著者

Kakeya I. Wada T. Nakamura R. Kadowaki K.

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.72, pp.014540, 2005-07

被引用文献数

14 10

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Josephson plasma excitations in the high-Tc superconductor Bi2Sr2CaCu2O8+delta have been investigated in a wide microwave frequency region (9.8–75 GHz)—in particular, in a magnetic field applied parallel to the ab plane of the single crystal. In sharp contrast to the case for magnetic fields parallel to the c axis or tilted from the ab plane, it was found that there are two kinds of resonance modes, which are split in energy and possess two distinctly different magnetic field dependences. One always lies higher in energy than the other and has a shallow minimum at about 0.8 kOe, then increases linearly with magnetic field. On the other hand, another mode begins to appear only in a magnetic field (from a few kOe and higher) and has a weakly decreasing tendency with increasing magnetic field. By comparing with a recent theoretical model the higher-energy mode can naturally be attributed to the Josephson plasma resonance mode propagating along the primitive reciprocal lattice vector of the Josephson vortex lattice, whereas the lower-frequency mode is assigned to the novel phase collective mode of the Josephson vortex lattice, which has never been observed before.

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

---

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

---

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

---

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

---

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

Nomura S. Yamaguchi M. Akazaki T. Tamura H. Maruyama T. Miyashita S. Hirayama Y.

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.76, no.20, pp.201306, 2007-11

被引用文献数

8 7

---

Both the electron and the optically created hole effective masses are found to be density dependent in a two-dimensional electron system of a GaAs/Al0.33Ga0.67As back-gated quantum well by magnetophotoluminescence spectroscopy. We show that the density-dependent electron effective mass increases with a decrease in the electron density (ns) to ns<1×1011 cm−2. It is found that the electron effective masses determined from the lowest and the second Landau levels are larger than those from the higher Landau levels. The hole effective mass is found to increase with a decrease in ns and the hole is found to localize at ns<3×1010 cm−2. We observe an upward convex curve of the photoluminescence peak energy at 2<ν<3 depending on the electron-hole distance divided by the magnetic length. These results clearly show the important roles of both electron-electron and electron-hole interactions in the recombination of a valence hole with a high-quality two-dimensional electron system.

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

---

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

---

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

---

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

---

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

---

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

---

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

---

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

---

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.