Y. Hatsugai

University of Tsukuba, Tsukuba, Ibaraki, Japan

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Publications (91)229.8 Total impact

  • Source
    Toshikaze Kariyado · Yasuhiro Hatsugai
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    ABSTRACT: Mechanical graphene, which is a spring-mass model with the honeycomb structure, is investigated. The vibration spectrum is dramatically changed by controlling only one parameter, spring tension at equilibrium. In the spectrum, there always exist Dirac cones at K- and K'-points. As the tension is modified, extra Dirac cones are created and annihilated in pairs. When the time reversal symmetry is broken by uniform rotation of the system, creation and annihilation of the Dirac cones result in a jump of the appropriately defined Chern number. Then, a flip of the propagation direction of the chiral edge modes takes place, which gives an experimental way to detect the topological transition. This is a bulk-edge correspondence of the classical system. We also demonstrate the other important concept, symmetry protection of the topological states, is at work in the classical system. For the time reversal invariant case, the topological edge modes exist for the fixed boundary condition but not for the free boundary condition. This contrast originates from the symmetry breaking at the free boundary.
  • Source
    Y Hatsugai · K Shiraishi · H Aoki
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    ABSTRACT: In order to analytically capture and identify peculiarities in the electronic structure of silicene, the Weaire–Thorpe (WT) model, a standard model for treating three-dimensional (3D) silicon, is applied to silicene with a buckled 2D structure. In the original WT model for four hybridized sp 3 orbitals on each atom along with inter-atom hopping, the band structure can be systematically examined in 3D, where flat (dispersionless) bands exist as well. For examining silicene, here we re-formulate the WT model in terms of the overlapping molecular-orbital (MO) method which enables us to describe flat bands away from the electron–hole symmetric point. The overlapping MO formalism indeed enables us to reveal an important difference: while in 3D the dipersive bands with cones are sandwiched by doubly-degenerate flat bands, in 2D the dipersive bands with cones are sandwiched by triply-degenerate and non-degenerate (nearly) flat bands, which is consistent with the original band calculation by Takeda and Shiraishi. Thus there emerges a picture for why the whole band structure of silicene comprises a pair of dispersive bands with Dirac cones with each of the bands touching a nearly flat (narrow) band at Γ. We can also recognize that, for band engineering, the bonds perpendicular to the atomic plane are crucial, and that ferromagnetism or structural instabilities are expected if we can shift the chemical potential close to the flat bands.
    New Journal of Physics 02/2015; 17(2). DOI:10.1088/1367-2630/17/2/025009 · 3.67 Impact Factor
  • Source
    Takahiro Fukui · Yasuhiro Hatsugai
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    ABSTRACT: We argue that the entanglement Chern number proposed recently is an invariant under the adiabatic deformation of a gapped many-body groundstate into {\it disentangled/purified} one, which means a partition of the Chern number (disentangled Chern number) into subsystems. We generalize the idea to another topological number, the Z$_2$ Berry phase for a system with particle-hole symmetry, and apply it to a groundstate in a weak topological phase where the Chern number is vanishing but it has, nevertheless, edge states. This entanglement Berry phase is especially useful for characterizing random systems with non trivial edge states.
    Journal of the Physical Society of Japan 01/2015; 84(4). DOI:10.7566/JPSJ.84.043703 · 1.48 Impact Factor
  • Source
    Toshikaze Kariyado · Yasuhiro Hatsugai
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    ABSTRACT: Topological properties of the spin-1/2 dimerized Heisenberg ladder are investigated focusing on the plateau phase whose magnetization is a half of the saturation value that appears in the applied magnetic field. Although the applied magnetic field removes most of the symmetries of the system, there is a symmetry protected topological phase supported by the spatial inversion symmetry in the 1/2-plateau phase. The Z2 Berry phase that is associated with a symmetry respecting boundary and is quantized into 0 and \pi is used to give a symmetry protected topological order parameter. Edge states are also analyzed to confirm the bulk-edge correspondence. In addition, a symmetry breaking boundary is considered. Then, we observe a unique type of quantization of the Berry phase, a quantization into +-\pi/2 of the Berry phase. In this case, the bulk-edge correspondence is also unique, namely, there emerge "polarized" edge states for the case with +-\pi/2 quantization. We also evaluate the entanglement entropy by the infinite time-evolving block decimation (iTEBD) to complement the Berry phase based arguments. Further, a different type of the topological order parameter is extracted from the matrix product state representation of the ground state given by the iTEBD.
    Physical Review B 12/2014; 91(21). DOI:10.1103/PhysRevB.91.214410 · 3.74 Impact Factor
  • Source
    Y. Hatsugai · K. Shiraishi · H. Aoki
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    ABSTRACT: In order to analytically capture and look for peculiarities in the electronic structure of silicene, Weaire-Thorp(WT) model, a standard model for treating three-dimensional (3D) silicon, is applied to silicene with the buckled 2D structure. A particular interest is that in the original WT model for four hybridized $sp^3$ orbitals on each atom and inter-atom hopping, the band structure can be systematically examined, which contains flat (dispersionless) bands. If we re-formulate the model in terms of the more general "overlapping molecular orbital" theory due to Hatsugai and Maruyama, we can generically treat the flat bands away from the electron-hole symmetric point in multi-orbital models for the first time. This enables us to newly interpret why the whole band structure of silicene comprises dispersive bands with Dirac cones and (nearly) flat bands. The algebraic formulation enables us to pin point an important difference from 3D, where the dipersive bands with cones are sandwiched by doubly-degenerate flat ones, that in 2D the dipersive bands with cones are sandwiched by triply-degenerate and non-degenerate (nearly) flat bands, which is consistent with the original band calculation by Takeda and Shiraishi. For the band engineering the bonds perpendicular to the atomic plane are crucial. A ferromagnetism is expected if we can shift the chemical potential close to the flat bands.
  • Source
    Yasuhiro Hatsugai · Tohru Kawarabayashi · Hideo Aoki
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    ABSTRACT: Anomalously sharp (delta-function-like) $n=0$ Landau level in the presence of disorder is usually considered to be a manifestation of the massless Dirac fermions in magnetic fields. This property persists even when the Dirac cone is tilted, which has been shown by Kawarabayashi et al. [Phys. Rev. B {\bf 83}, 153414 (2011)] to be a consequence of a "generalized chiral symmetry". Here we pose a question whether this property will be washed out when the tilted Dirac fermion becomes massive. Surprisingly, while the massive case with split $n=0$ Landau levels may seem to degrade the anomalous sharpness, the levels do remain delta-function-like. This has been shown analytically in terms of the Aharonov-Casher argument extended to the massive tilted Dirac ferimions. A key observation is that the conventional and generalized chiral operators are related with each other via a non-unitary transformation, with which the split, nonzero-energy $n=0$ wave functions of the massive system can be identified as a gauge-transformed zero-mode wave functions of the massless system. This is confirmed from a numerical result for a model tight-binding system. A message is that the chiral symmetry, rather than a simpler notion of the sublattice symmetry, is essential for the robustness of the $n=0$ Landau level, which is why the chiral symmetry remains applicable even to massive case.
    Physical Review B 10/2014; 91(8). DOI:10.1103/PhysRevB.91.085112 · 3.74 Impact Factor
  • Source
    Takahiro Fukui · Yasuhiro Hatsugai
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    ABSTRACT: If an extensive partition in two dimensions yields a gapful entanglement spectrum of the reduced density matrix, the Berry curvature based on the corresponding entanglement eigenfunction defines the Chern number. We propose such an entanglement Chern number as a useful, natural, and calculable topological invariant, which is potentially relevant to various kinds of topological ground states. We show that it serves as an alternative topological invariant for time reversal invariant systems, and as a new topological invariant for a weak topological phase of a superlattice Wilson-Dirac model. In principle, the entanglement Chern number can be also effective for interacting systems such as topological insulators in contrast to the $Z_2$ invariants.
    Journal of the Physical Society of Japan 08/2014; 83(11). DOI:10.7566/JPSJ.83.113705 · 1.48 Impact Factor
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    ABSTRACT: We propose classification schemes for characterizing two-dimensional topological phases with nontrivial weak indices. Here, "weak" implies that the Chern number in the corresponding phase is trivial, while the system shows edge states along specific boundaries. As concrete examples, we analyze different versions of the so-called Wilson-Dirac model with (i) anisotropic Wilson terms, (ii) next nearest neighbor hopping terms, and (iii) a superlattice generalization of the model, here in the tight-binding implementation. For types (i) and (ii) a graphic classification of strong properties is successfully generalized for classifying weak properties. As for type (iii), weak properties are attributed to quantized Berry phase pi along a Wilson loop.
    Physical Review B 05/2014; 90(15). DOI:10.1103/PhysRevB.90.155443 · 3.74 Impact Factor
  • Source
    Toshikaze Kariyado · Yasuhiro Hatsugai
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    ABSTRACT: Symmetry protected quantization of the Berry phase is discussed in relation to edge states. Assuming an existence of some adiabatic process which protects quantization of the Berry phase, non trivial Berry phase $\gamma=\pm 2\pi\rho$ ($\rho$ is a local filling of particles) for the bulk suggests appearance of edge states with boundaries. We have applied this generic consideration for Bloch states of some two dimensional model with massless Dirac fermions where $\gamma=\pm\pi/2$ implies the edge states. Entanglement entropy is evaluated for the models and its relation to the bulk-edge correspondence of Dirac fermions is discussed as well.
    Physical Review B 04/2014; 90(8). DOI:10.1103/PhysRevB.90.085132 · 3.74 Impact Factor
  • Daichi Seki · Yuji Hamamoto · Yasuhiro Hatsugai
    Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
  • Hiroki Sakamoto · Yasuhiro Hatsugai · Hideo Aoki · Tohru Kawarabayashi
    Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
  • Toshikaze Kariyado · Yasuhiro Hatsugai
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    ABSTRACT: Electronic structure of fermionic Shastry–Sutherland model, which is recently revealed to have a rich phase diagram, is investigated further in detail. We find that if the spin–orbit coupling exists, the quantum spin Hall insulator phase is also possible, in addition to the phases discovered in the previous study. Furthermore, first-principles calculation is performed for an existing material SrCu2(BO3)2 that has Shastry–Sutherland type lattice network, and the obtained results confirm those established with the tight-binding model.
    Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
  • Source
    Toshikaze Kariyado · Yasuhiro Hatsugai
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    ABSTRACT: The fermionic Shastry-Sutherland model has a rich phase diagram, including phases with massless Dirac fermions, a quadratic band crossing point, and a pseudospin-1 Weyl fermion. Berry phases defined by the one-dimensional momentum as a parameter are quantized into 0 or pi due to the inversion symmetry combined with the time reversal, or existence of the glide plane, which also protects the massless Dirac cones with continuous parameters. This is the symmetry protected Z2 quantization. We have further demonstrated the Z2 Berry phases generically determine the existence of edge states in various phases and with different types of the boundaries as the bulk-edge correspondence of the massless Dirac fermion systems.
    Physical Review B 07/2013; 88(24). DOI:10.1103/PhysRevB.88.245126 · 3.74 Impact Factor
  • Source
    Takahiro Fukui · K. -I. Imura · Yasuhiro Hatsugai
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    ABSTRACT: We explore novel topological phases realized in a superlattice system based on the Wilson-Dirac model. Our main focus is on a two-dimensional analogue of weak topological insulator phases. We find such phases as those characterized by gapless edge states that are protected by symmetry but sensitive to the orientation of the edge relative to the superlattice structure. We show that manifest and hidden reflection symmetries protect such weak topological phases, and propose bulk Z2 indices responsible for the topological protection of the edge states.
    Journal of the Physical Society of Japan 04/2013; 82(7). DOI:10.7566/JPSJ.82.073708 · 1.48 Impact Factor
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    ABSTRACT: Classical and quantum dynamics are important limits for the understanding of the transport characteristics of interacting electrons in nanodevices. Here we apply an intermediate semiclassical approach to investigate the dynamics of two interacting electrons in a planar nanochannel as a function of Coulomb repulsion and electric field. We find that charge is mostly redistributed to the channel edges and that an electric field enhances the particle-like character of electrons. These results may have significant implications for the design and study of future nanodevices.
    Applied Physics Express 04/2013; 6(6). DOI:10.7567/APEX.6.065201 · 2.57 Impact Factor
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    ABSTRACT: Influence of Coulomb blockade on electron scattering by a quantum dot has been theoretically investigated using a multielectron wave packet simulation technique based on the time-dependent Hartree-Fock approximation. In our simulation, the bound states of electrons in the dot are self-consistently determined. We confirmed that Koopman's theorem keeps its validity only for weak Coulomb interactions. Moreover, we show that the maximum number of electrons trapped in the dot does depend on the strength of Coulomb interactions. Consequently, the transmission and reflection probabilities of an incident wave packet toward the dot are strongly influenced by the number of trapped electrons in the dot. (C) 2013 The Japan Society of Applied Physics
    Japanese Journal of Applied Physics 04/2013; 52(4S):04CJ06. DOI:10.7567/JJAP.52.04CJ06 · 1.06 Impact Factor
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    Y Hatsugai · T Morimoto · T Kawarabayashi · Y Hamamoto · H Aoki
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    ABSTRACT: Chiral symmetry, fundamental in the physics of graphene, guarantees the existence of topologically stable doubled Dirac cones and anomalous behaviors of the zero-energy Landau level in magnetic fields. Its crucial role, especially its manifestation in optical responses and many-body physics in graphene, is explained in this paper. We also give an overview of multilayer graphene from the viewpoint of the optical properties and their relation with chiral symmetry.
    New Journal of Physics 03/2013; 15(3):035023. DOI:10.1088/1367-2630/15/3/035023 · 3.67 Impact Factor
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    ABSTRACT: We have investigated the effect of Coulomb interaction on electron transport in a one-dimensional nanoscale structure using a multi-electron wave packet approach. To study the time evolution, we numerically solve the time-dependent Hartree-Fock equation, finding that the electron wave packet dynamics strongly depends on the Coulomb interaction strength. When the Coulomb interaction is large, each electron wave packet moves separately in the presence of an electric field. With weak Coulomb interaction, however, the electron wave packets overlap, forming and moving as one collective wave packet.
    31st International Conference on the Physics of Semiconductors (ICPS); 01/2013
  • D. Lidsky · J. Shiraishi · Y. Hatsugai · M. Kohmoto
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    ABSTRACT: We generalize the model of Hatsugai and Kohmoto [J. Phys. Soc. Jpn, 61, 2056 (1992)] and find ground states which do not show the properties of Fermi liquids. We work in two space dimensions, but it is straightforward to generalize to higher dimensions. The ground state is highly degenerate and there is no discontinuity in the momentum distribution; i.e., there is no Fermi surface. The Green’s function generically has a branch cut.
  • M. Yamanaka · Y. Hatsugai · M. Kohmoto
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    ABSTRACT: Phase diagram of the S = 1

Publication Stats

2k Citations
229.80 Total Impact Points

Institutions

  • 2008–2015
    • University of Tsukuba
      • Centre for Computational Sciences
      Tsukuba, Ibaraki, Japan
  • 1990–2007
    • The University of Tokyo
      • • Department of Applied Physics
      • • Institute for Solid State Physics
      Tokyo, Tokyo-to, Japan
  • 2006
    • Ibaraki University
      Mito-shi, Ibaraki, Japan
  • 1993
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States