研究者業績

大槻 東巳

オオツキ トウミ  (Ohtsuki Tomi)

基本情報

所属
上智大学 理工学部機能創造理工学科 教授
(兼任)応用データサイエンス学位プログラム運営委員長
学位
理学士(東京大学)
理学修士(東京大学)
理学博士(東京大学)

研究者番号
50201976
J-GLOBAL ID
200901008827204586
researchmap会員ID
1000082891

外部リンク

量子ホール効果に関する研究で東京大学で理学博士を取る。博士研究員としてドイツ滞在中に,Anderson転移に関する数値スケーリング法について学ぶ。これを応用し,準位統計のスケーリングを大阪大学,東邦大学で行う。上智大学に来てからは大規模数値計算による,局在長,2端子コンダクタンスの研究を行っている。最近はトポロジカル絶縁体・超伝導体の研究を主に行っている。2016年以降,物性物理学における機械学習の応用をテーマに研究を続けている。

教育;力学,電磁気学,線形代数,統計力学,固体物理の講義を行ってきた。大学院ではナノスケールの物理や相転移の物理を教えている。
研究;不規則電子系における量子輸送現象を研究している。特にAnderson転移の性質を大規模数値計算で調べてきた。最近はスピンが絡んだ輸送現象の研究も行っている。

(研究テーマ)
アンダーソン転移の数値的研究
スピンに関連した量子輸送現象の研究

物性物理学における機械学習の応用


論文

 205
  • Yukinori Yoshimura, Koji Kobayashi, Tomi Ohtsuki, Ken-Ichiro Imura
    SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 16(1) 14403 2015年2月  査読有り招待有り
    The concept of the topological insulator (TI) has introduced a new point of view to condensedmatter physics, relating a priori unrelated subfields such as quantum (spin, anomalous) Hall effects, spin-orbit coupled materials, some classes of nodal superconductors, superfluid He-3, etc. From a technological point of view, TIs are expected to serve as platforms for realizing dissipationless transport in a non-superconducting context. The TI exhibits a gapless surface state with a characteristic conic dispersion (a surface Dirac cone). Here, we review peculiar finite-size effects applicable to such surface states in TI nanostructures. We highlight the specific electronic properties of TI nanowires and nanoparticles, and in this context we contrast the cases of weak and strong TIs. We study the robustness of the surface and the bulk of TIs against disorder, addressing the physics of Dirac and Weyl semimetals as a new research perspective in the field.
  • 小林 浩二, 大槻 東巳, 井村 健一郎, 野村 健太郎
    日本物理学会講演概要集 70 1137-1137 2015年  
  • 劉 尚, 大槻 東巳, 進藤 龍一
    日本物理学会講演概要集 70 1142-1142 2015年  
  • 和田 未来, 小林 浩二, 大槻 東巳
    日本物理学会講演概要集 70 1105-1105 2015年  
  • Takayuki Goto, Kazuki Matsui, Tadashi Adachi, Tomi Ohtsuki, Ngoc Han Tu, Yoichi Tanabe, Katsumi Tanigaki, Isao Watanabe, Zaher Salman, Andreas Suter, Thomas Prokscha
    20TH INTERNATIONAL CONFERENCE ON MAGNETISM, ICM 2015 75 100-105 2015年  査読有り
    A new method utilizing the low energy muon spin rotation (mu SR) technique to investigate the spin structure within the surface layer of topological insulator has been proposed. In order to detect the spin polarization parallel with the surface, one applies a weak field B-par, which, by breaking the time reversal symmetry, is expected to induce a net magnetization perpendicular the surface. This will affect the profile of muon spin depolarization spectra. Preliminary measurements on the topological insulator Bi1.5Sb0.5TeSe2 with tetradymite structure are also shown.
  • 伊久美 隼人, 小林 浩二, 大槻 東巳
    日本物理学会講演概要集 70 1396-1396 2015年  
  • 吉村 幸徳, 小林 浩二, 大槻 東巳, 井村 健一郎
    日本物理学会講演概要集 70 1271-1271 2015年  
  • 和田 未来, 小林 浩二, 大槻 東巳
    日本物理学会講演概要集 70 1276-1276 2015年  
  • 小林 浩二, 大槻 東巳, 井村 健一郎
    日本物理学会講演概要集 70 1275-1275 2015年  
  • Masaru Sakai, Yuta Inose, Tomi Ohtsuki, Kazuhiro Ema, Katsumi Kishino, Toshiharu Saiki
    JAPANESE JOURNAL OF APPLIED PHYSICS 53(3) 030301 2014年3月  査読有り
    The first near-field optical imaging of light localization in a GaN nanocolumn system was performed. The sample used was a randomly arranged GaN nanocolumn with high aspect ratio. We attached an InGaN single quantum well at the apex of each GaN nanocolumn as an illuminant antenna and observed luminescence from the illuminant using an aperture-type scanning near-field optical microscope. By this technique, we directly obtained optical images of luminescence and its spatial distribution for the GaN nanocolumn system. These images, along with histogram analysis, excitation wavelength dependence, and numerical calculations, offer evidence of Anderson localization of light. (C) 2014 The Japan Society of Applied Physics
  • Koji Kobayashi, Tomi Ohtsuki, Ken-Ichiro Imura, Igor F. Herbut
    PHYSICAL REVIEW LETTERS 112(1) 016402-016402 2014年1月  査読有り
    The quantum phase transition between the three dimensional Dirac semimetal and the diffusive metal can be induced by increasing disorder. Taking the system of a disordered Z(2) 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 nu and the dynamical critical exponent z 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.
  • Keith Slevin, Tomi Ohtsuki
    NEW JOURNAL OF PHYSICS 16(1) 015012-015012 2014年1月  査読有り招待有り
    We report a careful finite size scaling study of the metal-insulator transition in Anderson's model of localization. We focus on the estimation of the critical exponent v that describes the divergence of the localization length. We verify the universality of this critical exponent for three different distributions of the random potential: box, normal and Cauchy. Our results for the critical exponent are consistent with the measured values obtained in experiments on the dynamical localization transition in the quantum kicked rotor realized in a cold atomic gas.
  • Sadashige Matsuo, Kensaku Chida, Daichi Chiba, Teruo Ono, Keith Slevin, Kensuke Kobayashi, Tomi Ohtsuki, Cui-Zu Chang, Ke He, Xu-Cun Ma, Qi-Kun Xue
    Physical Review B - Condensed Matter and Materials Physics 88(15) 155438-1-245438-6 2013年10月28日  査読有り
    We report on conductance fluctuation in quasi-one-dimensional wires made of epitaxial Bi2Se3 thin film. We found that this type of fluctuation decreases as the wire length becomes longer and that the amplitude of the fluctuation is well scaled to the coherence, thermal diffusion, and wire lengths, as predicted by conventional universal conductance fluctuation (UCF) theory. Additionally, the amplitude of the fluctuation can be understood to be equivalent to the UCF amplitude of a system with strong spin-orbit interaction and no time-reversal symmetry. These results indicate that the conductance fluctuation in Bi2Se3 wires is explainable through UCF theory. This work verifies the scaling relationship of UCF in a system with strong spin-orbit interaction. © 2013 American Physical Society.
  • Sadashige Matsuo, Kensaku Chida, Daichi Chiba, Teruo Ono, Keith Slevin, Kensuke Kobayashi, Tomi Ohtsuki, Cui-Zu Chang, Ke He, Xu-Cun Ma, Qi-Kun Xue
    PHYSICAL REVIEW B 88(15) 2013年10月  査読有り
    We report on conductance fluctuation in quasi-one-dimensional wires made of epitaxial Bi2Se3 thin film. We found that this type of fluctuation decreases as the wire length becomes longer and that the amplitude of the fluctuation is well scaled to the coherence, thermal diffusion, and wire lengths, as predicted by conventional universal conductance fluctuation (UCF) theory. Additionally, the amplitude of the fluctuation can be understood to be equivalent to the UCF amplitude of a system with strong spin-orbit interaction and no time-reversal symmetry. These results indicate that the conductance fluctuation in Bi2Se3 wires is explainable through UCF theory. This work verifies the scaling relationship of UCF in a system with strong spin-orbit interaction.
  • Koji Kobayashi, Tomi Ohtsuki, Ken-Ichiro Imura
    Physical Review Letters 110(23) 236803 2013年6月5日  
    A global phase diagram of disordered weak and strong topological insulators is established numerically. As expected, the location of the phase boundaries is renormalized by disorder, a feature recognized in the study of the so-called topological Anderson insulator. Here, we report unexpected quantization, i.e., robustness against disorder of the conductance peaks on these phase boundaries. Another highlight of the work is on the emergence of two subregions in the weak topological insulator phase under disorder. According to the size dependence of the conductance, the surface states are either robust or "defeated" in the two subregions. The nature of the two distinct types of behavior is further revealed by studying the Lyapunov exponents. © 2013 American Physical Society.
  • Koji Kobayashi, Tomi Ohtsuki, Ken-Ichiro Imura
    PHYSICAL REVIEW LETTERS 110(23) 2013年6月  査読有り
    A global phase diagram of disordered weak and strong topological insulators is established numerically. As expected, the location of the phase boundaries is renormalized by disorder, a feature recognized in the study of the so-called topological Anderson insulator. Here, we report unexpected quantization, i.e., robustness against disorder of the conductance peaks on these phase boundaries. Another highlight of the work is on the emergence of two subregions in the weak topological insulator phase under disorder. According to the size dependence of the conductance, the surface states are either robust or "defeated" in the two subregions. The nature of the two distinct types of behavior is further revealed by studying the Lyapunov exponents.
  • Yuta Inose, Hiroaki Ueda, Kazuhiro Ema, Ramesh Vadivelu, Yusuke Igawa, Katsumi Kishino, Tomi Ohtsuki
    Optics InfoBase Conference Papers 2013年  
    We report observations of lasing phenomena in the regularly arranged InGaN/GaN nanocolumns. We show several types of laser actions, namely random lasing and distributed feedback lasing resulting from the randomness of the sample configurations. © OSA 2013.
  • Yuta Inose, Kazuhiro Ema, Masaru Sakai, Akihiko Kikuchi, Katsumi Kishino, Tomi Ohtsuki
    PHYSICS OF SEMICONDUCTORS 1566 548-+ 2013年  査読有り
    We conducted numerical and experimental studies on Anderson localization of light in two-dimensional random systems of semiconductor columns. We investigated finite size scaling of the localization effect as a function of localization length and system size from the calculation. We therefore obtained a localization parameter map. We also report observations of random lasing in the semiconductor samples. We show that the occurrence of the laser action have a strong relationship to the localization length by comparisons of the map with the experimental results
  • K. Kobayashi, T. Ohtsuki, K. Slevin
    LOCALISATION 2011: PROCEEDINGS OF THE SATELLITE CONFERENCE OF LT 26 11 114-119 2013年  
    We have estimated the critical exponent describing the divergence of the localization length at the metal-quantum spin Hall insulator transition. The critical exponent for the metal-ordinary insulator transition in quantum spin Hall systems is known to be consistent with that of topologically trivial symplectic systems. However, the precise estimation of the critical exponent for the metal-quantum spin Hall insulator transition proved to be problematic because of the existence, in this case, of edge states in the localized phase. We have overcome this difficulty by analyzing the second smallest positive Lyapunov exponent instead of the smallest positive Lyapunov exponent. We find a value for the critical exponent nu = 2.73 +/- 0.02 that is consistent with that for topologically trivial symplectic systems.
  • Ken-Ichiro Imura, Mayuko Okamoto, Yukinori Yoshimura, Yositake Takane, Tomi Ohtsuki
    Physical Review B 86(24) 245436 2012年12月28日  
  • Ken-Ichiro Imura, Mayuko Okamoto, Yukinori Yoshimura, Yositake Takane, Tomi Ohtsuki
    PHYSICAL REVIEW B 86(24) 2012年12月  査読有り
    The nontrivialness of a topological insulator (TI) is characterized either by a bulk topological invariant or by the existence of a protected metallic surface state. Yet, in realistic samples of finite size, this nontrivialness does not necessarily guarantee the gaplessness of the surface state. Depending on the geometry and on the topological indices, a finite-size energy gap of different nature can appear, and, correspondingly, exhibit various scaling behaviors of the gap. The spin-to-surface locking provides one such gap-opening mechanism, resulting in a power-law scaling of the energy gap. Weak and strong TIs show different degrees of sensitivity to the geometry of the sample. As a noteworthy example, a strong TI nanowire of a rectangular-prism shape is shown to be more gapped than that of a weak TI of precisely the same geometry. DOI: 10.1103/PhysRevB.86.245436 PACS number(s): 73.22.-f, 73.20.At, 72.80.Sk
  • Slevin, K., Ohtsuki, T.
    Conductor-Insulator Quantum Phase Transitions 9780199592593 2012年  
  • Ohtsuki Tomi, Kobayashi Koji, Slevin Keith
    素粒子論研究 119(4) D192-D201 2012年  
    The Anderson transition is a disorder driven quantum phase transition between metallic and insulating phases. In contrast to the common belief that two dimensional (2D) systems are always insulating and that the Anderson transition does not occur in 2D, in certain universality classes 2D systems can be metallic. We review the recent development of the theory of the Anderson transition in 2D. There are ten universality classes: three Wigner-Dyson classes, three chiral universality classes, and four Bogoliubov-de Gennes classes. We report results for critical exponents and distributions of conductance for the symplectic universality class. We emphasize that, on the one hand, the existence of a topological insulating phase does not alter the value of the critical exponent, while on the other, it strongly affects the form of the conductance distribution at the transition.
  • K. Slevin, T. Ohtsuki
    International Journal of Modern Physics Conference Series 11 60--69 2012年1月1日  
  • SLEVIN Keith, OHTSUKI Tomi
    数理解析研究所講究録別冊 27 141-147 2011年7月  
  • Y. Inose, M. Sakai, K. Ema, A. Kikuchi, K. Kishino, T. Ohtsuki
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 2011年  査読有り
    The possibility of Anderson localization of light in random dielectric systems has been discussed over the last three decades [1]. However, theoretical and experimental studies for the light localization in random media are much more difficult than those in photonic crystals. Therefore, observation of the light localization has been realized only recently [2]. As a result, it is fair to say the wave localization phenomenon is not well understood. © 2011 IEEE.
  • Masaru Sakai, Yuta Inose, Tomi Ohtsuki, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 2011年  査読有り
    In disordered materials, the combination of multiple light scattering and optical interference induces the localization of light. This phenomenon is called Anderson localization which is widely observed in electron systems with random potentials. Experimental studies of Anderson localization of light have been performed over the last three decades [1]. However, most of those studies attempted to secure the evidence of light localization by macroscopic observation of the light scattered by disordered materials. Recently, we proposed the direct observation of light localization using GaN nanocolumn samples by near-field scanning optical microscopy (NSOM), and presented the evidence of the light localization by a histogram analysis of NSOM image and by showing the wavelength dependence of the position of the localized spots in NSOM images [2,3]. We also observed random lasing in GaN nanocolumn samples and discussed the lasing property from a viewpoint of Anderson localization [4]. In this study, we extracted the two-dimensional (2D) spatial dependence from the NSOM image to estimate the localization length in GaN nanocolumns. © 2011 IEEE.
  • Y. Inose, M. Sakai, K. Ema, A. Kikuchi, K. Kishino, T. Ohtsuki
    PHOTONIC AND PHONONIC PROPERTIES OF ENGINEERED NANOSTRUCTURES 7946 2011年  
    In the two-dimensional random system composed of a disordered array of a dielectric cylindrical column ensemble, Anderson localization of light is possible. We show localization parameter maps for the light localization adopting parameters of gallium nitride nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate parametric dependence of the localization characteristics on the light frequency, the radius of the columns, and the filling fraction of the columns. To obtain the maps, we have simulated temporal light diffusion in random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We conclude that the main mechanism for localization varies continuously with the column filling fraction from Mie resonance of single column to Bragg-like diffraction of the column ensemble.
  • Yuta Inose, Masaru Sakai, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino, Tomi Ohtsuki
    PHYSICAL REVIEW B 82(20) 205328 2010年11月  
    In the two-dimensional random system composed of a disordered array of dielectric cylindrical columns, Anderson localization of light occurs. To obtain frequency dependence of the light localization characteristics, we have simulated temporal diffusion of electromagnetic waves in such a random system adopting parameters of actual nanosized semiconductor samples with a high filling fraction of the columns, using the finite-difference time-domain method. We have investigated diffusion length, autocorrelation function of light energy density, and time variation in total energy within the system at several frequencies. We obtain universal behavior of light localization phenomenon as a function of the light localization length and system size, from which we estimate frequency dependence of the localization length. In addition, we show that the frequency dependence of the localization effect depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section of a single scatterer.
  • K. Kobayashi, T. Ohtsuki, H. Obuse, K. Slevin
    PHYSICAL REVIEW B 82(16) 165301 2010年10月  
    We study numerically the charge conductance distributions of disordered quantum spin-Hall (QSH) systems using a quantum network model. We have found that the conductance distribution at the metal-QSH insulator transition is clearly different from that at the metal-ordinary insulator transition. Thus the critical conductance distribution is sensitive not only to the boundary condition but also to the presence of edge states in the adjacent insulating phase. We have also calculated the point-contact conductance. Even when the two-terminal conductance is approximately quantized, we find large fluctuations in the point-contact conductance. Furthermore, we have found a semicircular relation between the average of the point-contact conductance and its fluctuation.
  • Masaru Sakai, Yuta Inose, Kazuhiro Ema, Tomi Ohtsuki, Hiroto Sekiguchi, Akihiko Kikuchi, Katsumi Kishino
    APPLIED PHYSICS LETTERS 97(15) 151109 2010年10月  
    We report observations of random laser action in self-organized GaN nanocolumns. We have measured three samples with different filling fractions and investigated the dependence of the lasing property on the random configuration of nanocolumns. Numerical calculations based on a finite-difference time-domain method have also been performed and the comparison with the experimental results shows a clear relationship between the strength of light localization and the occurrence of random laser action. (C) 2010 American Institute of Physics. [doi:10.1063/1.3495993]
  • B. Kramer, A. MacKinnon, T. Ohtsuki, K. Slevin
    INTERNATIONAL JOURNAL OF MODERN PHYSICS B 24(12-13) 1841-1854 2010年5月  
    This chapter describes the progress made during the past three decades in the finite size scaling analysis of the critical phenomena of the Anderson transition. The scaling theory of localization and the Anderson model of localization are briefly sketched. The finite size scaling method is described. Recent results for the critical exponents of the different symmetry classes are summarised. The importance of corrections to scaling are emphasised. A comparison with experiment is made, and a direction for future work is suggested.
  • Yuta Inose, Masaru Sakai, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino, Tomi Ohtsuki
    THIRD INTERNATIONAL WORKSHOP ON THEORETICAL AND COMPUTATIONAL NANOPHOTONICS - TACONA-PHOTONICS 2010 1291 118-+ 2010年  査読有り
    We present frequency dependence of Anderson localization of light in finite-sized two-dimensional random systems. We have investigated time variation of total energy within a system composed of dielectric columns, by adopting parameters of actual nano-sized semiconductor crystals with a high filling fraction of the columns. We have succeeded in obtaining single parameter scaling of the localization effect. In addition, we show that the frequency dependence of the localization in the system depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section for a single column.
  • M. Kohda, J. Ohe, H. Sanada, M. Yamamoto, T. Ohtsuki, J. Nitta
    ULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XIV 7600 76001B 2010年  査読有り
    In two-dimensional electron gas (2DEG), spatial gradient of effective magnetic field due to spin orbit interaction yields spin dependent force. By taking this advantage, Stern-Gerlach spin filter in 2DEG has been proposed for generating spin polarized currents without any external magnetic fields and ferromagnetic materials [Phys. Rev. B 72, 041308(R) (2005)]. In order to demonstrate the spin filtering effect, detection of spin polarized electrons becomes of crucial importance. Here, we propose an electrical detection of spin filtering by introducing an in-plane magnetic field in mesoscopic Stern-Gerlach spin filter. In-plane magnetic field induces spin polarized electrons due to Zeeman splitting, generating the imbalance between up-spin and down-spin currents after the spin separation. Calculated spin separation angle becomes 10 degrees based on experimentally accessible parameters. Time evolution of wave packet shows the spin separation as well as the charge imbalance under the in-plane magnetic field. By fabricating Y-branch shaped narrow wire structure with two split gate electrodes at the junction, spin filtering effect can be detected as the magnitude difference of each branch currents. Gate bias dependence of each branch current was measured in B-ex = +/- 15 T at T = 4.2 K.
  • Koji Kobayashi, Tomi Ohtsuki, Keith Slevin
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78(8) 084708-084708-5 2009年8月  
    We study the transport properties of disordered two-dimensional electron systems with a perfectly conducting channel. We introduce an asymmetric Chalker-Coddington network model and numerically investigate the point-contact conductance. We find that the behavior of the conductance in this model is completely different from that in the symmetric model. Even in the limit of a large distance between the contacts, we find a broad distribution of conductance and a non-trivial power law dependence of the averaged conductance on the system width. Our results are applicable to systems such as zigzag graphene nano-ribbons where the numbers of left- and right-going channels are different.
  • 小林浩二, 大槻東巳, K. Slevin
    Journal of the Physical Society of Japan 78(8) 084708-084708-5 2009年7月27日  
  • Keith Slevin, Tomi Ohtsuki
    PHYSICAL REVIEW B 80(4) 041304-1-041304-4 2009年7月  
    We report an estimate nu = 2.593 [2.587,2.598] of the critical exponent of the Chalker-Coddington model of the integer quantum Hall effect that is significantly larger than previous numerical estimates and in disagreement with experiment. This suggests that models of noninteracting electrons cannot explain the critical phenomena of the integer quantum Hall effect.
  • Yuta Inose, Tomi Ohtsuki, Hideyuki Kunugita, Kazuhiro Ema, Masaru Sakai, Akihiko Kikuchi, Katsumi Kishino
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16) 193 2009年  査読有り
    We show pseudogap maps of Anderson localization of light adopting the parameters of self-organized nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate the parametric dependence of the localization effect. To obtain the maps, we simulated light propagation in open random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We found that the shape of the pseudogaps is close to the one of bandgaps in photonic crystals. We conclude that strong localization of light occurs because of interference by average Bragg diffraction, not strong Mie resonant peaks.
  • K. Kobayashi, K. Hirose, H. Obuse, T. Ohtsuki, K. Slevin
    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 2 150 2009年  
    We study the transport properties of disordered electron systems that contain perfectly conducting channels. Two quantum network models that belong to different universality classes, unitary and symplectic, are simulated numerically. The perfectly conducting channel in the unitary class can be realized in zigzag graphene nano-ribbons and that in the symplectic class is known to appear in metallic carbon nanotubes. The existence of a perfectly conducting channel leads to novel conductance distribution functions and a shortening of the conductance decay length.
  • K. Hirose, OHTSUKI TOMI, K. Slevin
    Physica E: Low-dimensional Systems and Nanostructures 40(5) 1677-1680 2008年3月  
  • Tohru Kawarabayashi, Yoshiyuki Ona, Tomi Ohtsuki, Stefan Kettemann, Alexander Struck, Bernhard Kramer
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 40(5) 1072-1074 2008年3月  
    The two-terminal conductance of quantum Hall wires in the presence of spatially correlated disorder is investigated numerically. It is found that the conductance plateau transitions shift to higher energies than the corresponding Landau subband centers. By performing the analysis on individual samples, it is found that the critical energy of the plateau transition depends on the disorder realization, but the distance between successive critical energies is independent of the sample and is equal to the bulk Landau level separation. (C) 2007 Elsevier B.V. All rights reserved.
  • A. Struck, T. Kawarabayashi, A. Zhuravlev, T. Ohtsuki, B. Kramer, S. Kettemann
    Physica Status Solidi (B) Basic Research 245(2) 393-408 2008年2月  
    The quantum phase diagram of disordered quantum wires in a strong magnetic field is reviewed. For uncorrelated disorder potential the 2-terminal conductance, as calculated with the numerical transfer matrix method, shows zero temperature discontinuous transitions between exactly integer plateau values and zero. This is explained by the dimensional crossover of the bulk localisation length, which drives a transition from delocalised to localised edge states. In the thermodynamic limit, fixing the aspect ratio of the wire, there is a transition from the one dimensional chiral metal of extended edge states to localisation along the wire. In the vicinity of this chiral metal insulator transition (CMIT), states are identified which are superpositions of edge states with opposite chirality. The bulk contribution of such states is found to decrease with increasing wire width. Based on exact diagonalisation results for the eigenstates and their participation ratios, we conclude that these states are characteristic for the CMIT, and have the appearance of nonchiral edges states. Thereby these states are distinguishable from other states in the quantum Hall wire, namely, extended edge states, two-dimensionally (2D) localized, quasi-1D localized, and 2D critical states. In the presence of spatially correlated random potential we find with the numerical transfermatrix method that the potential correlation results in a shift of quantized conductance plateaus in long wires proportional to the strength of the random potential. This shift is found to be insensitive to the strength of magnetic fields and the same for all plateaus. A semiclassical explanation of this effect is proposed. We conclude with an outlook on modfications of the quantum phase diagram due to the spin degree of freedom of the electrons and their interactions. We discuss the stability of the phase diagram at finite temperature. © 2008 Wiley-VCH Verlag GmbH &amp Co. KGaA.
  • Tohru Kawarabayashi, Yoshiyuki Ono, Tomi Ohtsuki, Stefan Kettemann, Alexander Struck, Bernhard Kramer
    PHYSICAL REVIEW B 75(23) 235317-1-235317-5 2007年6月  
    Quantum transport properties in quantum Hall wires in the presence of spatially correlated random potential are investigated numerically. It is found that the potential correlation reduces the localization length associated with the edge state, in contrast to the naive expectation that the potential correlation increases it. The effect appears as the sizable shift of quantized conductance plateaus in long wires, where the plateau transitions occur at energies much higher than the Landau band centers. The scale of the shift is of the order of the strength of the random potential and is insensitive to the strength of magnetic fields. Experimental implications are also discussed.
  • Jun-ichiro Ohe, Tomi Ohtsuki, Bernhard Kramer
    PHYSICAL REVIEW B 75(24) 245313-1-245313-5 2007年6月  
    A Hall effect due to spin chirality in mesoscopic systems is investigated. We consider a four-terminal Hall system including local spins with geometry of a vortex domain wall, where strong spin chirality appears near the center of the vortex. The Fermi energy of the conduction electrons is assumed to be comparable to the exchange coupling energy where the adiabatic approximation ceases to be valid. Our results show a Hall effect where a voltage drop and a spin current arise in the transverse direction, which is shown to survive in the presence of weak disorder. The similarity between this Hall effect and the conventional spin Hall effect in systems with spin-orbit interaction is pointed out.
  • J. -i. Ohe, T. Ohtsuki, B. Kramer
    PHYSICS OF SEMICONDUCTORS, PTS A AND B 893 1263-+ 2007年  査読有り
    A Hall effect due to the spin chirality in mesoscopic systems is investigated numerically. We consider 4-terminal Hall systems including local spins with vortex domain wall geometry, where strong spin chirality appears near the vortex center. The Fermi energy of the conduction electrons is assumed to be comparable to the exchange coupling energy where the adiabatic approximation cannot be applied. Our results show Hall effect where voltage drop and spin current arise in transverse direction.
  • Yoichi Asada, Keith Slevin, Tomi Ohtsuki
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 34(1-2) 228-231 2006年8月  査読有り
    Electron transport phenomena in disordered electron systems with spin-orbit coupling in two dimensions and below are studied numerically. The scaling hypothesis is checked by analyzing the scaling of the quasi-1D localization length. A logarithmic increase of the mean conductance is also confirmed. These support the theoretical prediction that the two-dimensional metal in systems with spin-orbit coupling has a perfect conductivity. Transport through a Sierpinski carpet is also reported. (c) 2006 Elsevier B.V. All rights reserved.
  • Tohru Kawarabayashi, Tomi Ohtsuki
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 34(1-2) 104-107 2006年8月  査読有り
    Conductance fluctuations in two-dimensional random magnetic fields are investigated numerically in the case where the mean and the fluctuation of the random magnetic fields are of the same order. The conductance is evaluated by means of the Landauer formula. It is found that for a system with edge states, the conductance fluctuation exhibits clearly a Shubnikov-de Haas type oscillation in the weak field regime. (c) 2006 Elsevier B.V. All rights reserved.
  • Yamamoto, M., Dittmer, K., Kramer, B., Ohtsuki, T.
    Physica E: Low-Dimensional Systems and Nanostructures 32(1-2 SPEC. ISS.) 2006年  
  • Y Asada, K Slevin, T Ohtsuki
    PHYSICAL REVIEW B 73(4) 041102 2006年1月  
    We investigate the possibility of an Anderson transition below two dimensions in disordered systems of noninteracting electrons with symplectic symmetry. Numerical analysis of energy level statistics and conductance statistics on Sierpinski carpets with spin-orbit coupling indicates the occurrence of an Anderson transition below two dimensions.
  • M Yamamoto, J Ohea, T Ohtsuki, J Nitta, B Kramer
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 29(3-4) 490-494 2005年11月  査読有り
    We investigate numerically the spin polarization of the current in the presence of Rashba spin-orbit interaction (RSOI) in a 3-terminal conductor. We use equation-of-motion method to simulate the time evolution of the wave packet and focus on single-channel transport. A T-shaped conductor with uniform RSOI proposed by Kiselev and Kim and a Y-shaped conductor with nonuniform RSOI are considered. In the T-shaped conductor, the strength of RSOI is assumed to be uniform. We have found that the spin polarization becomes nearly 100% with little loss of conductance for sufficiently strong spin-orbit coupling. This is due to the spin-dependent group velocity of electrons at the junction which causes the spin separation. In the Y-shaped conductor, the strength of RSOI is modulated perpendicular to the charge Current. A spatial gradient of effective magnetic field due to the nonuniform RSOI causes the Stern-Gerlach type spin separation. The direction of the polarization is perpendicular to the Current and parallel to the spatial gradient. Again almost 100% spin polarization can be realized by this spin separation. (c) 2005 Elsevier B.V. All rights reserved.

MISC

 139

書籍等出版物

 11
  • 大槻 東巳 (担当:共著, 範囲:p. 13-30 (第1章))
    朝倉書店 2019年10月10日 (ISBN: 9784254131291)
    機械学習を使って物理学で何ができるのかを解説した著書。大槻・真野の分担は機械学習,深層学習が物理に何を起こそうとしているかを波動関数の解析を例に解説した,第1章である。
  • Ohtsuki Tomi (担当:共著, 範囲:109-157)
    Wiley-Scrivener 2019年4月9日 (ISBN: 9781119407294)
    Topological properties are sometimes emergent or enforced by the breaking of translational invariance. Here, in this chapter we discuss dimensional crossover of topological properties in thin films of topological insulators (TI) and Weyl semi- metals, electronic properties on the surface of TI nanoparticles and TI nanowires as a constrained electronic system. To discuss the effects of disorder is another highlight of this chapter. We cast on the unusual robustness of Dirac and Weyl semimetal phases against disorder, then the discussion is turned to a novel type of quantum criticality emergent from this unusual robustness, leading us to formu- late the scaling theory of semimetal-metal transition. The concept of topological matter dose not fade under circumstances of absent translational invariance; it is on the contrary, emergent or enforced under such circumstances.
  • 大槻 東巳
    Wiley online library 2019年3月12日 (ISBN: 9781119407317)
  • 大槻 東巳 (担当:共著, 範囲:86-110)
    Oxford University Press 2012年7月18日 (ISBN: 9780199592593)
  • 大槻 東巳 (担当:共著, 範囲:295-305)
    朝倉書店 2012年5月10日 (ISBN: 9784254131031)

共同研究・競争的資金等の研究課題

 27

その他

 6
  • 2005年10月 - 2008年3月
    文部科学省の現代GP,グローバル社会における系統的科学英語教育の支援を受け,カリキュラム委員長として30科目以上にわたる表記の科目群を整えた。また実際に物理系の科目を,パワーポイント,WEBサイト,DVD(The mechanical universe and beyond..)を用い,リスニング,ライティング,リーディングを教えた。\nまた本プログラムは30科目以上の講義からなるが,そのカリキュラム委員長を務めた。さらに各種の電子教材を作成した。
  • 1998年4月 - 2007年9月
    ナノスケールの物理学に関する講義ノートを電子化し,学生に配付した。これにより学生は予習,復習を容易に行えるようになった。さらに細かい記法と複雑な式を正確に学べるようになった。
  • 1995年4月 - 2006年9月
    線形代数学を物理学科向けに講義した。講義ノートを工夫して作成し,全員が簡単にダウンロードできるようにした。
  • 2003年10月 - 2006年3月
    電磁気学の講義ノートを電子化した。これにより学生は講義に集中できるようになった。また,予習,復習が容易になった。
  • 1999年4月 - 2001年9月
    物理学を学ぶ上で必要な数学と,それをふまえた体系的な力学を講義ノートを電子化して教授した。学生は詳しい講義ノートをダウンロード,印刷することで内容に集中できるようになった。