Y. Hatsugai

University of Tsukuba, Tsukuba, Ibaraki, Japan

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Publications (76)208.37 Total impact

  • [Show abstract] [Hide abstract]
    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.
    05/2014;
  • 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.
    04/2014;
  • 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.
    07/2013;
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    T. Fukui, K. -I. Imura, Y. 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; · 2.09 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). · 2.73 Impact Factor
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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. · 4.06 Impact Factor
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    ABSTRACT: We study the transport dynamics of semiclassical electrons in graphene in the Klein tunneling regime by scattering wave packets off a potential step while taking the full graphene Hamiltonian into account. Besides establishing the basic transmission characteristics for zigzag and armchair step edges, our numerical simulation suggests that the wave packet dynamics is substantially affected by the Berry curvature, which induces lateral shifts near the step edge reminiscent of the Hall effect of light. This anomalous correction can be relevant for recent Klein tunneling experiments.
    11/2012;
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    ABSTRACT: The effect of disorder on the Landau levels of massless Dirac fermions is examined for the cases with and without the fermion doubling. To tune the doubling a tight-binding model having a complex transfer integral is adopted to shift the energies of two Dirac cones, which is theoretically proposed earlier and realizable in cold atoms in an optical lattice. In the absence of the fermion doubling, the $n=0$ Landau level is shown to exhibit an anomalous sharpness even if the disorder is uncorrelated in space (i.e., large K-K' scattering). This anomaly occurs when the disorder respects the chiral symmetry of the Dirac cone.
    08/2012;
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    ABSTRACT: We investigate the time evolution simulation of the spin torque transfer based on the spin polarized electron wave packet in the impurity Anderson model. The time dependence of the spin expectation value of the local spin site can be interpreted in terms of two site electron problem. The high energy contribution such as charge fluctuation appears in the dynamics of the spin torque transfer.
    Journal of the Physical Society of Japan 04/2012; 81(4):4706-. · 2.09 Impact Factor
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    ABSTRACT: We investigated multi-electron wave packet dynamics considering Coulomb interaction under applied electric field by solving the time-dependent Hartree--Fock equation. We confirm that the Coulomb interaction works to prolong the lifetime of the wave packets. Moreover, we find that the applied electric field also prolongs the wave packet lifetime. This indicates that the particle nature of electrons will be dominant in future nanodevices under high electric field.
    Japanese Journal of Applied Physics 02/2012; 51(2). · 1.07 Impact Factor
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    ABSTRACT: We have theoretically investigated the wave packet dynamics method for the write characteristics of the nanoscale magnetic tunnel junction with synthetic ferrimagnets based on the microscopic quantum electron model. In this study, we have performed numerical simulation of two bands tight binding electron model. In order to take into account the electron--electron correlation, we consider the on-site Coulomb interaction, Hund exchange coupling and finite electric field. In our simulation, we employ the time dependent molecular field approximation. Based on the simulation using wave packets, we have clarified the role of ferrimagnet in the nanoscale magnetic tunnel junction.
    Japanese Journal of Applied Physics 02/2012; 51(2). · 1.07 Impact Factor
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    Y. Hamamoto, Y. Hatsugai, H Aoki
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    ABSTRACT: We study a many-body ground state of graphene in perpendicular magnetic fields. Chiral symmetry in graphene enables us to determine the many-body ground state, which turns out to be a doubly degenerate chiral condensate for the half-filled (undoped) case. In the ground state a prominent charge accumulation emerges along zigzag edges. We also show that gapless excitations are absent despite the presence of the robust edge modes, which is consistent with the Chern number C = 0.
    Journal of Physics Conference Series 08/2011;
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    Y. Hatsugai, I. Maruyama
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    ABSTRACT: Adiabatic $Z_Q$ invariants by quantized Berry phases are defined for gapped electronic systems in $d$-dimensions ($Q=d+1$). This series includes Polyacetylene, Kagome and Pyrochlore lattice respectively for $d=1,2$ and 3. The invariants are quantum $Q$-multimer order parameters to characterize the topological phase transitions by the multimerization. This fractional quantization is protected by the global $Z_Q$ equivalence. As for the chiral symmetric case, a topological form of the $Z_2$-invariant is explicitly given as well.
    EPL (Europhysics Letters) 09/2010; · 2.26 Impact Factor
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    H Watanabe, Y Hatsugai, H Aoki
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    ABSTRACT: The quantum Hall effect in graphene is regarded to be involving half-integer topological numbers associated with the massless Dirac particle, this is usually not apparent due to the doubling of the Dirac cones. Here we theoretically consider two classes of lattice models in which we manipulate the Dirac cones with either (a) two Dirac points that have mutually different energies, or (b) multiple Dirac cones having different Fermi velocities. We have shown, with an explicit calculation of the topological (Chern) number for case (a) and with an adiabatic argument for case (b) that the results are consistent with the picture that a single Dirac fermion contributes the half-odd integer series (... -3/2, -1/2, 1/2, 3/2, ...) to the Hall conductivity when the Fermi energy traverses the Landau levels.
    Journal of Physics Conference Series 09/2010; 334(1).
  • I Maruyama, Y Hatsugai
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    ABSTRACT: Quantized Berry phases proposed as local order parameters for gapped quantum liquids at zero temperature are studied to detect the Kondo singlet phase which is known as a theoretical prototype of the Kondo insulators. We deal with the Kondo lattice model and the periodic Anderson model as two basic models for heavy fermions. We show that the Berry phase is actually quantized as trivial or non-trivial value, i.e., 0 or π, and analytically obtained both in the strong coupling limit of the Kondo lattice model and in the weak coupling limit of the periodic Anderson model. In addition, the Heisenberg Kondo lattice model is studied numerically by the quantized Berry phase.
    Journal of Physics Conference Series 03/2009; 150(4):042116.
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    Y. Hatsugai
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    ABSTRACT: There are two types of edge states in graphene with/without magnetic field. One is a quantum Hall edge state, which is topologically protected against small perturbation. The other is a chiral zero mode that is localized near the boundary with/without magnetic field. The latter is also topological but is guaranteed to be at zero energy by the chiral symmetry, which is also responsible for massless Dirac-like dispersion. Conceptual roles of the edge states are stressed and reviewed from the point of view of the bulk-edge correspondence and topological order.
    Solid State Communications 01/2009; · 1.53 Impact Factor
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    I. Maruyama, T. Hirano, Y. Hatsugai
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    ABSTRACT: A spin-1/2 frustrated two-leg ladder with four-spin exchange interaction is studied by quantized Berry phases. We found that the Berry phase successfully characterizes the Haldane phase in addition to the rung-singlet phase, and the dominant vector-chirality phase. The Hamiltonian of the Haldane phase is topologically identical to the S = 1 antiferromagnetic Heisenberg chain. Decoupled models connected to the dominant vector-chirality phase revealed that the local object identified by the non-trivial (pi) Berry phase is the direct product of two diagonal singlets.
    Journal of Physics Conference Series 01/2009; 145(1).
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    T Fukui, T Fujiwara, Y. Hatsugai
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    ABSTRACT: We show that the Z$_2$ invariant, which classifies the topological properties of time reversal invariant insulators, has deep relationship with the global anomaly. Although the second Chern number is the basic topological invariant characterizing time reversal systems, we show that the relative phase between the Kramers doublet reduces the topological quantum number Z to Z$_2$. Comment: 4 pages, typos corrected
    Journal of the Physical Society of Japan 09/2008; · 2.09 Impact Factor
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    I. Maruyama, Y. Hatsugai
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    ABSTRACT: We have numerically studied a non-adiabatic charge transport in the quantum Hall system pumped by a magnetic flux, as one of the simplest theoretical realizations of non-adiabatic Thouless pumping. In the adiabatic limit, a pumped charge is quantized, known as Laughlin's argument in a cylindrical lattice. In a uniform electric field, we obtained a formula connecting quantized pumping in the adiabatic limit and no-pumping in the sudden limit. The intermediate region between the two limits is determined by the Landau gap. A randomness or impurity effect is also discussed. Comment: 4 pages, 6 figures
    Journal of Physics Conference Series 06/2008;
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    ABSTRACT: A spin-(1/2) two-leg ladder with four-spin ring exchange is studied by quantized Berry phases, used as local-order parameters. Reflecting local objects, nontrivial (pi) Berry phase is founded on a rung for the rung-singlet phase and on a plaquette for the vector-chiral phase. Since the quantized Berry phase is topologically invariant for gapped systems with the time-reversal symmetry, topologically identical models can be obtained by the adiabatic modification. The rung-singlet phase is adiabatically connected to a decoupled rung-singlet model and the vector-chiral phase is connected to a decoupled vector-chiral model. Decoupled models reveal that the local objects are a local singlet and a plaquette singlet, respectively.
    Physical review. B, Condensed matter 06/2008; · 3.77 Impact Factor

Publication Stats

1k Citations
208.37 Total Impact Points

Institutions

  • 2008–2013
    • 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
  • 1993–1998
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States