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

Kondo Takahiro Honma Yujiro Oh Junepyo Machida Takahiro Nakamura Junji

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.82, no.15, pp.153414, 2010-10

被引用文献数

50

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We report nonuniform and long-range electronic perturbation of graphite near the Fermi level by the point defect based on the measurements with scanning tunneling spectroscopy. The states propagate with threefold symmetry perpendicular to the zigzag edges at the point defect, indicating the formation of the nonbonding states. The propagation continues for 3–4 nm from the defect and is accompanied by oscillations in the state energy and state intensity with a periodicity of (√3×√3)R30°. The oscillation in the state intensity is ascribed to the disruption in the π-conjugated system in graphite around the defect, i.e., edge-states propagation from the defect while the oscillation in the state energy is ascribed to electron-electron interactions between the nonbonding π electronic states and the standing-wave caused by π bands of graphite.

著者

Maeshima N. Hino K. Yonemitsu K.

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.82, no.16, pp.161105, 2010-10

被引用文献数

5 5

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We study photoinduced ultrafast coherent oscillations originating from orbital degrees of freedom in the one-dimensional two-orbital Hubbard model. By solving the time-dependent Schrödinger equation for the numerically exact many-electron wave function, we obtain time-dependent optical response functions. The calculated spectra show characteristic coherent oscillations that vary with the frequency of probe light. A simple analysis for the dominant oscillating components clarifies that these photoinduced oscillations are caused by the quantum interference between photogenerated states. The oscillation attributed to the Raman-active orbital excitations (orbitons) clearly appears around the charge-transfer peak.

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.

著者

Kamiya Katsumasa Okada Susumu

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.83, no.15, pp.155444, 2011-04

被引用文献数

17 11

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We report total-energy electronic-structure calculations based on density functional theory performed on single-stranded DNA (ssDNA) encapsulated in single-walled carbon nanotubes (SWCNTs). We find that the encapsulation reaction is exothermic for nanotubes with diameters greater than 1.33 nm. The energy gain is calculated to be in the range of 0.8–1.5 eV/nm, depending on tube diameter, base sequences, and ssDNA structure. In optimal ssDNA-SWCNT hybrid-system geometries, the polar groups of ssDNA, i.e. the POH moiety in its backbone, are located adjacent to the wall of the nanotube. The electronic structure of the hybrid system is qualitatively similar to a simple sum of those of an isolated ssDNA molecule and an empty SWCNT. However, detailed analysis of the electronic structure of the hybrid system reveals that the encapsulation of ssDNA into a SWCNT affects the electronic structures of both the ssDNA and the SWCNT.

著者

Koshelev A. E. Buzdin A. I. Kakeya I. Yamamoto T. Kadowaki K.

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.83, no.22, pp.224515, 2011-06

被引用文献数

9

---

In strongly anisotropic layered superconductors in tilted magnetic fields, the Josephson vortex lattice coexists with the lattice of pancake vortices. Due to the interaction between them, the dissipation of the Josephson vortex lattice is very sensitive to the presence of the pancake vortices. If the c-axis magnetic field is smaller than the corresponding lower critical field, the pancake stacks are not formed but the individual pancakes may exist in the fluctuational regime either near the surface in large-size samples or in the central region for small-size mesas. We calculate the contribution of such fluctuating pancake vortices to the c-axis conductivity of the Josephson vortex lattice and compare the theoretical results with measurements on small mesas fabricated out of Bi2Sr2CaCu2O8+δ crystals. A fingerprint of fluctuating pancakes is a characteristic exponential dependence of the c-axis conductivity observed experimentally. Our results provide strong evidence of the existence of the fluctuating pancakes and their influence on the Josephson vortex lattice dissipation.

著者

Das Pradip Suzuki Yusuke Tachiki Masashi Kadowaki Kazuo

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.83, no.22, pp.220513, 2011-06

被引用文献数

44

---

We report on the observation of bulk superconductivity from dc magnetization measurements in a cylindrical single crystal of CuxBi2Se3. The magnitude of the magnetization in the Meissner state is very small, and the magnetic-field dependence of the magnetization just above the lower critical field Hc1 is very different from those of usual type-II superconductors. We studied the character of the vortex state theoretically in a spin-triplet pairing superconductor and compared it with the experimental results. The results showed that the superconductivity observed in CuxBi2Se3 is consistent with the spin-triplet pairing superconductivity with odd parity. We also observed a rapid relaxation phenomenon of the superconducting diamagnetism.

著者

Takagi Yoshiteru Okada Susumu

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.84, no.3, pp.035406, 2011-07

被引用文献数

9 7

---

We investigated the electronic structure of semiconducting single-walled carbon nanotubes (CNTs) adsorbed on the (111) surfaces of Au, Ag, Pt, and Pd and on the (0001) surfaces of Mg by first-principles calculations. Our calculations show that the electronic structure of the CNTs adsorbed on the metal surfaces strongly depends on the metal species. We found that on Pd surfaces, the characteristic one-dimensional electronic structure of the CNTs is totally disrupted by the strong hybridization between the π state of the CNTs and the d state of the Pd surfaces. In sharp contrast, on the Au surfaces, the CNTs retain the one-dimensional properties of their electronic structure. The distribution of the total valence charge of the CNTs on the Pd surfaces also shows a strong covalent nature between the CNTs and the surfaces. Our calculations show the importance of metal electrodes in designing CNT electronic devices.

著者

Sasaoka Kenji Yamamoto Takahiro Watanabe Satoshi Shiraishi Kenji

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.84, no.12, pp.125403, 2011-09

被引用文献数

5

---

The alternating-current response of a quantum point contact (QPC) is numerically investigated using the nonequilibrium Green function method combined with an effective mass theory. We found that the susceptance of a QPC increases stepwise with increasing gate voltage, when the width of the quantization plateau in the gate voltage-conductance curve is narrower than the width of the region where the conductance changes gradually. We also show that the height of a susceptance step is proportional to the ac-bias frequency. These simulation results are in excellent qualitative agreement with recent experimental results. Moreover, we found that the transition from capacitive susceptance to inductive susceptance occurs with increasing gate voltage. The capacitive-inductive transition point is independent of the ac-bias frequency, but it does depend on the contact width.

著者

Asai Hidehiro Tachiki Masashi Kadowaki Kazuo

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.85, no.6, pp.064521, 2012-02

被引用文献数

42 18

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In this study, we numerically investigate the terahertz radiation from mesa-structured intrinsic Josephson junctions (IJJs) using a three-dimensional calculation model. We assume an in-phase mode of the phase differences and calculate electromagnetic fields inside and outside of the IJJs simultaneously. We consider the appearance of a hot spot in the mesa where jc locally decreases and investigate the change of the radiation power with varying hot-spot positions. The radiation powers for three different hot-spot positions are calculated as functions of voltage. We observe strong radiation when the ac Josephson frequency satisfies the cavity resonance condition. Transverse-magnetic modes TMm,n whose indices m and n are even appear regardless of the positions of hot spots. Meanwhile, the TMm,n cavity modes whose m or n are odd appear only when the hot spots break the reflectional symmetry of the mesa structure. Moreover, we calculate the radiation patterns emitted by the IJJs at these cavity resonance conditions. The radiation patterns reflect the existence of two types of internal modes, that is, a uniform background mode and a cavity resonance mode.

著者

Sivasubramanian V. Kojima S.

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.85, no.5, pp.054104, 2012-02

被引用文献数

20 7

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Brillouin spectra measurements have been carried out from 800 to 100 K for Pb(Sc1/2Ta1/2)O3 in back-scattering geometry. The frequency of the longitudinal acoustic (LA) phonon begins to show marked softening followed by an increase in its width below the intermediate temperature T* = 450 K. The gradual increase in the width of the LA phonon below T* is attributed to the electrostrictive coupling between the local polarization and strain. The frequency of the LA phonon mode exhibits anomalous behavior around 295 K, the phase transition temperature. The width of the LA phonon exhibits a sharp Landau-Khalatnikov-like maximum at 295 K. A central peak begins to appear around T* and exhibits critical slowing down upon further cooling. The relaxation times calculated for the LA phonon and central peak indicate the order-disorder nature of the phase transition.

著者

Maeshima Nobuya Hino Ken-ichi

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.85, no.20, pp.205305, 2012-05

被引用文献数

6 2

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An excitonic Floquet state in semiconductor superlattices (SLs) driven by an intense monochromatic laser is examined based on multichannel scattering theory, where this state is supported by a photon sideband attributed to a SL joint miniband. It is shown that the excitonic Fano resonance (FR) is caused by the coupling between photon sidebands, and thus one obtains the possibility of dynamic quantum control of this exciton state by means of laser tuning. This FR differs from the conventional FR observed in the original SLs without laser irradiation in that the latter FR results from the static Coulomb coupling between different SL joint minibands. Both the q parameter and the spectral width Γ associated with the excitonic FR of concern vary notably with a change of laser strength. In particular, it is found that the FR is made stable, that is, 1/

著者

Tomimoto Shinichi Kawana Keisuke Murakami Akira Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.85, no.23, pp.235320, 2012-06

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We have experimentally studied the spin dynamics of excitons, electrons, and trions in charge-tunable InP/InGaP quantum dots (QDs) excited by picosecond resonant laser pulses by observing the time-resolved Kerr rotation. In singly charged QDs, inversion of the spin polarization direction of doped electrons is found to be caused simply by variation in the pulse intensity, which is accompanied by an abrupt change of the spin coherence time. This phenomenon is reproduced by density-matrix calculations allowing for the reaction on the QD electron-trion four-level system during its coherent radiation emission. This result means that the optical coherence is another critical factor affecting electron spin coherence.