contribute to thelarge negative thermo-optic coefficient in metal halide perovskites.
Furthermore, we studied the impact of the thermo-optic effect on the optical response of CH3NH3PbCl3 underpump light illumination. A high-sensitivity interferometric technique revealed that the refractive index largely
decreases even under weak illumination [6]. The observed results were successfully explained by the analysis
based on the phonon emission of photogenerated carriers and thermal diffusion, together with the large negative
thermo-optic coefficient. This analysis also clarified that CH3NH3PbCl3 shows a very low thermal conductivity
and consequent efficient local heating, resulting in the large change of the refractive index. Our study on thethermo-optic effects elucidated the interplay between the unique electronic structures and optical responses in
halide perovskite semiconductors.

References[1] Y. Kanemitsu and T. Handa, Jpn. J. Appl. Phys. 57, 090101 (2018).
[2] T. Handa, A. Wakamiya, and Y. Kanemitsu, APL Mater. 7, 080903 (2019).
[3] T. Handa et al., J. Phys. Chem. Lett. 8, 954 (2017)[4] T. Handa et al., J. Phys. Chem. C 121, 16158 (2017).
[5] T. Handa et al., Phys. Rev. Mater. 2, 075402 (2018).
[6] T. Handa et al., Sci. Adv. 5, eaax0786 (2019).
[7] T. Handa et al., in preparation.