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Ju Hyun Park, Ho-Youl Jung, Chang Hyeon Park
2008 Volume 77 Issue 5 Pages
053001
Published: May 15, 2008
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In this Letter, the design problem of state estimator for a class of discrete-time neural networks is studied. A delay-independent linear matrix inequality (LMI) criterion for existence of the estimator is derived by using the Lyapunov method. The criterion can be easily solved by various convex optimization algorithms. A numerical example with simulation results is given to show the effectiveness of proposed method.
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Takeshi Akinaga, Masako Sugihara-Seki, Tomoaki Itano
2008 Volume 77 Issue 5 Pages
053401
Published: May 15, 2008
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An electrostatic model for osmotic flow through circular cylindrical pores is developed to describe the reflection coefficient for the membrane transport in the presence of surface charges on the pore wall and the solute. For a spherical solute placed at an arbitrary radial position in the pore, the electrical potential was computed by a spectral element method applied to the Poisson–Boltzmann equation together with the condition of electrical neutrality. The interaction energy between the surface charges was used to estimate the osmotic reflection coefficient. The proposed model predicts that even for a small Debye length compared to the pore radius, the repulsive electrostatic interaction between the surface charges could significantly increase the osmotic flow through the pore.
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Yoshiaki Kato, Masashi Kosaka, Hiromi Nowatari, Yuta Saiga, Atsushi Ya ...
2008 Volume 77 Issue 5 Pages
053701
Published: May 15, 2008
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We present the highly frustrated phenomena observed in YbAl
3C
3 with a spin gap. The inelastic neutron spectra of low-lying magnetic excitations at low temperatures are quite similar to those of SrCu
2(BO
3)
2 and (CuCl)LaNb
2O
7 that exhibit a dimer ground state in a two-dimensional quantum spin system. The structure of the first excited triplet state is directly clarified by inelastic neutron scattering experiments with a higher energy resolution. Furthermore, it becomes clear that the characteristic magnetic excitations remain within a high-temperature region. The result of the specific heat measurement also shows an anomaly associated with the formation of dimerized Yb ions. We conclude that a long-range magnetic order is suppressed by a large geometrical spin frustration, resulting in the stabilization of the dimer ground state.
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Toshiyuki Fujita, Yoshikuni Fujimoto, Seitaro Mitsudo, Toshitaka Ideha ...
2008 Volume 77 Issue 5 Pages
053702
Published: May 15, 2008
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We have studied a commensurate to incommensurate transition in the novel helimagnet CuB
2O
4 by means of high-frequency ESR. In the commensurate weak ferromagnetic (CWF) state, two ESR modes have been found at low temperatures, which are explained in terms of an electron paramagnetic resonance of the Cu
2+ spins at 8
d-sites and an antiferromagnetic resonance of the magnetically ordered Cu
2+ spins at 4
b-sites with the Dzyaloshinskii–Moriya (DM) interaction. The DM interaction among the Cu
2+ spins at 4
b-sites that carries the CWF state is evaluated quantitatively. The incommensurate helimagnetic state is discussed in terms of the magnetic frustration.
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Yuji Matsumoto, Motoki Sugi, Noriaki Kimura, Takemi Komatsubara, Haruy ...
2008 Volume 77 Issue 5 Pages
053703
Published: May 15, 2008
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We report the de Haas–van Alphen (dHvA) effect studies above metamagnetic transition fields in Ce
xLa
1−xRu
2Si
2. Three dHvA oscillations β′, γ′, and δ are observed for all the samples from
x=0.0 to 1.0. The frequencies and effective masses of these oscillations vary smoothly with the Ce concentration from LaRu
2Si
2 to CeRu
2Si
2. The signal magnitude is reduced only by a factor of approximately 1/10 at most even in alloys with a high concentration of Ce. These behaviors are the same as those observed in the alloys of the compounds with localized f electrons, indicating that the f electron maintains localized nature above the metamagnetic transitions.
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Hiroto Adachi, Manfred Sigrist
2008 Volume 77 Issue 5 Pages
053704
Published: May 15, 2008
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The effects of low carrier density and carrier compensation on mixed-state thermal transport are investigated beyond the quasiclassical approximation. It is shown that, contrary to the usual observations, the interplay of the two effects leads to an
increase in the thermal conductivity immediately below the upper critical field
Hc2. Our result can account for the anomalous behavior of the mixed-state thermal conductivity near
Hc2 recently observed in URu
2Si
2.
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Atsushi Ishizumi, Emiko Jojima, Aishi Yamamoto, Yoshihiko Kanemitsu
2008 Volume 77 Issue 5 Pages
053705
Published: May 15, 2008
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The controlled doping of impurities into semiconductor nanocrystals is essential for producing and enhancing optical and magnetic functionalities. We fabricated Mn
2+-doped CdS nanocrystals overcoated with a ZnS shell layer (CdS:Mn/ZnS core/shell nanocrystals) and studied photoluminescence (PL) properties of CdS:Mn/ZnS core/shell nanocrystals with different Mn
2+ concentrations. The intensity and decay profile of the Mn
2+ intra-3d transition PL are determined by the Mn
2+ concentration. Our findings suggest that the Mn
2+ concentration dependence of the PL dynamics is caused by exchange-coupled Mn pairs in nanocrystals.
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Yohta Hara, Kazuya Miyagawa, Kazushi Kanoda, Mina Shimamura, Biao Zhou ...
2008 Volume 77 Issue 5 Pages
053706
Published: May 15, 2008
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We present the results of a
1H NMR study of the single-component molecular conductor, [Au(tmdt)
2]. A steep increase in the NMR line width and a peak formation of the nuclear spin–lattice relaxation rate, 1⁄
T1, were observed at around 110 K. This behavior provides clear and microscopic evidences for a magnetic phase transition at considerably high temperature among organic conductors. The observed variation in 1⁄
T1 with respect to temperature indicates the highly correlated nature of the metallic phase.
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Shin-ichi Hikino, Michiyasu Mori, Saburo Takahashi, Sadamichi Maekawa
2008 Volume 77 Issue 5 Pages
053707
Published: May 15, 2008
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We propose the phase dynamics induced by spin waves in a superconductor/ferromagnet/superconductor (SC/FM/SC) Josephson junction. The resistively shunted junction (RSJ) model, which describes the dynamics of superconducting phase difference, is extended to include the spin wave excitation by ferromagnetic resonance (FMR) using the gauge invariant phase difference between two
s-wave superconductors. The current–voltage characteristics show step structures when the magnetization in FM is driven by tuning the microwave frequency to FMR in the SC/FM/SC junction. The result presents a new route to observe the spin wave excitation using the Josephson effect.
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Takuro Katsufuji, Takehito Suzuki, Haruki Takei, Masao Shingu, Kenichi ...
2008 Volume 77 Issue 5 Pages
053708
Published: May 15, 2008
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Spinel FeV
2O
4 with orbital degrees of freedom both at the Fe
2+ and V
3+ ions exhibits successive structural phase transitions. We studied this structural and magnetic properties as well as the correlation between the two. We found that the crystalline domains in the tetragonal or orthorhombic phases can be aligned by the applied magnetic field, inducing a large magnetostriction (∼1%). The competition and cooperation of Fe
2+ and V
3+ orbitals is the possible origin of these intriguing properties in FeV
2O
4.
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Shoji Ishibashi, Kiyoyuki Terakura, Hideo Hosono
2008 Volume 77 Issue 5 Pages
053709
Published: May 15, 2008
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The electronic and magnetic properties of the mother material LaOFeAs of new superconductors have been carefully studied using first-principles electronic structure calculations based on the generalized gradient approximation in the density functional theory. The present calculation predicts that the ground state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment alignment leading to orthorhombic symmetry of the crystal. In this particular magnetic state, the density of states at the Fermi level is very small. On the other hand, LaOFeP has turned out to be paramagnetic and a good metal. Implications of the results regarding the experimental observations will also be presented.
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Yuta Saiga, Kazuyuki Matsubayashi, Tetsuya Fujiwara, Masashi Kosaka, S ...
2008 Volume 77 Issue 5 Pages
053710
Published: May 15, 2008
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We have measured the electrical resistivity of a single crystal of YbCo
2Zn
20 at pressures up to 2.37 GPa and at temperatures from 50 mK to 300 K. Above a critical pressure
Pc (∼1 GPa), we have found a resistivity anomaly at
TM (∼0.15 K at 1 GPa) that increases with the pressure. At the ambient pressure, the system shows a nonmagnetic ground state described by the Fermi-liquid model. The
T2 coefficient of the electrical resistivity
A strongly increases with the pressure upon approaching
Pc. However, in the vicinity of
Pc, the temperature dependence of the resistivity deviates from the Fermi-liquid description. These observations suggest that the application of hydrostatic pressure induces a magnetically ordered state for
P≥
Pc and
T≤
TM.
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Yasuo Narumi, Yoshikazu Tanaka, Noriki Terada, Martin Rotter, Koichi K ...
2008 Volume 77 Issue 5 Pages
053711
Published: May 15, 2008
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The results of magnetization and synchrotron X-ray diffraction measurements on Gd
5Ge
3 subject to a pulsed high magnetic field are reported. When the magnetic field,
H, is applied parallel to the
a-axis, a sudden increase in magnetization,
M, has been observed at ∼3.8 T. For
H||
c,
M increases monotonically with
H. The magnetization becomes saturated above about 30 T for the two orientations. The lattice constant,
a elongates, while
c shrinks with increasing
H. We calculate the exchange energy under applied magnetic fields based on the proposed magnetic structure assuming an antiferromagnetic exchange interaction in the
a–
a plane and a ferromagnetic one along the
c-axis. We are successful in explaining the opposing behavior of the lattice deformations. Our calculation shows that the change in the lattice constants is proportional to
M2, in agreement with the observation.
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Kwok Wing Chow, Chun Kit Lam, Kaliyaperumal Nakkeeran, Boris Malomed
2008 Volume 77 Issue 5 Pages
054001
Published: May 15, 2008
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A class of complex Ginzburg–Landau (CGL) equations with variable coefficients is solved exactly by means of the Hirota bilinear method. Two novel features, elaborated in recent works on the bilinear method, are incorporated. One is a modified definition of the bilinear operator, which has been used to construct pulse, hole and front solutions for equations with constant coefficients. The other is the usage of time- or space-dependent wave numbers, which was employed to handle nonlinear Schrödinger (NLS) equations with variable coefficient. One-soliton solutions of the CGL equations with variable coefficients are obtained in an analytical form. A restriction imposed by the method is that the coefficient of the second-order dispersion must be real. However, nonlinear, loss (or gain) is permitted. A simple example of an exponentially modulated dispersion profile is worked out in detail to illustrate the principle. The competition between the linear gain and nonlinear loss, and
vice versa, is investigated. The analytical solutions for solitary pulses are tested in direct simulations. The amplified pulses are very robust, provided that the linear gain is reasonably small. The results may be implemented in soliton fiber lasers.
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Michio Otsuki
2008 Volume 77 Issue 5 Pages
054002
Published: May 15, 2008
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We perform an extensive molecular dynamics simulation in order to elucidate the rheological properties of fluids confined in a small periodic box. We find that the shear viscosity η decreases as \\dotγ
−2⁄3 with the shear rate \\dotγ when the system size
L is relatively small, while η is independent of \\dotγ when the system size is sufficiently large. By performing a systematic finite-size scaling analysis, we demonstrate that this system size dependence can be observed when the system size becomes comparable to the length scale that characterizes the cooperative motion of particles.
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Hidetsugu Sakaguchi, Hironobu Takeshita
2008 Volume 77 Issue 5 Pages
054003
Published: May 15, 2008
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We numerically study the symmetry breaking instabilities of vortex patterns in a rotating harmonic potential using a type of Ginzburg–Landau equation. The configurations of vortex lattices change markedly by the symmetry-breaking instabilities, and then, some vortices move away from the confinement potential, which leads to the annihilation of vortices. The symmetry-breaking instabilities and the instabilities of vortex nucleation determine the parameter region of stable vortex patterns. We verify that the symmetry-breaking instabilities also occur in a type of complex Ginzburg–Landau equation.
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Kenji Kajiwara, Yasuhiro Ohta
2008 Volume 77 Issue 5 Pages
054004
Published: May 15, 2008
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Casorati determinant solution to the non-autonomous discrete KdV equation is constructed by using the bilinear formalism. We present three different bilinear formulations which have different origins.
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Takao Yoshinaga, Kentaro Kan
2008 Volume 77 Issue 5 Pages
054005
Published: May 15, 2008
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We examine spatially periodic solutions of a nonlinear evolution equation describing the Marangoni instability of a planar liquid sheet with both free boundaries. The equation includes energy supply and dissipation terms and exhibits long wave instability. Numerical analysis shows that steady, oscillating and blowup solutions exist for particular parameters of the equations. The reduced amplitude equation explains that the steady and blowup properties of the solutions are determined by the parameter ratios in the original equation.
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Hayato Shiba, Nobuyasu Ito
2008 Volume 77 Issue 5 Pages
054006
Published: May 15, 2008
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Heat conduction in three-dimenisional nonlinear lattice models is studied using nonequilibrium molecular dynamics simulations. We employ the Fermi–Pasta–Ulam-β model, in which nonlinearity exists in the interaction of the biquadratic form. It is confirmed that the thermal conductivity, the ratio of the energy flux to the temperature gradient, diverges with increasing system size up to 128×128×256 lattice sites. This size corresponds to nanoscopic to mesoscopic scales of approximately 100 nm. From these results, we conjecture that the energy transport in insulators with perfect crystalline order exhibits anomalous behavior. The effects of the lattice structure, random impurities, and the natural length in interactions are also examined. We find that fcc lattices display stronger divergence than simple cubic lattices. When impurity sites of infinitely large mass, which are thus fixed, are randomly distributed, such divergence vanishes.
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Akiko Masaki, Sota Tsukada, Hiroyuki Mori
2008 Volume 77 Issue 5 Pages
054301
Published: May 15, 2008
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Boson–fermion mixture confined on an optical lattice is numerically studied. Mott-plateau like behavior is observed under certain conditions in the density profile of the bosons and fermions as one sees in bosonic atoms in a trap potential. In the Mott plateau area, the total number of the bosons and fermions at each site is kept to an integer value but both local density of the bosons and that of the fermions take some intermediate values. Unlike bosonic or fermionic Mott insulators, the bosons and fermions can move even in the Mott plateau area by exchanging their positions. With Monte Carlo simulations we demonstrate the formation of the Mott plateau, examine the property of the bosons and fermions by measuring the densities and local density fluctuations, and give discussion on this rather new state.
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Shigeo Kida, Kentaro Nakayama
2008 Volume 77 Issue 5 Pages
054401
Published: May 15, 2008
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A new kind of helical flow structure is observed in a precessing sphere. The Navier–Stokes equation for an incompressible viscous fluid of viscosity ν is solved numerically by a spectral method in a sphere of radius
a which rotates with a constant spin angular velocity Ω
s around an axis which rotates, orthogonally to itself, with another constant precession angular velocity Ω
p. A variety of flow state is realized depending on two control parameters
Re=
a2Ω
s⁄ν and Γ=Ω
p⁄Ω
s. Here we report the detailed flow structures at
Re=10, 200, 330, and 500 for a fixed value of Γ=0.1, for which the flow is stationary in the precession frame. It is found numerically that each streamline winds helically on a torus, the shape of which becomes more and more complicated as
Re increases. The spherical region is divided into many domains by the sense of streamline torsion. The number of domains varies depending on
Re.
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Wei Chung Lo, Lei Wang, Baowen Li
2008 Volume 77 Issue 5 Pages
054402
Published: May 15, 2008
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Thermal transistor is an efficient heat control device which can act as a heat switch as well as a heat modulator. In this paper, we study systematically one- and two-dimensional thermal transistors. In particular, we show how to improve significantly the efficiency of the one-dimensional thermal transistor. The study is also extended to the design of two-dimensional thermal transistor by coupling different anharmonic lattices such as the Frenkel–Kontorova and the Fermi–Pasta–Ulam lattices. Analogy between anharmonic lattices and single-walled carbon nanotube is drawn and possible experimental realization with multi-walled nanotube is suggested.
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Koji Yamauchi, Yukiharu Ohsawa
2008 Volume 77 Issue 5 Pages
054501
Published: May 15, 2008
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Shock formation processes arising from a collision of two plasmas in an external magnetic field
B0 are studied with electromagnetic particle simulations, for the case in which one plasma (exploding plasma) has a high velocity
v0 while the other (surrounding plasma) is at rest with a lower density; the ion gyroradius for the speed
v0 is much greater than the width of the shock transition region. The ion motion across
B0 induces strong electric field in the direction −
v0×
B0, which accelerates the surrounding ions in this direction in the early phase. Then, the change in the velocities of the two groups of ions due to the magnetic force leads to the formation of two pulses near the front of the exploding ions, which evolve into forward and reverse shock waves. The effect of the angle θ between
v0 and
B0 on plasma behavior is examined. The two pulses appear in time of the order of the ion gyroperiod. As θ decreases from 90
°, however, it takes a longer time for the two pulses to form.
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Takuro Kawasaki, Ken-ichi Ohshima
2008 Volume 77 Issue 5 Pages
054601
Published: May 15, 2008
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An X-ray structural study of single crystals of Mn
xTiS
2 (
x=0.13, 0.18, and 0.26) was performed to investigate both the nature of the chemical bond from the electron density distribution (EDD) obtained by the maximum entropy method and local atomic arrangements of intercalated Mn atoms. Mn atoms were intercalated into octahedral sites in the van der Waals gap layer, and interlayer spacing and the distance between each layer were increased by increasing the number of Mn atoms without any change in layered structure. The EDD revealed that covalent bonding exists between Ti and S atoms, and no covalent electrons were found between Mn and S atoms. It is further realized by X-ray diffuse scattering measurements that intercalated Mn atoms are correlated with each other three-dimensionally. In Mn
0.13TiS
2 and Mn
0.18TiS
2, an in-plane ordered structure for the low-temperature phase is expected to be of the 2×2-type in a Wood notation. On the other hand, a \\sqrt3×\\sqrt3
R30°-type ordered structure is expected to coexist with a 2×2-type ordered structure in Mn
0.26TiS
2.
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Shinya Kitamura
2008 Volume 77 Issue 5 Pages
054602
Published: May 15, 2008
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The numerical calculation for Ge/Si(001) heteroepitaxial growth is performed. We adopt the most widely used Stillinger–Weber potential, and the island energies of the three types, two-dimensional island, pyramid and dome, are explored as a function of the lateral size. These island energies are compared with each other to find the island morphology which has the lowest energy. Then, a growth history of the most stable growth mode is searched. Although the result reproduces qualitatively the Stranski–Krastanov growth as observed in the experiments, quantitative differences between our result and experiments in the critical wet layer thickness and the island morphology are found.
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Noriki Terada, Setsuo Mitsuda, Yoshikazu Tanaka, Yoshikazu Tabata, Koi ...
2008 Volume 77 Issue 5 Pages
054701
Published: May 15, 2008
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The results of synchrotron X-ray diffraction measurements on a single crystal of the triangular lattice antiferromagnet CuFeO
2 under zero and non-zero applied magnetic fields are reported. We find four satellite reflections at (0,3+
q,1⁄2), (0,4−
q,1⁄2), (0,4−2
q,0), and (0,3+2
q,0) with the incommensurate wave number
q∼0.415 in the ferroelectric incommensurate (FEIC) phase which appears in the magnetic field,
H, between 7 and 13 T at low temperatures. In the partially disordered (PD) phase which exists in the temperature,
T, range between 11 and 14 K, we find two satellite reflections at (0,4−
q,1⁄2) and (0,4−2
q,0) with the incommensurate wave number
q∼0.4. The
T and
H dependence of these satellite reflections are studied. We interpret that the reflections observed in the FEIC phase arise from incommensurate lattice modulations caused by a magnetoelastic coupling with the underlying magnetic structure. The observation of the reflection (0,4−
q,1⁄2) in finite fields and the (0,4−2
q,0) reflection at 0≤
H in the PD phase is also explained by the same model. A calculation of the X-ray diffraction intensity assuming the displacement of oxygen ions only successfully explains the observation. We discuss the relevance of these satellite reflections to the ferroelectricity.
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Yositake Takane, Katsunori Wakabayashi
2008 Volume 77 Issue 5 Pages
054702
Published: May 15, 2008
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We study conductance fluctuations in disordered quantum wires with unitary symmetry focusing on the case in which the number of conducting channels in one propagating direction is not equal to that in the opposite direction. We consider disordered wires with
N+
m left-moving channels and
N right-moving channels. In this case,
m left-moving channels become perfectly conducting, and the dimensionless conductance
g for the left-moving channels behaves as
g→
m in the long-wire limit. We obtain the variance of
g in the diffusive regime by using the Dorokhov–Mello–Pereyra–Kumar equation for transmission eigenvalues. It is shown that the universality of conductance fluctuations breaks down for
m≠0 unless
N is very large.
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Ken-ichi Sasaki, Riichiro Saito
2008 Volume 77 Issue 5 Pages
054703
Published: May 15, 2008
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The magnetism by the edge states in graphene is investigated theoretically. An instability of the pseudo-spin order of the edge states induces ferrimagnetic order in the presence of the Coulomb interaction. Although the next nearest-neighbor hopping can stabilize the pseudo-spin order, a strong Coulomb interaction makes the pseudo-spin unpolarized and real spin polarized. The magnetism of the edge states makes two peaks of the density of states in the conduction and valence energy bands near the Fermi point. Using a continuous model of the Weyl equation, we show that the edge-induced gauge field and the spin dependent mass terms are keys to make the magnetism of the edge states. A relationship between the magnetism of the edge states and the parity anomaly is discussed.
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Yasuhiro Yamada, Koichiro Tanaka
2008 Volume 77 Issue 5 Pages
054704
Published: May 15, 2008
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The dielectric response of ferroelectric Sr
1−xCa
xTiO
3 (SCT) under photoexcitation has been investigated. We observed photoinduced Debye dispersion, which is similar to that observed in nominally pure SrTiO
3. The temperature dependence of photoinduced Debye dispersion in SCT indicates that the origin of photoinduced Debye dispersion is the interfacial polarization between conductive and insulative regions. We assumed that conductive regions with mobile photocarriers are partly generated by photoexcitation and showed that the experimental results are well reproduced by the conductive-region model. We also observed a photoinduced reduction in ferroelectric phase transition temperature. This can be quantitatively explained by the reduction of Coulomb interaction between Ca-induced-dipoles due to the screening effect by mobile photocarriers.
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Shigeyuki Takagi, Jun-ichi Hoshino, Hidekazu Tomono, Kazuo Tsumuraya
2008 Volume 77 Issue 5 Pages
054705
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We investigate the electron transfer from a dissociatively adsorbed H
2 molecule to a Au(111) surface using the first-principles methods. A fractional electron transfers from a molecule to a substrate, and potential energy increases during the process. The initial energy increase coincides with that of the isolated, separated, and positively charged H
2 molecule calculated by the real-space density functional method. The barrier formation is due to the destabilization of the molecule induced by the electron transfer. The electronegativity difference between the adsorbate and the substrate determines the direction of the electron transfer.
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Masatoshi Onoue, Fumiyuki Ishii, Tamio Oguchi
2008 Volume 77 Issue 5 Pages
054706
Published: May 15, 2008
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We have performed first-principles calculations for the half-Heusler type ternary compounds
MNiSn (
M=Ti, Zr, and Hf). To discuss the relationship between the electronic and thermoelectric properties, we evaluate Seebeck coefficients from the calculated band structure within the Boltzmann transport theory. The temperature and carrier concentration dependences of the Seebeck coefficients are discussed in terms of the density of states and Fermi velocity near the band edges. The calculated Seebeck coefficients for all these materials are found to be about −300 μV/K at a low
n-type carrier concentration at room temperature, being in good agreement with experiments.
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Yasuhiro Tada, Norio Kawakami, Satoshi Fujimoto
2008 Volume 77 Issue 5 Pages
054707
Published: May 15, 2008
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We study the pairing symmetry of the noncentrosymmetric heavy fermion superconductors CeRhSi
3 and CeIrSi
3 under pressures, which are both antiferromagnets at ambient pressure. We solve the Eliashberg equation by means of the random phase approximation and find that the mixed state of extended
s- and
p-wave rather than the
d+
f wave state could be realized by enhanced antiferromagnetic spin fluctuations. It is elucidated that the gap function has line nodes on the Fermi surface and the resulting density of state in the superconducting state shows a similar character to that of usual
d-wave superconductors, resulting in the NMR relaxation rate 1⁄(
T1T) that exhibits no coherence peak and behaves like 1⁄(
T1T)∝
T2 at low temperatures.
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Takahisa Kouno, Shuji Ogata
2008 Volume 77 Issue 5 Pages
054708
Published: May 15, 2008
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The activation energy for oxygen diffusion in strained silicon crystal is investigated using the hybrid quantum-classical simulation scheme in combination with the nudged elastic band method. The electronic density-functional theory is applied to a local region containing the oxygen atom, while the classical inter-atomic potential, to the rest of the system. The system is stretched to three mutually perpendicular directions at a wide range of degree between −2 and 9%. We thereby find that the activation energy changes by between −0.4 and 0.2 eV depending sensitively on both direction and degree of the stretch, and that the peripheral atoms located far from the oxygen atom in the system contribute little to the change. Microscopic mechanisms of the strain-dependence of the activation energy are elucidated through combined analyses about the atomic and electronic structures.
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Akira Endo, Yasuhiro Iye
2008 Volume 77 Issue 5 Pages
054709
Published: May 15, 2008
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The amplitude and phase of Shubnikov–de Haas oscillations have been analyzed in detail for two-dimensional electron gases subjected to a weak unidirectional periodic potential modulation. The amplitude is suppressed, accompanied by inversion of the phase, at the maximum bandwidth conditions at low magnetic fields. The suppression is gradually taken over by the enhancement with the increase of the magnetic fields. The suppression and the enhancement are attributable to the collisional and the diffusion contribution of the modulated potential to the conductivity, respectively, the former (the latter) being dominant at low (high) magnetic fields. A theoretical calculation that takes the two types of contributions into account shows semi-quantitative agreement with experimental traces.
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Takuya Kato, Toshiyuki Maruyama, Shigeki Okitsu, Hideaki Sakata
2008 Volume 77 Issue 5 Pages
054710
Published: May 15, 2008
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Scanning tunneling microscopy and spectroscopy have been performed on superconducting La
2−xSr
xCuO
4 (
x=0.06, 0.12, 0.16, and 0.21) at 4.2 K. Atomically resolved surface topography has been observed on the
ab surface, which exhibits a one-dimensional structure corresponding to the buckling pattern of the orthorhombic phase. The tunneling spectra of all samples exhibit kinks at about ±5 meV in addition to the energy gap, indicating the existence of two energy scales. While the gap magnitude varies on the length scale of a few nm and steadily increases upon underdoping, the kink energy is spatially uniform on the surface and almost independent of the doping level.
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Jin Nakamura, Nobuyoshi Yamada, Kazuhiko Kuroki, Tamio Oguchi, Kozo Ok ...
2008 Volume 77 Issue 5 Pages
054711
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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Carbon- and boron-2
p states of superconducting and non-superconducting boron-doped diamond (BDD) samples are measured using soft X-ray emission and absorption spectroscopy (XES and XAS) near C- and B-K edges. The electronic structure of B 2
p does not show a marked boron-doping dependence, except that a considerable amount of in-gap state in empty states is observed. In C-K XAS spectra, two peaks
H and
I are observed around the Fermi level. The
H peak is attributed to 2
p state of carbon first-nearest neighboring to the dopant boron atoms (1NN-C), and the
I peak to 2
p state of carbon further than 1NN-C from an impurity boron. Incoherent (normal-excitation) XES spectra do not show a large chemical shift by B-doping, but its intensity just below the valence band maximum (VBM) decreases with B-doping. It cannot be interpreted within a simple rigid band model. An elastic peak of
I-excited C-K XES spectrum and the width of
I-peak of C-K XAS spectrum suggest that impurity state is localized in non-superconducting BDD but is not in superconducting BDD. Namely impurity state of superconducting BDD is merged with the valence band, and holes in the merged state play an important role in the occurrence of superconductivity of BDD.
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Shingo Yonezawa, Shuichi Kusaba, Yoshiteru Maeno, Pascale Auban-Senzie ...
2008 Volume 77 Issue 5 Pages
054712
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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We have studied the onset temperature of the superconductivity
Tconset of the organic superconductor (TMTSF)
2ClO
4, by precisely controlling the direction of the magnetic field
H. We compare the results of two samples with nearly the same onset temperature but with different scattering relaxation time τ. We revealed a complicated interplay of a variety of pair-breaking effects and mechanisms that overcome these pair-breaking effects. In low fields, the linear temperature dependences of the onset curves in the
H–
T phase diagrams are governed by the orbital pair-breaking effect. The dips in the in-plane field-angle φ dependence of
Tconset, which were only observed in the long-τ sample, provides definitive evidence that the field-induced dimensional crossover enhances the superconductivity if the field direction is more than about 19° away from the
a axis. In the high-field regime for
H||
a, the upturn of the onset curve for the long-τ sample indicates a new superconducting state that overcomes the Pauli pair-breaking effect but is easily suppressed by impurity scatterings. The Pauli effect is also overcome for
H||
b′ by a realization of another state for which the maximum of
Tconset(φ) occurs in a direction different from the crystalline axes. The effect on
Tconset of tilting the applied field out of the conductive plane suggests that the Pauli effect plays a significant role in determining
Tconset. The most plausible explanation of these results is that (TMTSF)
2ClO
4 is a singlet superconductor and exhibits Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) states in high fields.
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Yasuhiro H. Matsuda, Toshiya Inami, Kenji Ohwada, Yuto Murata, Hiroyuk ...
2008 Volume 77 Issue 5 Pages
054713
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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The magnetic-field-induced valence transition in EuNi
2(Si
1−xGe
x)
2 (
x=0.82, 0.85) has been studied by X-ray absorption spectroscopy at low temperatures. The field dependence of the Eu valence is directly observed for the first time. We find a significant decrease in the valences at high magnetic fields above 40 T. The valence of Eu changes from
v*=2.75 at a zero field to
v*=2.33 at 40 T for
x=0.82. The results are analyzed using a theoretical model. The hybridization parameter
V between the Eu
2+ and Eu
3+ states and the energy separation Δ
E are determined, which are crucial for quantum mechanical mixing.
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Naoki Kase, Takahiro Muranaka, Jun Akimitsu
2008 Volume 77 Issue 5 Pages
054714
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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A new superconducting ternary germanide, Y
3Pt
4Ge
6, was discovered at the transition temperature of 2.6 K by the electrical resistivity and magnetic susceptibility measurements. The magnetization versus magnetic field (
M–
H) curve indicated that Y
3Pt
4Ge
6 is a type-II superconductor. The lower critical field
Hc1 and the upper critical field
Hc2 were estimated to be 53 Oe and 2.73 kOe. The coherence length ξ(0)
GL, penetration depth λ(0)
GL, and Ginzburg–Landau (GL) parameter κ(0) obtained from these values were 347 Å, 3230 Å, and 9.3, respectively. Specific heat measurements revealed that Y
3Pt
4Ge
6 is a weak-coupling BCS superconductor with an isotropic superconducting gap. The electron–phonon mass enhancement parameter λ
γ was calculated to be 0.19, which was consistent with the weak-coupling limit inferred from the specific jump Δ
Cp⁄γ
nTc.
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Erhan Cengiz, Gokhan Apaydin, Volkan Aylikci, Necati Kaya, Engin Tiras ...
2008 Volume 77 Issue 5 Pages
054801
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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In this study, the L
1→L
2, L
1→L
3, and L
2→L
3 Coster–Kronig (CK) transition probabilities (
f12,
f13, and
f23) for Hf compounds were investigated. The samples were excited by 59.5 keV γ-rays from a
241Am annular radiative source. L X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV. In addition, it was observed a chemical effect on the CK transition probabilities for Hf compounds. The semi-empirical values of
f12,
f13, and
f23 CK transition probabilities have been compared with the semi-empirical, theoretical and experimental data for pure Hf, respectively.
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Eiichi Yagi, Kazuhiro Hirabayashi, Yoichi Murakami, Shigetoshi Koike, ...
2008 Volume 77 Issue 5 Pages
054802
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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To locate hydrogen in Nb alloyed with 0.3 at. % oxygen, which is the alloy system for the tunnelling motion of hydrogen to be observed, channelling experiments have been performed at room temperature utilizing a nuclear reaction
1H(
11B,α)αα with a
11B beam of about 2 MeV. It has been demonstrated that for a hydrogen concentration of
CH=0.003 (
CH=[H]⁄[M]; hydrogen-to-metal-atom ratio), H atoms are located at sites displaced by 0.5–0.6 Å from tetrahedral (
T) sites towards their nearest-neighbour lattice points, while for
CH=0.019, most of the H atoms are located at
T sites and the remains are at the same sites as those observed for
CH=0.003. This result indicates the trapping of hydrogen by oxygen and the formation of O–H pairs. The number of H atoms trapped by one O atom is approximately unity. As locations of the trapped hydrogen involved in the tunnelling motion, three types of displaced sites, which are described above, with the same separation of about 0.82 Å between neighbouring equivalent sites around O atoms are discussed.
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Satohiro Tajima, Masato Inoue, Masato Okada
2008 Volume 77 Issue 5 Pages
054803
Published: May 15, 2008
Released on J-STAGE: December 28, 2011
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Translational-symmetric filters provide a foundation for various kinds of image processing. When a filtered image containing noise is observed, the original one can be reconstructed by Bayesian inference. Furthermore, hyperparameters such as the smoothness of the image and the noise level in the communication channel through which the image observed can be estimated from the observed image by setting a criterion of maximizing marginalized likelihood. In this article we apply a diagonalization technique with the Fourier transform to this image reconstruction problem. This diagonalization not only reduces computational costs but also facilitates theoretical analyses of the estimation and reconstruction performances. We take as an example the Mexican-hat shaped neural cell receptive field seen in the early visual systems of animals, and we compare the reconstruction performances obtained under various hyperparameter and filter parameter conditions with each other and with the corresponding performances obtained under no-filter conditions. The results show that the using a Mexican-hat filter can reduce reconstruction error.
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