著者

Hamamoto Yuji Kawarabayashi Tohru Aoki Hideo Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.88, no.19, pp.195141, 2013-11

被引用文献数

2 1

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Motivated by the recent experiments indicating a spin-unpolarized ν=0 quantum Hall state in graphene, we theoretically investigate the ground state based on the many-body problem projected onto the n=0 Landau level. For an effective model with the on-site Coulomb repulsion and antiferromagnetic exchange couplings, we show that the ground state is a doubly degenerate spin-resolved chiral condensate in which all the zero-energy states with up spin are condensed into one chirality, while those with down spin to the other. This can be exactly shown for an Ising-type exchange interaction. The charge gap due to the on-site repulsion in the ground state is shown to grow linearly with the magnetic field, in qualitative agreement with the experiments.

著者

Ryu Shinsei Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:01631829)

巻号頁・発行日

vol.63, no.23, pp.233307, 2001-05

被引用文献数

9 9

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The zero-mode wave function of a massless Dirac fermion in the presence of a random gauge field is studied. The density correlation function is calculated numerically, and found to exhibit a power law in the weak randomness with a disorder-dependent exponent. It deviates from the power law, and the disorder dependence becomes frozen in the strong randomness. A classical statistical system is employed through the replica trick to interpret the results, and the direct evaluation of the replica limit is demonstrated numerically. Analytical expressions of the correlation function and the free energy are also discussed with the replica symmetry breaking and the Liouville field theory.

著者

Ryu Shinsei Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:01631829)

巻号頁・発行日

vol.65, no.3, pp.033301, 2001-12

被引用文献数

18

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The Dirac fermion in the random chiral models is studied, which includes the random gauge field model and the random hopping model. We focus on a connection between continuum and lattice models to give a clear perspective for the random chiral models. Two distinct structures of density of states around zero energy, one is a power-law dependence on energy in the intermediate energy range and the other is a peak at zero energy, are revealed by an extensive numerical study for large systems up to 250×250. For the random hopping model, the above findings reconcile previous inconsistencies between the lattice and the continuum models.

著者

Fukui Takahiro Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.75, no.12, pp.121403, 2007-03

被引用文献数

97 54

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For generic time-reversal-invariant systems with spin-orbit couplings, we clarify a close relationship between the Z2 topological order and the spin Chern number (SChN) in the quantum spin-Hall effect. It turns out that a global gauge transformation connects sectors with different SChNs (even integers) modulo 4, which implies that the SChN and Z2 topological orders yield the same classification. We present a method of computing the SChN and demonstrate it in single and double planes of graphene.

著者

Katsura Hosho Hirano Takaaki Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.76, no.1, pp.012401, 2007-07

被引用文献数

46 43

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We investigate the entanglement properties of the valence-bond-solid states with generic integer spin S. Using the Schwinger boson representation of the valence-bond-solid states, the entanglement entropy, the von Neumann entropy of a subsystem, is obtained exactly and its relationship with the usual correlation function is clarified. The saturation value of the entanglement entropy, 2 log2(S+1), is derived explicitly and is interpreted in terms of the edge-state picture. The validity of our analytical results and the edge-state picture is numerically confirmed. We also propose an application of the edge state as a qubit for quantum computation.

著者

Yamamoto Susumu Fujiwara Takeo Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.76, no.16, pp.165114, 2007-10

被引用文献数

26 16

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Electronic structure of stripe ordered La2−xSrxNiO4 is investigated. The system with x=(1/3) is insulator, in LSDA+U calculations, and shows charge and spin stripe, consistent with the experimental results. A highly correlated system of x=(1/2) is studied by using exact diagonalization of the multiorbital many-body Hamiltonian derived from LDA calculations and including on-site and intersite Coulomb interactions. The fluctuation of the residual spin on Ni3+ (hole) site couples with the charge fluctuation between Ni3+ and Ni2+ states and this correlation lowers the total energy. The resultant ground state is an insulator with charge and spin stripe of the energy gap 0.9 eV, consistent with the observed one. The on-site Coulomb interaction stabilizes integral valency of each Ni ion (Ni3+ and Ni2+), but does not induce the charge order. Two quantities, intersite Coulomb interaction and anisotropy of hopping integrals, play an important role to form the charge and spin stripe order in a system of x=(1/2).

著者

Song Hui Maruyama Isao Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.76, no.13, pp.132202, 2007-10

被引用文献数

7 10

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We investigate the integer quantum Hall system in a two dimensional lattice model with spatially correlated disorder by using the efficient method to calculate the Chern number proposed by Fukui et al. [J. Phys. Soc. Jpn. 74, 1674 (2005)]. Distribution of charge density indicates that the extended states at the center of each Landau band have percolating current paths, which are topologically equivalent to the edge states that exist in a system with boundaries. As increasing the strength of disorder, the floating feature is observed in an averaged Hall conductance as a function of filling factor. Its relation to the observed experiments is also discussed.

著者

Hatsugai Yasuhiro

出版者

IOP Publishing

雑誌

Journal of physics. Condensed matter (ISSN:09538984)

巻号頁・発行日

vol.19, pp.145209, 2007-04

被引用文献数

32 13

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Recently, by using quantized Berry phases, a prescription for a local characterization of gapped topological insulators has been given (Hatsugai 2006 Preprint cond-mat/0603230). It requires that the ground state is gapped and is invariant under some anti-unitary operation. A spin liquid which is realized as a unique ground state of the Heisenberg spin system with frustrations is a typical target system, since pairwise exchange couplings are always time-reversal invariants even with frustrations.As for a generic Heisenberg model with a finite excitation gap, we locally modify the Hamiltonian by a continuous SU(2) twist only at a specific link and define the Berry connection by the derivative. Then the Berry phase evaluated by the entire many-spin wavefunction is used to define the local topological order parameter at the link. We numerically apply this scheme for several spin liquids and show its physical validity. For example, it implies that the Haldane phase of the S = 1 chains is characterized by uniform π quantized Berry phases.

著者

Nakamura Yoshiaki Ichikawa Masakazu Watanabe Kentaro Hatsugai Yasuhiro

出版者

American Institute of Physics

雑誌

Applied physics letters (ISSN:00036951)

巻号頁・発行日

vol.90, no.15, pp.153104, 2007-04

被引用文献数

40 33

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A scanning tunneling microscopic study revealed quantum fluctuation of tunneling currents in individual Ge quantum dots (QDs) on SiO2/Si. This was due to the charging energy change in the QDs caused by single-electron transfer from or into the QDs. The observed electron discharging time of approximately milliseconds agreed with the propagation model of the electron wave packets from the QDs to the Si substrates by a tunneling effect rather than by passing through voids in the SiO2 smaller than electron de Broglie wavelength.

著者

Hamamoto Yuji Aoki Hideo Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.86, no.20, pp.205424, 2012-11

被引用文献数

11

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The role of chiral symmetry in many-body states of graphene in strong magnetic fields is theoretically studied with the honeycomb lattice model. For a spin-split Landau level where the leading electron-electron interaction is the nearest-neighbor repulsion, a chiral condensate is shown to be, within the subspace of the n=0 Landau level, an exact many-body ground state having a finite gap, for which a calculation of the Chern number reveals that the ground state is a Hall insulator with a topological degeneracy of two. The topological nature of the ground state is shown to manifest itself as a Kekuléan bond order along armchair edges, while the pattern melts in the bulk due to quantum fluctuations. These can be regarded as a realization of the bulk-edge correspondence that is peculiar to a chiral-symmetric system. We have also obtained the ground state when point defects are introduced in the honeycomb lattice to reveal how the presence or absence of the chiral symmetry affects the defect states.

著者

Arikawa Mitsuhiro Maruyama Isao Hatsugai Yasuhiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.82, no.7, pp.073105, 2010-10

被引用文献数

10

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A crossover between the Bose-Einstein condensation (BEC) and BCS superconducting state is described topologically in the chiral symmetric fermion system with attractive interaction. Using a local Z2 Berry phase, we found a quantum phase transition between the BEC and BCS phases without accompanying the bulk gap closing.