## Basic Information

### Major Professional Backgrounds

- 2008/04/01, 2010/02/28, Tohoku University, Department of Physics, Assistant Professor
- 1999/10/01, 2000/09/30, Universite Paris XI, LPTMS: Laboratoire de Physique Theorique et des Models Statistiques, Visiting Researcher
- 2000/10/01, 2002/09/30, Universite Paris XI, LPS: Laboratoire de Physique des Solides
- 2004/04/01, 2007/02/28, RIKEN, Condensed Matter Theory Laboratory
- 2007/03/01, 2008/03/31, University of Tokyo, Institute for Solid State Physics, Researcher
- 2002/10/01, 2003/12/31, Universitaet Karlsruhe, TFP: Institut fur Theoretische Festkoerperphysik
- 2007/07/01, 2007/07/31, Universite Aix-Marseille II / Universite de la Mediterranee, Centre de Physique Theorique (CNRS), Visiting Professor

### Educational Backgrounds

- The University of Tokyo, Graduate School, Division of Engineering, Department of Applied Physics, Japan, 1997/04, 1999/09
- The University of Tokyo, Graduate School, Division of Engineering, Department of Applied Physics, Japan, 1995/04, 1997/03
- The University of Tokyo, Faculty of Science, Department of Physics, Japan, 1991/04, 1995/03
- Kaisei Senior High School, Japan, 1988/04, 1991/03

### Academic Degrees

- Doctor of Engineering, The University of Tokyo
- Master of Engineering, The University of Tokyo

### Educational Activity

- 【Bachelor Degree Program】School of Engineering : Cluster 2(Electrical, Electronic and Systems Engineering)

### In Charge of Primary Major Programs

- Electronic Devices and Systems

### Research Fields

- Mathematical and physical sciences;Physics;Condensed matter physics I

### Research Keywords

- topological insulator
- Weyl semimetal
- Dirac electrons
- quantum transport

## Research Activities

### Academic Papers

- ★, Generalized bulk-edge correspondence for non-Hermitian topological systems, Phys. Rev. B, 100, 165430-1-165430-8, 20191031
- ★, Short Ballistic Josephson Coupling in Planar Graphene Junctions with Inhomogeneous Carrier Doping, PHYSICAL REVIEW LETTERS, 077701, 20180216
- Comparative study of Weyl semimetal and topological/Chern insulators: Thin-film point of view, PHYSICAL REVIEW B, 94(23), 20161212
- ★, Quantum phase transition in disordered topological quantum matter - boundary of topologically distinct states -, Solid State Physics, 51(10), 567, 20161015
- Dimensional crossover of transport characteristics in topological insulator nanofilms, PHYSICAL REVIEW B, 92(23), 20151203
- Manipulating quantum channels in weak topological insulator nanoarchitectures, PHYSICAL REVIEW B, 92(19), 20151123
- Engineering Dirac electrons emergent on the surface of a topological insulator, SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS, 16(1), 201502
- Noninvasive Metallic State, JPS Conf. Proc., 4, 013005, 20150115
- Characterizing weak topological properties: Berry phase point of view, PHYSICAL REVIEW B, 90(15), 20141023
- One-dimensional topological insulator: A model for studying finite-size effects in topological insulator thin films, PHYSICAL REVIEW B, 89(12), 20140324
- ★, Density of States Scaling at the Semimetal to Metal Transition in Three Dimensional Topological Insulators, PHYSICAL REVIEW LETTERS, 112(1), 20140107
- Perfectly conducting channel on the dark surface of weak topological insulators, PHYSICAL REVIEW B, 88(4), 20130703
- Unified Description of Dirac Electrons on a Curved Surface of Topological Insulators, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 82(7), 201307
- Symmetry Protected Weak Topological Phases in a Superlattice, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 82(7), 201307
- Disordered Weak and Strong Topological Insulators, PHYSICAL REVIEW LETTERS, 110(23), 20130605
- Criticality of the metal-topological insulator transition driven by disorder, PHYSICAL REVIEW B, 87(20), 20130531
- Protection of the surface states in topological insulators: Berry phase perspective, PHYSICAL REVIEW B, 87(20), 20130507
- Finite-size energy gap in weak and strong topological insulators, PHYSICAL REVIEW B, 86(24), 20121228
- Spherical topological insulator, PHYSICAL REVIEW B, 86(23), 20121213
- Majorana bound state of a Bogoliubov-de Gennes-Dirac Hamiltonian in arbitrary dimensions, NUCLEAR PHYSICS B, 854(2), 306-320, 20120111
- Dirac Electrons on a Sharply Edged Surface of Topological Insulators, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 81(9), 201209
- Quasiclassical Theory of the Josephson Effect in Ballistic Graphene Junctions, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 81(9), 201209
- Interfacial charge and spin transport in Z(2) topological insulators, PHYSICAL REVIEW B, 83(12), 20110309
- Josephson Current through a Planar Junction of Graphene, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 80(4), 201104
- Disorder-Induced Multiple Transition Involving Z(2) Topological Insulator, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 80(5), 201105
- ★, Weak topological insulator with protected gapless helical states, PHYSICAL REVIEW B, 84(3), 20110727
- Spin Berry phase in anisotropic topological insulators, PHYSICAL REVIEW B, 84(19), 20111102
- Spin Berry phase in the Fermi-arc states, PHYSICAL REVIEW B, 84(24), 20111212
- Flat edge modes of graphene and of Z2 topological insulator, Nanoscale Res Lett., 6(1), 358 (1-6), 20110401
- Flat edge modes of graphene and of Z(2) topological insulator, NANOSCALE RESEARCH LETTERS, 6, 20110421
- Zigzag edge modes in a Z(2) topological insulator: Reentrance and completely flat spectrum, PHYSICAL REVIEW B, 82(8), 20100827
- Analytic Theory of Edge Modes in Topological Insulators, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 79(12), 201012

### Publications such as books

- 2019, Advanced Topological Insulators, This book is the first pedagogical synthesis of the field of topological insulators and superconductors, one of the most exciting areas of research in condensed matter physics. Presenting the latest developments, while providing all the calculations necessary for a self-contained and complete description of the discipline, it is ideal for researchers and graduate students preparing to work in this area, and it will be an essential reference both within and outside the classroom. The book begins with the fundamental description on the topological phases of matter such as one, two- and three-dimensional topological insulators, and methods and tools for topological material's investigations, topological insulators for advanced optoelectronic devices, topological superconductors, saturable absorber and in plasmonic devices. Advanced Topological Insulators provides researchers and graduate students with the physical understanding and mathematical tools needed to embark on research in this rapidly evolving field., This book is the first pedagogical synthesis of the field of topological insulators and superconductors, one of the most exciting areas of research in condensed matter physics. Presenting the latest developments, while providing all the calculations necessary for a self-contained and complete description of the discipline, it is ideal for researchers and graduate students preparing to work in this area, and it will be an essential reference both within and outside the classroom. The book begins with the fundamental description on the topological phases of matter such as one, two- and three-dimensional topological insulators, and methods and tools for topological material's investigations, topological insulators for advanced optoelectronic devices, topological superconductors, saturable absorber and in plasmonic devices. Advanced Topological Insulators provides researchers and graduate students with the physical understanding and mathematical tools needed to embark on research in this rapidly evolving field., Topological Matter in the Absence of Translational Invariance (Chap. 4), topological Insulator, Scrivener Publishing, 2019, Scholarly Book, Joint work, 英語, Koji Kobayashi, Tomi Ohtsuki and Ken-Ichiro Imura, 9781119407294, 414, 109-158

### Invited Lecture, Oral Presentation, Poster Presentation

- 22pB101-2, JPS 73rd Annual Meeting, 2018/03/22, Without Invitation, Japanese
- Higher-order topological insulator, Ken Imura, Emergent Condensed-Matter Physics 2018 (ECMP2018), 2018/03/06, With Invitation, English, Chair: Akio Kimura (Hiroshima University, ECMP) Organizers: Takahiro Onimaru, Takeshi Matsumura, Arata Tanaka, Ken Imura, 401N, AdSM, Hiroshima University (Higashi-Hiroshima campus)
- Multipole topological insulator and pumping, T. Kiriki, K.-I. Imura, S. Hayashi, Y. Yoshimura, T. Nakanishi, Emergent Condensed-Matter Physics 2018 (ECMP2018), 2018/03/05, Without Invitation, English, Chair: Akio Kimura (Hiroshima University, ECMP) Organizers: Takahiro Onimaru, Takeshi Matsumura, Arata Tanaka, Ken Imura, AdSM, Hiroshima University (Higashi-Hiroshima campus)
- P1: “Higher-order topological insulators and pumping”, Ken Imura (Hiroshima Univ.), BEC2018 "Variety and universality of bulk-edge correspondence in topological phases: From solid state physics to transdisciplinary concepts", 2018/01/05, Without Invitation, Japanese, Organizers: Hideo Aoki (AIST/Univ. of Tokyo) Takahiro Fukui (Ibaraki Univ.) Mikio Furuta (Univl. of Tokyo) * Yasuhiro Hatsugai (Univ. of Tsukuba) Satoshi Iwamoto (Univ. of Tokyo) Tohru Kawarabayashi (Toho Univ.) Akio Kimura (Hiroshima Univ.) Yoshiro Takahashi (Kyoto Univ.) (alphabetical, * chair) Local organizers: Ken-ichiro Imura (Hiroshima Univ.) Toshikaze Kariyado (NIMS) Shuta Nakajima (Kyoto Univ.), University of Tsukuba
- "Bulk-edge correspondence in topological transport and pumping", Ken Imura (Hiroshima), Novel Quantum States in Condensed Matter 2017, 2017/10/26, With Invitation, English, Sumio Ishihara (Tohoku), Hirokazu Tsunetsugu (ISSP), Masao Ogata (Tokyo), Yukitoshi Motome (Tokyo), Takami Tohyama (TUS), Shuichi Murakami (Tokyo Tech), Yoichi Yanase (Kyoto), Masatoshi Sato (YITP, Chair), Keisuke Totsuka (YITP), Ippei Danshita (YITP), Manfred Sigrist (ETH Zurich), Leon Balents (UCSB), Yukawa Institute for Theoretical Physics, Kyoto University
- P51: Topological spin pumping: effects of disorder, adiabaticity and bulk-edge correspondence, Tatsuya Kiriki, Ken Imura, Arata Tanaka, Junjiro Kanamori Memorial International Symposium – New Horizon of Magnetism –, 2017/09/28, Without Invitation, English, Chair: Hiroshi Katayama-Yoshida (CSRN, Graduate School of Engineering, The University of Tokyo) Co-Chair: Hisazumi Akai (ISSP, The University of Tokyo), Koshiba Hall, The University of Tokyo, Tokyo, Japan, The Nobel prize of physics in 2016 has been attributed to three theorists, who introduced the idea of topology in condensed-matter physics. One of the highlighted aspects was the topological interpretation of quantum Hall effect (QHE). The concept of topological pumping is a variant of this idea, applied to a temporal evolution of the system, instead of in the (crystal) momentum space. In QHE quantization of the Hall plateaus is due to an underlying topological order, encoding the nontrivial phase property of bulk wave function in two space dimensions (2D). In topological pumping the same applies to the pumped charge or spin in the corresponding 1D system under a time-dependent potential. The original idea of this topological pumping can be found in classical literatures [1,2]. Yet, to our knowledge no clear experimental demonstration had been reported until in 2015 two experimental groups embodied this idea in the system of cold atoms [3]. Note that the original proposal was based on an electronic system. One of the issues yet to be investigated in the experimental studies is on the role of (i) disorder and (ii) spin in pumping. Here, we consider the effects of (i) disorder in the system of (ii) spin pumping. The adiabaticity and the bulk-edge correspondence (BEC) are two major ingredients indispensable in the realization of topological pumping [4,5]. We study the validity of spin pumping both in the adiabatic limit (snapshot picture) and in the standard temporal evolution scheme to reveal the role of BEC in spin pumping. We compare slightly different roles of disorder in QHE [6] and pumping both in the charge and spin versions of them. Here, we will focus on the analogy of quantum spin Hall (QSH) effect [7] and topological spin pumping. We discuss possible spintronic applications of our analysis to spin and charge pumping in Weyl and Dirac semimetal systems such as TaAs [8]. References [1] D. J. Thouless, Phys. Rev. B 27, 6083 (1983). [2] R. B. Laughlin, Phys. Rev. B 23, 5632 (1981). [3] S. Nakajima, et al., Nature Physics 12, 296; ibid., M. Lohse et al., 350 (2016). [4] Y. Hatsugai and T. Fukui, Phys. Rev. B 94, 041102 (2016). [5] K.-I. Imura, Y. Yoshimura, T. Fukui and Y. Hatsugai, arXiv:1706.04493. [6] T. Ohtsuki and Y. Ono, J. Phys. Soc. Jpn. 58, 956 (1989); T. Ando and H. Aoki, Physica B 184, 365 (1993). [7] M. Koenig, et al., Science 318, 766 (2007). [8] S.-Y. Su et al., Science 349, 628 (2016).
- P.721: Bulk-edge correspondence in topological transport and pumping, Imura K.-I., Yoshimura Y., Fukui T., Hatsugai Y., LT28: 28th International Conference on Low Temperature Physics, 2017/08/14, Without Invitation, English, Topologically nontrivial phases show protected surface or edge states. The existence of such surface states and the way how they appear is indeed uniquely determined by the bulk topological numbers. The bulk-edge correspondence (BEC) [1] refers to this one-to-one relation. Depending on the system in question, BEC manifests in different forms and govern the spectral and transport properties of topological insulators and semimetals. We have previously focused on the stability of surface states against lattice imperfections [2], cases of weak topological phases [3], and of Weyl semimetal thin films [4]. Quantization of pumped charge and spin is another manifestation of the nontrivial topological properties in the bulk. To quantify topological pumping, time evolution of the initial ground state is to be considered, but it is also useful to analyze “snapshots” of the system [5,6]. Here, using the prescription of Ref. [6], we study the robustness of topological pumping against (on-site) disorder. FIG. 1 shows time dependence of the system’s polarization, indicating that the pumped charge is unchanged in spite of the irregularities due to disorder. The snapshot picture reveals the role of edge states in topological pumping, providing also an interesting twist on the BEC picture. [1] Y. Hatsugai, Phys. Rev. Lett. 71, 3697 (1993). [2] K.-I. Imura and Y. Takane, Phys. Rev. B 87, 205409 (2013). [3] Y. Yoshimura, K.-I. Imura, T. Fukui and Y. Hatsugai, Phys. Rev. B 90, 155443 (2014). [4] Y. Yoshimura, W. Onishi, K. Kobayashi, T. Ohtsuki and K.-I. Imura, Phys. Rev. B. 94, 235414 (2016). [5] D. J. Thouless, Phys. Rev. B 27, 6083 (1983). [6] Y. Hatsugai and T. Fukui, Phys. Rev. B 94, 041102 (2016).
- Revisiting the Z2 properties of a Z2 topological insulator, Ken Imura, BEC4, 2016/03/25, With Invitation, Japanese, Yasuhiro Hatsugai
- From topological insulators to density-of-state scaling, Ken-Ichiro Imura, 2016/03/04, With Invitation, Japanese, CMSI, ISSP
- Chiral edge channels in weak and strong Chern insulators, Yukinori Yoshimura, Ken-Ichiro Imura, Takahiro Fukui and Yasuhiro Hatsugai, International Research Symposium on Chiral Magnetism, 2015/12/07, Without Invitation, English, JSPS Grant-in-Aid forScientific Research (S) (No. 25220803) "Toward a New Class Magnetism by Chemically-controled Chirality" + Hiroshima University "Center for Chiral Science”, Aster Plaza, Hiroshima, Japan, We report our recent study [1] on the new classification schemes for characterizing the so-called “weak” (*) Chern insulating (WCI) phases in two spatial dimensions that appear in different variations of the standard Wilson-Dirac model. Those variations that bear WCI phases include models with (i) anisotropic Wilson terms, (ii) next nearest neighbor hopping terms in the tight-binding implementation, and also (iii) its superlattice generalization [2]. Here, we show that for types (i) and (ii) a prescription introduced in Refs. [3] for classifying strong properties can be successfully generalized so as to classify the weak properties. As for type (iii), though a matrix nature that arises from the superlattice structure is antithetical to the use of the above prescription, we are still able to attribute its weak properties to a quantized Berry phase along a Wilson loop. The latter suggests that an even number of, i.e., two Dirac (crossing) points that appear in the edge spectrum of such WCI phases are topologically protected. In the figures below, we show the typical behavior of such a Wilson loop exponent in the SCI and WCI phases. (*) to be contrasted with the strong Chern insulator (SCI) [1] Y. Yoshimura, K.-I. Imura, T. Fukui and Y. Hatsugai, “Characterizing weak topological properties: Berry phase point of view”, Phys. Rev. B 90, 155443 (2014). [2] T. Fukui, K.-I. Imura and Y. Hatsugai, “Symmetry protected weak topological phases in a superlattice,” J. Phys. Soc. Jpn. 82 (2013) 073708. [3] Y. Hatsugai and S. Ryu, “Topological quantum phase transitions in superconductivity on lattices,” Phys. Rev. B 65, 212510 (2002).
- Thin film topological insulators: bulk-edge correspondence, dimensional crossover and superlattice generalization, K.-I. Imura, Physics of bulk-edge correspondence and its universality: from solid state physics to cold atoms; International workshop 2015, 2015/09/28, With Invitation, English, Yasuhiro Hatsugai (Univ. of Tsukuba), Univ. of Tsukuba, Bunko School Building, Tokyo Campus, 3-29-1 Otsuka, Bunkyo-ku, 112-0012 Tokyo
- P12: Dimensional crossover of topological properties: case of topological superlattice insulator thin films, Y. Yoshimura (Hiroshima Univ.), Physics of bulk-edge correspondence and its universality: from solid state physics to cold atoms; International workshop 2015, 2015/09/27, Without Invitation, English, Yasuhiro Hatsugai (Univ. of Tsukuba), Univ. of Tsukuba, Bunko School Building, Tokyo Campus, 3-29-1 Otsuka, Bunkyo-ku, 112-0012 Tokyo
- P11: Density of states scaling in Weyl/Dirac semimetals, K. Kobayashi (Sophia Univ.), Physics of bulk-edge correspondence and its universality: from solid state physics to cold atoms; International workshop 2015, 2015/09/27, Without Invitation, English, Yasuhiro Hatsugai (Univ. of Tsukuba), Univ. of Tsukuba, Bunko School Building, Tokyo Campus, 3-29-1 Otsuka, Bunkyo-ku, 112-0012 Tokyo
- Properties of the critical point in disordered Weyl/Dirac semimetals, K. Kobayashi, T. Ohtsuki, K.-I. Imura, and K. Nomura, 2015/09/19, Without Invitation, Japanese
- Dimensional crossover of topological features in topological insulator superlattice, Y. Yoshimura, K.-I. Imura, 2015/09/18, Without Invitation, Japanese
- Delocalization in disordered and thin-film topological insulators, Ken Imura, Delocalisation Transitions in Disordered Systems, 2015/07/30, With Invitation, English, Asia Pacific Center for Theoretical Physics, POSTECH, Systems of a topological insulator, in the presence of disorder, show multiple facets in the study of localization-delocalization transitions in disordered systems. A single Dirac cone on the surface of a strong topological insulator is immune to disorder and tends to be ultimately delocalized. Twinning of such Dirac cones in case of the weak topological insulator makes the system more fragile to disorder. Taking account of the effects of constrained geometry, i.e., those of the finite size, introduces yet another aspects into the study of delocalization transitions in topological insulator systems. In thin film geometries the weak topological insulator can be regarded as stacked layers of a quantum spin Hall system with helical edge modes. Such a system exhibits successive localization-delocalization transitions as the number of stacked layers is varied. Disorder sometimes turns an ordinary insulator to a topological insulator: case of the disorder-induced topological insulator. Geometrical constriction, e.g., imposing the system into a thin-film geometry, sometimes leads to a similar consequence. Here, we will discuss combined effects of such disorder and geometrical constriction in the delocalization transitions in topological insulator systems.
- Th-PE-102: Universal Critical Conductance Distributions in Disordered 2D Topological Insulators, K. Kobayashi, H. Ikumi, M. Wada, K.-I. Imura and T. Ohtsuki, EP2DS-21, 2015/07/30, Without Invitation, Japanese, Shingo Katsumoto (University of Tokyo), Koji Muraki (NTT BRL), Junsaku Nitta (Tohoku University), Sendai International Center, Conductance distributions at the phase boundaries: metal-insulator or insulator-insulator (but between topologically different insulators), contain much information on the nature of quantum critical point. Conventionally, the critical points are classified according to the symmetry of the random system, and the critical points in the same class share quantitatively the same properties. Those critical properties are expected to be universal (scale-free and detail independent), and become a good measure of the phase transition. Recently, it was revealed that the insulators are classified topologically into Z, Z2, and trivial, and there are variety of phase transitions even in the same symmetry class. We have shown previously [1] that the conductance distribution at the metal-quantum spin Hall insulator transition is different from that of the ordinary metal-insulator transition; it is affected by the presence or absence of edge state on the insulating side. Yet, in this work, we show that the critical conductance distribution is otherwise universal, i.e., it is uniquely determined, once the symmetry class and the topological number of the adjacent insulating phase(s) are specified. We have investigated the conductance distributions numerically with the transfer matrix method. We have employed Dirac-type tight-binding Hamiltonians, and found that the obtained critical conductance distributions coincide with those of network model (see Fig. 1). We note that the shape of distribution is clearly distinguishable from those of different symmetry class or topological number. We also show that in the 3D topological insulator nanofilms [2], the universal critical distribution is reproduced when the number of layers are small enough, and the shape is changed by reducing the number of layers reflecting the change of symmetry class; that means the conductance distributions are independent of the origin of the topological order (band inversion, magnetic field etc.). Other interesting critical properties, such as spin transport and multifractality of the critical points, will be also mentioned. References [1] K. Kobayashi, T. Ohtsuki, H. Obuse, and K. Slevin, Phys. Rev. B 82, 165301 (2010). [2] K. Kobayashi, Y. Yoshimura, K.-I. Imura, and T. Ohtsuki, arXiv:1409.1707 (2014).
- Mo-PE-106: Emergent conductive edge and surface states in topological insulator thin films, Yukinori Yoshimura, Koji Kobayashi, Ken-Ichiro Imura and Tomi Ohtsuki, EP2DS-17, 2015/07/27, Without Invitation, English, Shingo Katsumoto (University of Tokyo), Koji Muraki (NTT BRL), Junsaku Nitta (Tohoku University), Sendai International Center, Recently, much effort has been made to grow thin films of a topological insulator [1, 2]. Naturally, its primary purpose was to reduce the contribution of the bulk to transport quantities. In this work, we have performed a theoretical study of such a topological insulator (TI) thin film [3]. The thin film geometry allows for physically interpolating the two and three dimensions (2D and 3D) limits by changing the number of stacked layers (N) [4, 5]. We consider a standard example of AII symmetry class; a TI can occur both in 2D and 3D. We note that for these two cases, the Z2 topological numbers specify the TI phases. In 2D the TI phase is characterized by a single Z2 index (ν0), while in 3D there are in principle 16 different topological phases characterized by four distinct Z2 indices: one strong (ν0) and three weak (ν1, ν2, ν3). One way to characterize topological properties of a thin film is to consider 2D type Z2 index by regarding the film as an effective 2D system. We adapt Fu-Kane formula [6] to calculate the Z2 index of the system, and establish Z2 index maps by calculating the index (ν0) as a function of the number of stacked layers N and gap parameter m0 (see Fig. 1). We also perform numerical study of the conductance of TI thin films. As a results, 1. We have shown that topological features absent in the two limits can be “emergent” in the thin film geometry of a finite number of stacked layers (see Fig. 1). 2. Through numerical study of the conductance of TI thin films, we have revealed how the 2D topological character evolves to its 3D counterpart as the number of stacked layers is increased (see the “conductance maps” in Ref. [3]). ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼References Fig. 1. The Z2 topological index map. Red (white) regions represent ν0 = 1(ν0 = 0). [1] Y. Zhang et al., Nature Physics 6, 584 (2010). [2] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. Lett. 109, 066803 (2012). [3] K. Kobayashi, K.-I. Imura, Y. Yoshimura, and T. Ohtsuki,“ Dimensional crossover of transport charac- teristics in topological insulator nanofilms, ”arXiv:1409.1707. [4] W.-Y. Shan, H.-Z. Lu, and S.-Q. Shen, New J. Phys. 12, 043048 (2010). [5] K. Ebihara, K. Yada, A. Yamakage, and Y. Tanaka, Physica E: low-dimensional Systems and Nanostruc- tures 44, 885 (2012). [6] L. Fu and C. L. Kane, Phys. Rev. B 76, 045302 (2007).
- P26: Density of States Scaling in disordered Dirac and Weyl semimetals, Imura, Ken- Ichiro, Koji Kobayashi, Tomi Ohtsuki and Igor Herbut, Topological & Correlated Matter, 2015/06/29, Without Invitation, English, Gordon Research Conference, The Hong Kong University of Science and Technology Hong Kong, China
- Mesoscopic topological insulator, Ken Imura, New Perspectives in Spintronic and Mesoscopic Physics Jun 1-19, 2015, Kashiwa, Japan, 2015/06/05, With Invitation, Japanese, Takeo Kato, Yoshichika Otani, Institute for Solid State Physics, University of Tokyo, The workshop aims at harnessing two differently evolving but closely related sub-disciplines of condensed matter physics, i.e. mesoscopic and spintronic physics. Mesoscopic physics has been an active research field since the 1980s, and has enabled us to elucidate the quantum mechanical nature of electrons by utilizing modern nano-fabrication technologies for semiconductors. Recent theoretical and experimental developments have deepened insight on mesoscopic systems, and also strengthened relationship with other research fields such as nonequilibrium statistical mechanics, quantum information, many-body quantum theory, fundamental theory of quantum mechanics, and so on. Physics of spintronics has also been evolving since the discovery of giant magnetoresistance in 1988. It now covers all the spin-related phenomena such as pure spin currents, spin injection, spin transfer torque, spin Hall effect, and so on. The spintronic physics is not only a basis of practical technologies, but also provides a variety of fundamental concepts relevant to mesoscopic physics such as spin diffusion, spin currents, Berry phases, and phenomena originated from spin-orbit interaction. The goal of this workshop is to address important common future issues for both spintronic and mesoscopic physics by sharing recent theoretical and experimental developments in these research fields and to pave a way towards breakthrough in the interdisciplinary research area. The first week (Jun 1-5) focuses on recent developments of the following research topics; nonequilibrium properties, quantum Hall effects, quantum dots, dynamics, their application to quantum information, transport in a single atomic-layer systems such as graphene, and topological matter, circuit QED systems, Andreev scattering, many-body effect (Kondo effect), and heat transport. Spin related research topics will be chosen to stimulate discussion among researchers in both spintronic and mesoscopic physics. The second week (Jun 8-12) focuses on interdisciplinary topics. Two days (Jun 8,9) are devoted to seminars on spin related phenomena originated from spin-orbit interaction. The other three days are reserved for the international symposium (Jun 10-12). Key foreign and domestic researchers are invited to discuss various experimental and theoretical topics to find interesting common problems in this emergent interdisciplinary area and to grope for new direction of future study. For discussion from broader viewpoints, the symposium also treats related subjects such as oxide interfaces, NV center, surface transport and so on. The final week (Jun 15-19) focuses on spintronic physics. Recent developments of spin pumping dynamics, spin accumulation, spin-charge transformation, spin Hall effect, magnon transport, Rashba interaction, and nanomanets are discussed. In addition to phenomenology and theoretical studies of effective models, first-principles calculation of spin-related quantities is treated. Discussion with researchers in mesoscopic physics will be promoted by picking out fundamental research topics closely related to electron coherence. http://www.issp.u-tokyo.ac.jp/public/npsmp2015/
- PS-32: Emergent quantum spin Hall system in topological insulator nanofilms, Yukinori Yoshimura, Koji Kobayashi, Ken-Ichiro Imura and Tomi Ohtsuki, New Perspectives in Spintronic and Mesoscopic Physics, Kashiwa, Jun. 2015, 2015/06, Without Invitation, English
- Conductance distribution functions in disordered topological insulator nanofilms, K. Kobayashi, T. Ohtsuki, K.-I. Imura, 2015/03/21, Without Invitation, Japanese
- Emergent topological properties in topological insulator thin films, Y. Yoshimura, K. Kobayashi, K.-I. Imura, and T. Ohtsuki, Topotronics2015: The 1st International Workshop on the Topological Electronics, 2015/03/10, Without Invitation, English, Prof. Shinobu Hikami (OIST, Mathematical and Theoretical Physics Unit), OIST, Okinawa, Japan, Recently, much effort has been made to grow thin films of a topological insulator [1-2]. Naturally, its primary purpose was to reduce the contribution of the bulk to transport quantities. In this work, we have performed a theoretical study of such a topological insulator thin film [3]. In the presence of time reversal symmetry, we consider a standard example of AII symmetry class; a topological insulator (TI) can occur both in two and three spatial dimensions (2D and 3D). We note that for these two cases, construction of the Z2 topological numbers specifying the TI phases. In 2D the TI phase is characterized by a single Z2 index (ν0), while in 3D there are in principle 16 different topological phases characterized by four distinct Z2 indices: one strong and three weak. The thin film geometry allows for physically interpolating the 2D and 3D limits by changing the number of stacked layers [4-5]. In this work, 1) We show that topological features absent in the two limits can be “emergent” in the thin film geometry of a finite number of stacked layers. This has been done by focusing on the 2D type topological index (ν0) adapted for a thin film (see Fig. 1). 2) Through numerical study of the conductance of TI thin films, we have revealed how the 2D topological character evolves to its 3D counterpart as the number of stacked layers is increased (see the “conductance maps” in Ref. [3]). [1] Y. Zhang et al., Nature Physics 6, 584 (2010). [2] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. Lett. 109, 066803 (2012). [3] K. Kobayashi, K.-I. Imura, Y. Yoshimura, and T. Ohtsuki, “Dimensional crossover of transport characteristics in topological insulator nanofilms,” arXiv:1409.1707. [4] W.-Y. Shan, H.-Z. Lu, and S.-Q. Shen, New J. Phys. 12, 043048 (2010). [5] K. Ebihara, K. Yada, A. Yamakage, and Y. Tanaka, Physica E: low-dimensional Systems and Nanostructures 44, 885 (2012).
- S10.00005 : Dimensional crossover of transport characteristics in topological insulator nanofilms, Ken-Ichiro Imura, Yukinori Yoshimura, Koji Kobayashi, Tomi Ohtsuki, APS March Meeting 2015, 2015/03/05, Without Invitation, English, American Physical Society, Henry B.Gonzalez Convention Center, San Antonio, Texas, Recently, much effort has been made to grow thin films of a topological insulator. Naturally, its primary purpose was to reduce the contribution of the bulk to transport quantities. Here, we propose that searching for quantized transport in such TI thin films is an efficient way for probing non-trivial topological features encoded in the 3D bulk band structure. In a recent work (Kobayashi, KI, Yoshimura & Ohtsuki, arXiv:1409.1707), we have highlighted the following issues: 1) Transport characteristics of TI thin films is well understood by studying the conductance both in the edge and slab geometries. 2) We introduce ``conductance maps'' for revealing the dimensional crossover in such TI thin films. Quantization of the conductance occurs both in the edge and in the slab geometries, but not at the same time. 3) We focus on the even-odd feature in transport with respect to the number of stacked layers. We found parameter regimes in which the even-odd feature is broken by inversion of the finite-size gap associated with hybridization of the top and bottom surface wave functions. We propose that tuning the hybridization gap of a TI thin film and make it inverted is an effective way of realizing a 2D quantum spin Hall state.
- Q10.00015 : Density of states scaling at the semimetal to metal transition in three dimensional topological insulators, Igor Herbut (Simon Fraser University), Ken Imura (Hiroshima University) Tomi Ohtsuki (Sophia University) Koji Kobayashi (Sophia University), APS March Meeting 2015, 2015/03/04, Without Invitation, English, American Physical Society, Henry B.Gonzalez Convention Center, San Antonio, Texas, The quantum phase transition between the three dimensional Dirac semimetal and the diffusive metal can be induced by increasing disorder. Taking the system of disordered Z2 topological insulator as an important example, we compute the single particle density of states by the kernel polynomial method. We focus on three regions: the Dirac semimetal at the phase boundary between two topologically distinct phases, the tricritical point of the two topological insulator phases and the diffusive metal, and the diffusive metal lying at strong disorder. The density of states obeys a novel single parameter scaling, collapsing onto two branches of a universal scaling function, which correspond to the Dirac semimetal and the diffusive metal. The diverging length scale critical exponent and the dynamical critical exponent are estimated, and found to differ significantly from those for the conventional Anderson transition. Critical behavior of experimentally observable quantities near and at the tricritical point is also discussed. (K. Kobayashi et al, Phys. Rev. Lett. vol. 112, 016402 (2014))
- Q7.00003 : Phase diagrams of disordered 3D topological insulators and superconductors, Tomi Ohtsuki (Sophia University), Koji Kobayashi (Sophia Univeristy) Ken-Ichiro Imura (Hiroshima University) Ken Nomura (Tohoku University), APS March Meeting 2015, 2015/03/04, Without Invitation, English, American Physical Society, Henry B.Gonzalez Convention Center, San Antonio, Texas, A global phase diagram of disordered weak and strong topological insulators belonging to the class AII is obtained by numerically calculating the conductance, the Lyapunov exponents and the density of states. The location of the phase boundaries, i.e., the mass parameter, is renormalized by disorder, a feature recognized in the study of topological Anderson insulator. We report quantized conductance on the phase boundaries between topologically distinct phases, which is interpreted as the robustness of conductance against disorder. This robustness is also confirmed by the large-scale numerical calculation of the density of states, which remains parabolic up to certain strength of disorder with renormalized Dirac electron velocity. From the size dependence of the conductance, we also point out that the surface states of weak topological insulator are either robust or ''defeated''. The nature of the two distinct types of behavior is further revealed by studying the Lyapunov exponents. (K. Kobayashi et al., Phys. Rev. Lett. vol. 110, 236803 (2013)). We also obtain the phase diagram of disordered topological superconductors belonging to the class DIII. Similar renormalization of mass and velocity due to disorder is found.
- PB-17: Dimensional crossover of transport characteristics in topological insulator nanofilms, K.-I. Imura, K. Kobayashi, Y. Yoshimura and T. Ohtsuki, International Conference on Topological Quantum Phenomena 2014, 2014/12/18, Without Invitation, English, Innovative Research Area: "Topological Quantum Phenomena in Condensed Matter with Broken Symmetries", Kyoto University, Kyoto, A single Dirac cone emergent on the surface of a topological insulator (TI) is probed by surface sensitive measurements such as ARPES and STM, and serves as a smoking gun for making a distinction between topological and ordinary band insulators. Yet, an independent proof that the system is indeed topologically non-trivial should be given by a transport measurement, which is so far unsuccessful due to difficulty of separating bulk and surface contributions. Recently, much effort has been made to grow thin films of a topological insulator [1-3]. Naturally, its primary purpose was to reduce the contribution of the bulk to transport quantities. Here, we propose that searching for quantized transport in such TI thin films (see FIG. 1) is an effective way to probe the non-trivial topological feature encoded in the 3D bulk band structure. In this work [4], the following issues are highlighted: 1) Transport characteristics of TI thin films is well understood by studying the conductance both in the edge and slab geometries (FIG. 1, FIG. 2). 2) We introduce “conductance maps” (FIG. 2) for revealing the dimensional crossover in such TI thin films. Quantization of the conductance occurs both in the edge and in the slab geometries, but not at the same time. We demonstrate that such quantization regimes in the edge and slab geometries are both exclusive and complementary. 3) We focus on the even-odd feature in transport with respect to the number of stacked layers. We found parameter regimes in which the even-odd feature is broken by inversion of the finite-size gap associated with hybridization of the top and bottom surface wave functions. We propose that tuning the hybridization gap of a TI thin film and make it inverted is an effective way of realizing a 2D quantum spin Hall state. [1] Y. Zhang et al., Nature Physics 6, 584 (2010). [2] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. Lett. 109, 066803 (2012). [3] A. A. Taskin, F. Yang, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. B 89, 121302 (2014). [4] K. Kobayashi, K.-I. Imura, Y. Yoshimura and T. Ohtsuki, “Dimensional crossover of transport characteristics in topological insulator nanofilms,” arXiv:1409.1707.
- Dimensional Crossover of Helical Transport in Topological Insulator Nanofilms, K. Kobayashi, K.-I. Imura, Y. Yoshimura, T. Ohtsuki, International Research Symposium on Chiral Magnetism, 2014/12/07, Without Invitation, English, JSPS Grant-in-Aid forScientific Research (S) (No. 25220803) "Toward a New Class Magnetism by Chemically-controled Chirality" + Hiroshima University "Center for Chiral Science”, Aster Plaza, Hiroshima
- 8pAX-9: Transport properties and its layer number dependence of disordered topological insulator thin films, K. Kobayashi, T. Ohtsuki, K.-I. Imura, 2014/09/08, Without Invitation, Japanese
- Characterization of two-dimensional weak topological phases, Yukinori Yoshimura, Ken Imura, Takahiro Fukui, Yasuhiro Hatsugai, Les Houches School of Physics, session CIII “Topological aspects of condensed matter physics”, 2014/08/15, Without Invitation, English, Ecole de Physique des Houches, Ecole de Physique des Houches, Les Houches, Haute-Savoie, France
- Density of state scaling in the disordered three-dimensional topological insulators and superconductors, K. Kobayashi, T. Ohtsuki, K.-I. Imura, I. F. Herbut, K. Nomura, International Workshop "Quantum Disordered Systems: What's Next?", 2014/06/25, Without Invitation, English, IRSAMC, Univ. Toulouse, Institut de Mathématiques de Toulouse, Toulouse, France
- Protection of the surface states in topological insulators: Berry phase perspective, Ken-Ichiro Imura, Yositake Takane, IXth Rencontres du Vietnam, Nanophysics: from fundamentals to applications (the return), 2013/08/09, With Invitation, English, Rencontres du Vietnam, Rencontres du Vietnam, Quy Nhon, Vietnam, Topological insulator is a newly established exotic state of matter that has been intensively discussed in the field of condensed-matter nanophysics since the last couple of years. Though it is undistinguishable from ordinary band insulators in the bulk in the sense it has a gapped spectrum with its Fermi energy lying in that gap, on the surface it exhibits a protected gapless state, i.e., it behaves like a metal on its surface. The central issue that has been discussed so far was on the existence of such a metallic surface state protected by the topological non-triviality of the gapped bulk band structure. Here, we attempt to clarify the remaining question, ``why does such a gapless state appear only on the surface?'' In the lattice implementation of a topological insulator, one may be able to regard, e.g., an atomic-scale isolated closed object, like a cubic bubble in the bulk, or an atomic-scale rectangular-prism-shaped hole also as a surface. However, we know from (numerical) experiments that the protected surface state appears only on its macroscopic surfaces even in the case of sparse lattice systems, exhibiting no symptom of penetrating into the bulk. Why is the surface state noninvasive into the bulk? What prevents it from penetrating into the sparsely filled interior of the lattice models? We will argue that this is a consequence of the Berry phase pi associated with the spin connection characteristic to the topological insulator surface states.

### Awards

- 2015/01/08, Highly Cited Paper, Web of Science, Density of States Scaling at the Semimetal to Metal Transition in Three Dimensional Topological Insulators
- 2013/04/26, 2012 Highliy Cited Article, The Physical Society of Japan Editor-in-Chief

## Social Activities

### History as Peer Reviews of Academic Papers

- 2015, Topological Quantum Matter, Chief editor
- 2015, Nature Physics, Others, Reviewer, 1
- 2015, Nature Communications, Others, Reviewer, 2
- 2015, Physical Review X, Others, Referee, 1
- 2015, Physical Review Letters, Others, Referee, 2
- 2015, Physical Review B, Others, Referee, 3
- 2015, New Journal of Physics, Others, Reviewer, 1
- 2015, Scientific Reports (Nature Publishing Group), Others, Reviewer, 1
- 2014, Topological Quantum Matter, Chief editor
- 2014, Nature Physics, Others, Reviewer, 1
- 2014, Physical Review Letters, Others, Referee, 2
- 2014, Physical Review B, Others, Referee, 7
- 2014, New Journal of Physics, Others, Reviewer, 2
- 2014, Journal of the Physical Society of Japan, Others, Referee, 3
- 2014, Scientific Reports (Nature Publishing Group), Others, Reviewer, 1
- 2013, Physical Review Letters, Others, Referee, 2
- 2013, Physical Review B, Others, Referee, 8
- 2013, New Journal of Physics, Others, Reviewer, 1
- 2013, Journal of the Physical Society of Japan, Others, Referee, 2