Journal of the Physical Society of Japan
Online ISSN : 1347-4073
Print ISSN : 0031-9015
ISSN-L : 0031-9015
Volume 76, Issue 8
Displaying 1-45 of 45 articles from this issue
  • Jin-Feng Jia, Shao-Chun Li, Yan-Feng Zhang, Qi-Kun Xue
    2007 Volume 76 Issue 8 Pages 082001
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Atomically flat metal ultrathin films grown on semiconductor substrates form a quasi-two-dimensional (2D) electronic system, which offers a great opportunity to explore novel properties induced by quantum size effects (QSE). In such a system, the motion of electrons in the film plane is essentially free, however, in the film normal direction it is confined, which leads to the quantized electronic states, i.e., quantum well states (QWS). The formation of QWS induces the redistribution of electrons and changes the electronic structure of the metal films and thus modifies their properties. By controlling the film thickness—the width of the confinement potential well, the QWS, together with the physical and chemical properties of the films can be engineered. In this article, we will summarize our recent studies on this problem in the Pb/Si(111) system, and discuss the perspectives in this area.
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  • Seiji Miyashita, Shu Tanaka, Masaki Hirano
    2007 Volume 76 Issue 8 Pages 083001
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Recently, interesting nonmonotonic time evolution has been pointed out in the experiments by Jönsson et al. and Jonsson et al. and also in the numerical simulation by Takayama and Hukushima, where the magnetic susceptibility does not monotonically relax to the equilibrium value, but moves to the opposite side. We study the mechanism of this puzzling nonmonotonic dynamical property in a frustrated Ising model in which the equilibrium correlation exhibits nonmonotonic temperature dependence (reentrant type). We study the time evolution of the spin correlation function after a sudden change of temperature. We find that the value of the correlation function shows nonmonotonic relaxation, and we analyze the mechanisms of the nonmonotonicity. We also point out that competition between different configurations generally causes nonmonotonic relaxation.
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  • Yohei Tutiya, Masahiro Kanai
    2007 Volume 76 Issue 8 Pages 083002
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    In this letter, we present exact shock solutions of a coupled system of delay differential equations, which was introduced as a traffic-flow model called car-following model. We use the Hirota method, originally developed in order to solve soliton equations. The relevant delay differential equations have been known to allow exact solutions expressed by elliptic functions with periodic boundary conditions. In the present work, however, shock solutions are obtained with open boundaries, representing the stationary propagation of a traffic jam.
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  • Tomoya Adachi, Keishi Kanada, Takehiro Saito, Akira Oosawa, Takayuki G ...
    2007 Volume 76 Issue 8 Pages 083701
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    The spin–lattice relaxation rate T1−1 of 1H-NMR has been measured in (CH3)2CHNH3Cu(ClxBr1−x)3 with x=0.88, which is reportedly a gapped system with a singlet ground state from the previous macroscopic magnetization and specific heat measurements, in order to investigate the bond randomness effect microscopically in the gapped composite Haldane system (CH3)2CHNH3CuCl3. It was discovered that the spin–lattice relaxation rate T1−1 in the present system includes both fast and slow relaxation parts indicative of the gapless magnetic ground state and the gapped singlet ground state, respectively. We discuss the obtained results with the previous macroscopic magnetization and specific heat measurements together with the microscopic μSR experiments.
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  • Takashi Tayama, Yuta Isobe, Toshiro Sakakibara, Hitoshi Sugawara, Yuji ...
    2007 Volume 76 Issue 8 Pages 083702
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    The effect of La impurities on the low-field (A phase) and high-field (B phase) ordered states in PrFe4P12 has been studied using dc magnetization measurements on a series of Pr1−xLaxFe4P12 (x=0, 0.01, 0.05, and 0.15). It is found that the long-range A phase order evolves to a short-range one at a very small La concentration between x=0.01 and 0.05. We also find clear evidence for the existence of ferromagnetism in the sample with x≥0.05. Remarkably, the ferromagnetic ordering temperature TC increases from 1.05 K for x=0.05 to 1.45 K for x=0.15, although the Weiss temperature decreases with increasing x. These observations indicate the competition between the A phase and ferromagnetic state, and the role of nesting on the A phase transition is emphasized.
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  • Hiroaki Matsueda, Sumio Ishihara
    2007 Volume 76 Issue 8 Pages 083703
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Motivated by the photoinduced phase transition in perovskite manganites, we examine charge and spin dynamics induced by photoirradiation. We calculate the transient optical absorption spectra of the extended double-exchange model by the density matrix renormalization group method. A charge-ordered insulator (COI) becomes metallic just after photoirradiation, and the metallic state tends to recover the initial COI. This recovery is accompanied by the suppression of an antiferromagnetic correlation in the COI and is remarkable near a phase boundary in the ground state. We discuss the implications of the recent pump-probe reflection and magneto-optical Kerr spectroscopy data.
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  • Ning Kang, Eisuke Abe, Yoshiaki Hashimoto, Yasuhiro Iye, Shingo Katsum ...
    2007 Volume 76 Issue 8 Pages 083704
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We report a study of the Aharonov–Bohm (AB) type oscillation of weak field magnetoresistance in an anti-dot lattice (ADL) of a two-dimensional hole system in the presence of strong spin–orbit interaction. The Fourier transform of the oscillations reveals non-monotonic evolutions with the different range of harmonic component, which are indicative of the existence of additional phase factors. A simulation considering Berry’s phase and the phase arising from the spin–orbit shift in the momentum space shows qualitative agreement with the experiment.
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  • Takashi Hotta
    2007 Volume 76 Issue 8 Pages 083705
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We discuss multipole properties of filled skutterudites containing heavy lanthanide Ln from a microscopic viewpoint on the basis of a seven-orbital Anderson model. For Ln = Gd, in contrast to naive expectation, quadrupole moments remain in addition to main dipole ones. For Ln = Ho, we find an exotic state governed by octupole moment. For Ln = Tb and Tm, no significant multipole moments appear at low temperatures, while for Ln = Dy, Er, and Yb, dipole and higher-order multipoles are dominant. We briefly discuss possible relevance of these multipole states with actual materials.
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  • Naoyuki Tateiwa, Yoshinori Haga, Tatsuma D. Matsuda, Shugo Ikeda, Etsu ...
    2007 Volume 76 Issue 8 Pages 083706
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We studied the pressure-induced superconductor CeIrSi3 with the non-centrosymmetric tetragonal structure under high pressure. The electrical resistivity and ac heat capacity were measured in the same run for the same sample. The critical pressure was determined to be Pc=2.25 GPa , where the antiferromagnetic state disappears. The heat capacity Cac shows both antiferromagnetic and superconducting transitions at pressures close to Pc. On the other hand, the superconducting region is extended to high pressures of up to about 3.5 GPa, with the maximum transition temperature Tsc=1.6 K around 2.5–2.7 GPa. At 2.58 GPa, a large heat capacity anomaly was observed at Tsc=1.59 K. The jump of the heat capacity in the form of ΔCacCac(Tsc) is 5.7±0.1. This is the largest observed value among previously reported superconductors, indicating the strong-coupling superconductivity. The electronic specific heat coefficient at Tsc is, however, approximately unchanged as a function of pressure, even at Pc.
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  • Kazuhiko Kuroki, Ryotaro Arita
    2007 Volume 76 Issue 8 Pages 083707
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    In the present study, we pin down the origin of the coexistence of the large thermopower and the low resistivity in NaxCoO2. It is revealed that not just the density of states (DOS), the effective mass, nor the band width, but the peculiar shape of the a1g band referred to as the “pudding mold” type, which consists of a dispersive portion and a somewhat flat portion, is playing an important role in this phenomenon. The present study provides a new guiding principle for designing good thermoelectric materials.
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  • Hiroshi Yamagami, Dai Aoki, Yoshinori Haga, Yoshichika Onuki
    2007 Volume 76 Issue 8 Pages 083708
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Electronic band structure calculations were carried out for a new neptunium superconductor NpPd5Al2 using a relativistic linear-augmented plane wave method within a local density approximation. The Fermi energy in the band structure is found to be located just on the 5f-bands of the j=5⁄2 state with the extremely narrow band width and therefore the theoretical electronic specific heat coefficient γb is extremely large, being γb=74.0 mJ/(K2·mol), which is compared to γ=390 mJ/(K2·mol) estimated experimentally. NpPd5Al2 is an uncompensated metal. The main electron Fermi surface corresponds to a doughnut-like flat Fermi surface centered at the Γ point which is connected with cylindrical Fermi surfaces elongated along the X–W–P line.
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  • Shunsuke Takemura, Yoshiyuki Fukumoto
    2007 Volume 76 Issue 8 Pages 083709
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    The magnetic properties of the spin doughnut Mo75V20, which is a frustrated magnetic cluster with 20 spins, are studied by using the finite-temperature Lanczos method. Our model Hamiltonian consists of one ten-spin ring and two five-spin rings with nearest-neighbor antiferromagnetic couplings J and J′′. The five-spin rings are coupled to the ten-spin ring by an antiferromagnetic exchange interaction J′ so as to form isosceles triangles of one J-bond and two J-bonds. By fitting the theoretical magnetic susceptibility with the experimental one measured by Müller et al., the exchange parameters are estimated as J=388 K, J=163 K, and J′′=81 K. The most interesting feature of this system is that the two five-spin rings behave as two free spins at low temperatures, although J>J′′. We also study the temperature dependence of the specific heat and find three peaks at TJ, J′′, and J⁄100.
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  • Yositake Takane, Katsunori Wakabayashi
    2007 Volume 76 Issue 8 Pages 083710
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    To study electron transport in disordered wires with the channel-number imbalance between two propagating directions, we consider the three-edge Chalker–Coddington model consisting of one right-moving and two left-moving edge channels of length L coupled by random tunneling. Since the imbalance makes one left-moving channel being perfectly conducting, the dimensionless conductances g and g′ for the left-moving and right-moving channels, respectively, differ from each other and satisfy g=g′+1. Using a supersymmetry approach, we obtain the asymptotic form of the ensemble average ⟨g⟩−1=⟨g′⟩ which decays exponentially with increasing L. It is shown that the corresponding decay length is four times shorter than that for the two-edge case. This result is in quantitative agreement with the existing random-matrix theory.
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  • Shin-ya Komura, Yoshihiro Yamazaki
    2007 Volume 76 Issue 8 Pages 083801
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    A model for the collective motion of granular particles induced by an interaction with interface motion is constructed. By numerical simulation, it is found that the model can qualitatively reproduce labyrinthine patterns that are quite similar to those experimentally obtained in the drying process of a water-granule mixture. It is found that the characteristic scale of the patterns is proportional to the inverse of the strength of dynamic friction for the particles. Moreover, the stick-slip motion of the interfaces is confirmed in our model.
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  • Norio Inui, Kazuya Nakamura, Yusuke Ide, Norio Konno
    2007 Volume 76 Issue 8 Pages 084001
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    A quantum cellular automaton, which is extended from a classical cellular automaton with Wolfram’s rule 150, is studied. In contrast to the classical cellular automaton, all possible configurations exist in a quantum superposition before measurement. We show that measuring the state of only one qubit significantly aeffects the time evolution in the quantum cellular automaton. In particular, we demonstrate that, occasionally, repeating the measurement enhances the appearance of the configurations found in the classical cellular automaton. The occurrence of this enhancement is primarily determined by the results of the measurement in the early time steps, and it is sustained by a feedback mechanism.
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  • Tetsuo Kurosaki, Miki Wadati
    2007 Volume 76 Issue 8 Pages 084002
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We investigate dynamical properties of bright solitons with a finite background in the F=1 spinor Bose–Einstein condensate (BEC), based on an integrable spinor model which is equivalent to the matrix nonlinear Schrödinger equation with a self-focusing nonlineality. We apply the inverse scattering method formulated under nonvanishing boundary conditions. The resulting soliton solutions can be regarded as a generalization of those under vanishing boundary conditions. One-soliton solutions are derived in an explicit manner. According to the behaviors at the infinity, they are classified into two kinds, domain-wall (DW) type and phase-shift (PS) type. The DW-type implies the ferromagnetic state with nonzero total spin and the PS-type implies the polar state, where the total spin amounts to zero. We also discuss two-soliton collisions. In particular, the spin-mixing phenomenon is confirmed in a collision involving the DW-type. The results are consistent with those of the previous studies for bright solitons under vanishing boundary conditions and dark solitons. To summarize, we establish the robustness and the usefulness of the multiple matter-wave solitons in the spinor BECs.
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  • Yoshimasa Matsuno
    2007 Volume 76 Issue 8 Pages 084003
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We develop a systematic procedure for constructing the multisoliton solutions of the short pulse (SP) model equation which describes the propagation of ultra-short pulses in nonlinear media. We first introduce a novel hodograph transformation to convert the SP equation into the sine-Gordon (sG) equation. With the soliton solutions of the sG equation, the system of linear partial differential equations governing the inverse mapping can be integrated analytically to obtain the soliton solutions of the SP equation in the form of the parametric representation. By specifying the soliton parameters, we obtain the multiloop and multibreather solutions. We investigate the asymptotic behavior of both solutions and confirm their solitonic feature. The nonsingular breather solutions may play an important role in studying the propagation of ultra-short pulses in an optical fibre.
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  • Kouji Ueda, Roman Krcmar, Andrej Gendiar, Tomotoshi Nishino
    2007 Volume 76 Issue 8 Pages 084004
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Critical behavior of the Ising model is investigated at the center of large scale finite size systems, where the lattice is represented as the tiling of pentagons. The system is on the hyperbolic plane, and the recursive structure of the lattice makes it possible to apply the corner transfer matrix renormalization group method. From the calculated nearest neighbor spin correlation function and the spontaneous magnetization, it is concluded that the phase transition of this model is mean-field like. One parameter deformation of the corner Hamiltonian on the hyperbolic plane is discussed.
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  • Narihisa Matsumoto, Daisuke Ide, Masataka Watanabe, Masato Okada
    2007 Volume 76 Issue 8 Pages 084005
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Synaptic connections are known to change dynamically. High-frequency presynaptic inputs induce decrease of synaptic weights. This process is known as short-term synaptic depression. The synaptic depression controls a gain for presynaptic inputs. However, it remains a controversial issue what are functional roles of this gain control. We propose a new hypothesis that one of the functional roles is to enlarge basins of attraction. To verify this hypothesis, we employ a binary discrete-time associative memory model which consists of excitatory and inhibitory neurons. It is known that the excitatory–inhibitory balance controls an overall activity of the network. The synaptic depression might incorporate an activity control mechanism. Using a mean-field theory and computer simulations, we find that the synaptic depression enlarges the basins at a small loading rate while the excitatory–inhibitory balance enlarges them at a large loading rate. Furthermore the synaptic depression does not affect the steady state of the network if a threshold is set at an appropriate value. These results suggest that the synaptic depression works in addition to the effect of the excitatory–inhibitory balance, and it might improve an error-correcting ability in cortical circuits.
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  • Hidekazu Tsuji, Masayuki Oikawa
    2007 Volume 76 Issue 8 Pages 084401
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Oblique interaction of two solitons of the same amplitude in an extended Kadomtsev–Petviashvili (EKP) equation, which is a weakly two-dimensional generalization of an extended Korteweg–de Vries (EKdV) equation, is investigated. This interaction problem is solved numerically under the initial and boundary condition simulating the reflection problem of the obliquely incident soliton due to a rigid wall. The essential parameters are given by Q*aQ and Ω*≡Ω⁄a1⁄2. Here, Q is the coefficient of the cubic nonlinear term in the EKP quation, a the amplitude of the incident soliton and Ω≡tanθi, θi being the angle of incidence. The numerical solutions for various values of these parameters reveal the effect of the cubic nonlinear term on the behavior of the waves generated by the interaction. When Q* is small, the interaction property is very similar to that of the Kadomtsev–Petviashvili equation. Especially, for relatively small Ω*, a new wave of large amplitude and of soliton profile called “stem” is generated. On the other hand, when Q* is close to 6, no stem is generated owing to the existence of amplitude restriction for the soliton solution.
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  • Makoto Hasegawa, Kazuaki Hanada, Kohnosuke Sato, Kazuo Nakamura, Hidek ...
    2007 Volume 76 Issue 8 Pages 084501
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    For initial plasma production, an induction electric field generated by applying voltage to a poloidal field (PF) coil system is used to produce a Townsend avalanche breakdown. When the avalanche margins are small, as for the International Thermonuclear Experimental Reactor (ITER) in which the induction electric field is about 0.3 V/m, the assistance of radio-frequency waves (RF) is provided to reduce the induction electric field required for reliable breakdown. However, the conditions of RF-assisted breakdown are not clear. Here, the effects of both RF and induction electric field on the RF-assisted breakdown are evaluated considering the electron loss. When traveling loss is the dominant loss, a simple model of an extended Townsend avalanche is proposed. In this model, the induction electric field required for RF-assisted breakdown can be decreased to half that required for induction breakdown.
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  • Kimitaka Itoh, Shinichiro Toda, Akihide Fujisawa, Sanae-I. Itoh, Masat ...
    2007 Volume 76 Issue 8 Pages 084502
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    In this article, we discuss the turbulent transport of the momentum (component perpendicular to main magnetic field) in toroidal plasmas. The fluctuations in the quasi-linear response tend to induce the shear of mean poloidal flow, as in the case of driving the zonal flows. The case in which the shear of mean poloidal flow and the mesoscale zonal flows coexist is studied. Zonal flows, which have a more rapid growth rate than the shear of mean flow, reaches the nonlinear saturation state, and modify the interaction between microscopic fluctuations and the shear of mean flow. Taking into account the screening effect of zonal flows, the shear of mean poloidal flow is subject to diffusive damping. Responses to the external force are also studied. A new bifurcation (from the state where mesoscale zonal flows are dominant to the one where zonal flows are suppressed) is found.
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  • Nguyen Van Hung, Paolo Fornasini
    2007 Volume 76 Issue 8 Pages 084601
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Anharmonic effective pair potentials and effective local force constants have been studied for fcc metals, assuming an interaction Morse potential and taking into account the influence of nearest neighbours of absorber and backscatterer atoms. Analytical expressions for the first three extended X-ray absorption fine structure (EXAFS) cumulants, as well as for the atomic mean square displacements, have been derived as a function of the Morse parameters. Numerical results for copper and nickel are compared with experimental data. A good agreement is found for the second cumulant. Non-negligible discrepancies are instead found for the first and third cumulants, which are tentatively attributed to the central nature of the Morse potential, which neglects many-body effects.
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  • Minoru Soda, Taketo Moyoshi, Yukio Yasui, Masatoshi Sato, Kazuhisa Kak ...
    2007 Volume 76 Issue 8 Pages 084701
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Magnetic, thermal and neutron diffraction measurements have been carried out on single-crystal samples of LuBaCo4O7 with kagome and triangular lattices of Co spins. Anomalies of the magnetic susceptibility have been found at temperatures Ta1∼70 K and Ta2∼50 K. We have also found a second-order structural transition at Tc1∼165 K and a first-order structural transition at Tc2∼105 K (Tc2′∼80 K) with increasing (decreasing) temperature. Two groups of magnetic diffuse scattering, corresponding to the short-range correlations with the commensurate (C) and incommensurate (IC) periods, have been found to exist below the structural transition temperature Tc2 (Tc2′). The reflections with the C and IC periods originate from the moments on the kagome and triangular lattices, respectively. At 10 K, the magnetic correlation length is estimated to be ∼100 Å. The structural changes of LuBaCo4O7 and YBaCo4O7 differ significantly, which seems to induce the differences in their magnetic properties.
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  • Takashi Hotta
    2007 Volume 76 Issue 8 Pages 084702
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We discuss Kondo behavior of a conduction electron system coupled with local optical phonon by analyzing the Anderson–Holstein model with the use of a numerical renormalization group (NRG) method. There appear three typical regions due to the balance between Coulomb interaction Uee and phonon-mediated attraction Uph. For Uee>Uph, we observe the standard Kondo effect concerning spin degree of freedom. Since the Coulomb interaction is effectively reduced as UeeUph, the Kondo temperature TK is increased when Uph is increased. On the other hand, for Uee<Uph, there occurs the Kondo effect concerning charge degree of freedom, since vacant and double occupied states play roles of pseudo-spins. Note that in this case, TK is decreased with the increase of Uph. Namely, TK should be maximized for UeeUph. Then, we analyze in detail the Kondo behavior at Uee=Uph, which is found to be explained by the polaron Anderson model with reduced hybridization of polaron and residual repulsive interaction among polarons. By comparing the NRG results of the polaron Anderson model with those of the original Anderson–Holstein model, we clarify the Kondo behavior in the competing region of UeeUph.
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  • Jin-Young Son, Kou Takubo, Daisuke Asakura, James W. Quilty, Takashi M ...
    2007 Volume 76 Issue 8 Pages 084703
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We report an x-ray photoemission spectroscopy study of spin-crossover complex [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) to identify the electronic changes due to temperature-induced and photoinduced spin transitions. The energy difference between the Fe 2p and F 1s peaks is found to correlate with the low-spin (LS) to high-spin (HS) transition of [Fe(ptz)6](BF4)2. The Fe 2p3⁄2 spectrum of the HS state is accompanied by a charge-transfer satellite and is analyzed by configuration-interaction cluster-model calculations. Based on the electronic-structure parameters from the cluster-model analysis, the stability of the LS state against the HS state is calculated as a function of the Fe–N bond length. A drastic bond-length reduction is required to stabilize the LS state and is also related to the observed core-level shift. The postillumination spectral shape well reproduces the spectrum before the laser illumination, suggesting that the surface of the Fe complex is chemically stable against the laser illumination for the photoinduced spin transition.
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  • Masaki Maeda, Masahiko Washihira, Hiroyuki Hashimoto, Ikuo Suzuki
    2007 Volume 76 Issue 8 Pages 084704
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    An iterative method of determining nonlinear elastic constants of a rectangular ceramic bar poled and excited in the direction of its thickness in the length extensional mode has been developed. The nonlinear elastic compliance coefficients s111E and s1111E were successfully determined by the present iterative nonlinear elastic GB fitting method. The shifts and deformation of the resonance spectra at high measuring voltages were simulated by taking the elastic higher harmonics into account. The jump in the admittance spectra at higher applied voltages could be fitted by employing two sets of admittance formulas.
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  • Shohei Kawabata, Yukio Yasui, Yoshiaki Kobayashi, Masatoshi Sato
    2007 Volume 76 Issue 8 Pages 084705
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Experimental studies have been carried out on geometrically frustrated quantum spins of Cu2+ ions of Hg2Cu2F6S with pyrochlore structure. No magnetic and structural phase transitions have been observed in the temperature (T) range down to 2 K. The magnetic correlation length ξ deduced from the T dependence of the NMR longitudinal relaxation rate is found to be as small as the atomic distance even at T much lower than the Cu2+–Cu2+ exchange coupling energy J∼100 K. With decreasing T from ∼200 K, the NMR Knight shift first increases, has a maximum at ∼90 K and decreases rather rapidly. This low-T behavior suggests that the system approaches the spin-liquid state. The upturn of the uniform magnetic susceptibility observed in the very low T region with decreasing T is due to lattice imperfections, and effects of these imperfections can be considered to be an origin of the large broadening of the NMR spectra observed at low T.
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  • Tomohiro Otsuka, Eisuke Abe, Shingo Katsumoto, Yasuhiro Iye, Gyong L. ...
    2007 Volume 76 Issue 8 Pages 084706
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We have studied the Fano effect in a few-electron quantum dot side-coupled to a quantum wire. The conductance of the wire, which shows an ordinal staircase-like quantization without the dot, is modified through the interference (the Fano effect) and the charging effects. These effects are utilized to verify the exhaustion of electrons in the dot. The “addition energy spectrum” of the dot shows a shell structure, indicating that the electron confinement potential is fairly circular. A rapid sign inversion of the Fano parameter on the first conductance plateau with the change of the wire gate voltage has been observed, and explained by introducing a finite width of dot–wire coupling.
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  • Keizo Yasukawa, Youtarou Takahashi, Tohru Suemoto, Susumu Kurita
    2007 Volume 76 Issue 8 Pages 084707
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We report on the femtosecond time-resolved luminescence spectroscopy in a quasi-one-dimensional halogen-bridged mixed valence platinum complex, [Pt(en)2][Pt(en)2Br2](ClO4)4 at low temperatures. From the temperature dependence of the lifetime of the self-trapped excitons (STEs), we estimated the potential barrier height in the direction to the non-radiative path to be 99±10 meV. The observed time-evolutions of the luminescence excited by 1.57 eV photons at 4 K show complicated behavior, which is considerably different from that at room temperature. From Fourier transform of the time domain data, a low frequency mode was found at 0.7 THz in addition to the main mode at 3.4 THz, which has been assigned to the motion of the halogen ions in the self-trapped excitons. Incorporating two modes into the model based on the theory of resonant secondary radiation, the experimental data are well reproduced. This result shows that the wavepacket moves on the two-dimensional adiabatic potential surface of the self-trapped exciton, following a Lissajous-like trajectory. Under excitation with 3.14 eV photons, the degradation of the coherence and existence of long-lived excess vibrational energy were found. These results give information about the STE formation process from the free exciton state.
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  • Takehiro Saito, Takahiko Sasaki, Takao Suzuki, Akira Oosawa, Takayuki ...
    2007 Volume 76 Issue 8 Pages 084708
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    High-field magnetic torque measurement was carried out in the spin gap system (CH3)2CHNH3CuCl3. It was observed that the magnitude of the magnetic torque τ is almost zero until the critical field and then increases rapidly, which indicates the existence of magnetic quantum phase transition from the spin-gap phase to the field-induced magnetic ordered phase. From the temperature dependence of τ, a cusplike minimum indicative of the Bose–Einstein condensation (BEC) of magnons was observed for HC-plane, while a kink anomaly was observed for HA-plane at the field-induced transition temperature. It was found that this difference in behavior between HA-plane and HC-plane can be interpreted as the breaking of the magnon BEC picture due to a rotational symmetry breaking. An additional anomaly was also observed in the field-induced magnetic ordered phase at the intermediate angle of the applied magnetic field in the A-plane and discussed in terms of spin-flop transition, spin-reorientation transition, and spin–lattice correlations. In order to investigate spin–lattice correlations in (CH3)2CHNH3CuCl3, we performed magnetostriction measurement and observed that magnetostriction appears with the disappearance of the spin gap.
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  • Takeshi Aimi, Masatoshi Imada
    2007 Volume 76 Issue 8 Pages 084709
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We examine Gaussian-basis Monte Carlo (GBMC) method introduced by Corney and Drummond. This method is based on an expansion of the density-matrix operator \\hatρ by means of the coherent Gaussian-type operator basis \\hatΛ and does not suffer from the minus sign problem. The original method, however, often fails in reproducing the true ground state and causes systematic errors of calculated physical quantities because the samples are often trapped in some metastable or symmetry broken states. To overcome this difficulty, we combine the quantum-number projection scheme proposed by Assaad, Werner, Corboz, Gull, and Troyer in conjunction with the importance sampling of the original GBMC method. This improvement allows us to carry out the importance sampling in the quantum-number-projected phase-space. Some comparisons with the previous quantum-number projection scheme indicate that, in our method, the convergence with the ground state is accelerated, which makes it possible to extend the applicability and widen the range of tractable parameters in the GBMC method. The present scheme offers an efficient practical way of computation for strongly correlated electron systems beyond the range of system sizes, interaction strengths and lattice structures tractable by other computational methods such as the quantum Monte Carlo method.
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  • Shin-ichi Kimura, Noriaki Kimura, Haruyoshi Aoki
    2007 Volume 76 Issue 8 Pages 084710
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    The temperature dependence of the optical conductivity spectrum of an itinerant ferromagnetic material, ZrZn2, was obtained and the dynamical effective mass and the scattering rate were derived. A renormalized Drude peak with a heavy effective mass developed at low temperatures. The effective mass rapidly increased below hω=10 meV and at 10 K it was observed to be 3 times heavier than that at 300 K. The scattering rate also rapidly decreased below 10 meV at 10 K. These results indicate the creation of heavy quasiparticles at low temperatures due to spin fluctuations.
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  • Soichiro Sasaki, Koichi Kusakabe, Masaaki Geshi, Hitose Nagara, Naoshi ...
    2007 Volume 76 Issue 8 Pages 084711
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We have studied the magnetic properties of LaCo2 in a MgCu2 structure by first-principles electronic structure calculation. The Korringa–Kohn–Rostoker method is utilized to evaluate the spin-moment dependence of total energy at zero temperature. When the external magnetic field is zero, the ground state has no net spin as determined by local-spin density approximation (LSDA). The obtained result is the same when the energy functional of a generalized gradient approximation is used. The fixed spin moment method, which successfully confirms the metamagnetic transition of YCo2 in LSDA, is applied to LaCo2. Our calculation suggests the occurrence of metamagnetic transition in LaCo2.
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  • Daisuke Uematsu, Masatoshi Sato
    2007 Volume 76 Issue 8 Pages 084712
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Magnetic studies have been carried out on Cu3(TeO3)2Br2 with diamond lattice of spin 1/2 Cu2+ ions. By using the high temperature expansion of the magnetic susceptibility, the exchange coupling constants have been determined to be J1kB=280±4 K (antiferromagnetic) and J2kB=−23±1 K (ferromagnetic) for two kinds of Cu–Cu pairs formed by the connections with edge and diagonal lines of a diamond, respectively. It indicates that each diamond chain has the ferrimagnetic state in the reported phase diagram [Takano et al.: J. Phys.: Condens. Matter 8 (1996) 6405]. The observed magnetic behavior can be well understood by this characteristic. Because J2 is ferromagnetic, the present system does not have magnetic frustration effects.
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  • Takafumi Kashi, Yukio Yasui, Taketo Moyoshi, Masatoshi Sato, Naoki Iga ...
    2007 Volume 76 Issue 8 Pages 084713
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Measurements of the magnetization and specific heat have been carried out for Co2TeO3Cl2 with a sawtooth lattice having two crystallographically distinct Co2+ sites. A powder neutron diffraction study has revealed that two magnetic transitions take place at TN1∼73 K and TN2∼15 K. Below TN1, Co spins at the apical sites of the sawtooth lattice order antiferromagnetically with their directions parallel to the b-axis. Co spins at the basal sites order antiferromagnetically at TN2 with their directions canted in the ab-plane from the a-axis by Dzyaloshinski–Moriya interaction. At this transition, no change in the spin structure at the apical sites has been observed. Discussion on possible origins of these complicated spin structure is presented.
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  • Shun Tonooka, Hiroki Nakano, Koichi Kusakabe, Naoshi Suzuki
    2007 Volume 76 Issue 8 Pages 084714
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We have found a set of order parameters that characterize the plateau state of the (S=1,1⁄2) spin chain system. The order parameters are regarded as extensions of the string order parameter of the S=1 antiferromagnetic Heisenberg spin chain. The extension is made by mapping the Sz=1⁄2 (Sz=−1⁄2) state of an S=1⁄2 spin onto the Sz=0 (Sz=−1) state of an S=1 spin. Numerical data indicate that, when single-ion anisotropy D is introduced, all the order parameters decrease continuously and vanish in the large-D region. Our estimate of the critical value Dc for the phase transition agrees well with the phase boundary determined by a previous level spectroscopy analysis. We present estimates of three critical exponents that are associated with the order parameters. A scaling relation between the exponents holds well in the present system.
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  • Toshiyuki Izumi, Yoshiki Imai, Tetsuro Saso
    2007 Volume 76 Issue 8 Pages 084715
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Magnetization and energy gap of Kondo insulator YbB12 are calculated theoretically based on the previously proposed tight-binding model composed of Yb 5dε and 4f Γ8 orbitals. It is found that magnetization curves are almost isotropic, naturally expected from the cubic symmetry, but that the gap-closing field has an anisotropy: the gap closes faster for the field in (100) direction than in (110) and (111) directions, in accord with the experiments. This is qualitatively understood by considering the maximal eigenvalues of the total angular momentum operators projected on each direction of the magnetic field. But the numerical calculation based on the band model yields better agreement with the experiment.
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  • Masahiro Matsumura, Genki Hyoudou, Machiko Itoh, Harukazu Kato, Takash ...
    2007 Volume 76 Issue 8 Pages 084716
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    Sb nuclear quadrupole resonances (NQR) have been performed on the alkaline-earth filled skutterudite AFe4Sb12 (A=Ca, Sr, and Ba) being close to ferromagnetic instabiity. No hyperfine splitting in NQR spectrum even at the lowest temperature (1.5 K) confirms an absence of uniform ferromagnetic order in these compounds. Above about T*∼70 K, the nuclear spin–lattice relaxation rate divided by temperature, 1⁄T1T, follows an empirical relationship 1⁄T1T=a+bχspin with respect to the Curie Weiss (CW) like T-dependence of the static spin susceptibility χspin. The correlation between 1⁄T1T and χspin is consistent with the self-consistent renormalization (SCR) theory for the three-dimensional (3D) nearly ferromagnet despite the relatively large contribution of the first term not correlating to χspin. However, below about T*, 1⁄T1T exhibits a gap-like suppression with a gap magnitude of about 65 K. This unusual suppression of the low energy spin excitations is contrasting to the case in LaFe4Sb12, and a characteristic feature in the filled skutterudites with divalent filler ion. The pseudo-gap structure near the Fermi level is proposed for the hybridized Fe3d–Sb5p band (mainly partial p state) for the unusual suppression.
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  • Eiichi Matsuoka, Takashi Tayama, Toshiro Sakakibara, Zenji Hiroi, Naok ...
    2007 Volume 76 Issue 8 Pages 084717
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    From the measurements of magnetization and specific heat, we constructed BT phase diagrams of single-crystalline DyPd3S4 which is known to exhibit antiferroquadrupolar (AFQ) and antiferromagnetic (AFM) ordering at low temperatures. The phase diagrams are found to be highly anisotropic and re-entrant, which are typical of rare-earth compounds exhibiting multipolar ordering. The crystalline electric field (CEF) scheme of Dy3+ in DyPd3S4 was deduced from the specific heat and magnetization measurements of the Y-diluted compounds Dy1−xYxPd3S4 (0.1≤x≤0.9) and discussed in detail. The CEF ground state was determined to be the orbitally-degenerated quartet Γ67(1), and the overall splitting width was estimated to be about 104 K. No correlation was found between the anisotropy of TQ and that of the Zeeman splitting width of the ground quartet Γ67(1).
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  • Kohtaro Yamada, Takao Ohta
    2007 Volume 76 Issue 8 Pages 084801
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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    We investigate the properties of a planer interface separating a lamellar structure and a gyroid structure in AB-type diblock copolymers. Depending on the relative orientation between lamellar and interface planes, we find that there are three different structures of an interface. Motion of an interface invading the unstable state is also studied numerically.
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  • Hiroyuki Mitamura, Hiroaki Ueda, Hiroko Aruga Katori, Shojiro Takeyama ...
    2007 Volume 76 Issue 8 Pages 085001
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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  • Yoshiyuki Yoshida, Shin-Ichi Ikeda, Naoki Shirakawa
    2007 Volume 76 Issue 8 Pages 085002
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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  • Hirotaka Manaka, Yusuke Miyashita, Yusuke Watanabe, Takatsugu Masuda
    2007 Volume 76 Issue 8 Pages 085003
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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  • Takashi Uchida, Yoshiro Kakehashi
    2007 Volume 76 Issue 8 Pages 087001
    Published: August 15, 2007
    Released on J-STAGE: August 12, 2010
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