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Russell E. Walstedt, Shinsaku Kambe, Yo Tokunaga, Hironori Sakai
2007 Volume 76 Issue 7 Pages
072001
Published: July 15, 2007
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In this paper we present highlights from a 6-year program of NMR/NQR/AFNMR studies carried out on both actinide and ligand nuclear spins in oxides and intermetallic compounds of the actinide elements U, Np, and Pu. We begin with the large family of actinide 115 compounds, consisting of metallic antiferromagnets, metamagnets, Pauli paramagnets, and the superconducting Pu-based systems. Short summaries are presented of
69,71Ga NMR studies of the exotic metamagnet NpCoGa
5, the surprisingly inhomogeneous UCoGa
5, and the heavy-fermion “high-
Tc” superconductors PuTGa
5, T=Rh and Co. The case of microdomains in the metallic antiferromagnet UGa
3 is briefly reviewed. Next, we summarize the direct and indirect
235U NMR studies on USb
2 and URh
3. These are the first studies of their kind and the only studies of
235U NMR in a metallic environment. Under the heading of actinide oxides we recap our original studies of
235U and
17O NMR in UO
2, followed by a summary of our investigation of multipolar ordering in NpO
2 via
17O NMR, in which the
237Np NMR parameters were determined via cross-relaxation effects. And finally, we summarize our
31P study of the ferromagnetic transition in the filled Skutterudite UFe
4P
12.
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Victor K. Kuetche, Thomas B. Bouetou, Timoleon C. Kofane
2007 Volume 76 Issue 7 Pages
073001
Published: July 15, 2007
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We derive a new equation that is considered as a generalized Schäfer–Wayne short pulse equation (SWSPE). We then elicit a soliton solution to this equation using the (1+1)-dimensional coupled nonlinear dispersionless equations (CNLDEs). As a result, this solution may possess a nonzero angular momentum.
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Hiromi Otsuka
2007 Volume 76 Issue 7 Pages
073002
Published: July 15, 2007
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We discuss an effective field theory of a triangular-lattice three-spin interaction model defined by the \\mathbb
Zp variables. Based on the symmetry properties and the ideal-state graph concept, we show that the vector dual sine-Gordon model describes the long-distance properties for
p≥5; we then compare its predictions with the previous argument. To provide the evidences, we numerically analyze the eigenvalue structure of the transfer matrix for
p=6, and we check the criticality with the central charge
c=2 of the intermediate phase and the quantization condition of the vector charges.
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Yukio Kaneda, Koji Morishita
2007 Volume 76 Issue 7 Pages
073401
Published: July 15, 2007
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The statistics of energy dissipation rate \\ ildeε in turbulence and the ratio α
n≡ε
n+1⁄ε
n is studied using the data of direct numerical simulations of incompressible turbulence with up to 2048
3 grid points, where ε
n is the average of \\ ildeε over a sphere of radius
rn with
rn⁄
rn+1=const>1. The data do not support the well-known assumption of the scale-independence of the statistics of α
n nor that of the statistical independence of α
n’s from each other, but suggest the existence of a scale range where the correlation between logα
n and logα
n+k is insensitive to
n. A simple zero-cumulant approximation compatible with these observations yields an estimate of the moment 〈(ε
n)
p〉 for given 〈logα
n〉’s, in good agreement with the simulations for approximately |
p|≤4.
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Vladimir Bubanja, Shuichi Iwabuchi
2007 Volume 76 Issue 7 Pages
073601
Published: July 15, 2007
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We consider two serially connected tunnel junctions embedded in a dissipative electromagnetic environment. Within the Coulomb blockade region where the sequential single electron tunneling is suppressed, electron transport through the whole system is achieved via higher-order tunneling processes. The standard approach using perturbation theory to fourth order in the tunnel Hamiltonian gives divergent rates of these processes at the Coulomb blockade boundary. By using a diagrammatic technique, we perform an analytic summation over a perturbative expansion of infinite order. In this way, we remove the divergence and derive an expression for the cotunneling current applicable over the whole Coulomb blockade region.
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Yusril Yusuf, Naoki Minami, Shohei Yamaguchi, Dong-Uk Cho, P. E. Cladi ...
2007 Volume 76 Issue 7 Pages
073602
Published: July 15, 2007
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We study the relation between the shape anisotropy changes, Δα, and the optical birefringence, Δ
n, of dry and swollen rigid bi-functionally (8.0 mol %, 8A2) and flexible tri-functionally [5 mol % (5V3) and 7 mol % (7V3)] cross-linked liquid single crystal elastomers (LSCEs) as a function of temperature. 4-
n-pentyl-4-cyanobiphenyl (5CB) is used as the solvent. Plotting Δα as a function of Δ
n, a linear dependence is observed for all LSCE samples. This indicates that Δα results from the liquid crystal ordering. The largest Δα (∼0.2 at
T=30 °C) is observed in dry 8A2, which has a large frozen-in orientational order (FOO). The swollen samples show much smaller Δα, ∼0.03–0.05, because of a smaller modulus elasticity after the swelling process.
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Fumitatsu Iwase, Kazuya Miyagawa, Shigeki Fujiyama, Kazushi Kanoda, Sa ...
2007 Volume 76 Issue 7 Pages
073701
Published: July 15, 2007
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We report a
35Cl NQR investigation into the neutral–ionic (NI) phase transition in DMTTF-QCl
4 (4,4′-dimethyltetrathiafulvalene-
p-chloranil), the end material of the family containing an NI quantum transition system. Four distinct lines with comparable intensities were observed well below
Tc=65 K; this observation is consistent with the antiferroelectric nature of the ionic phase. The temperature dependences of the spectral shift and splitting below
Tc are moderate in contrast with the case of TTF-QCl
4. Low-energy charge-lattice fluctuations are observed around
Tc as a drastic increase of 1⁄
T1, allowing the observation of the quantum fluctuations in a chemically substituted complex. An analysis of the NQR spectra yields detailed temperature variations in the charge transfer and the concomitant lattice distortion around the NI transition.
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Taka-hisa Arima
2007 Volume 76 Issue 7 Pages
073702
Published: July 15, 2007
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A novel type of magnetic origin of ferroelectric polarization is proposed. In crystals with triclinic, monoclinic, or trigonal (rhombohedral) symmetries, the proper screw type of magnetic ordering can induce ferroelectricity through the variation in the metal–ligand hybridization with spin–orbit coupling. The ferroelectric polarization in delafossite compounds Cu(Fe,Al)O
2 is successfully explained by the mechanism. The relation between the polarization direction and the magnetic modulation vector is also predicted.
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Takanobu Jujo
2007 Volume 76 Issue 7 Pages
073703
Published: July 15, 2007
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We theoretically investigate intermodulation distortion in high-
Tc superconductors. We study the effect of nonmagnetic impurities on the real and imaginary parts of nonlinear conductivity. The nonlinear conductivity is proportional to the inverse of temperature owing to the dependence of the damping effect on energy, which arises from the phase shift deviating from the unitary limit. It is shown that the final-states interaction makes the real part predominant over the imaginary part. These effects have not been included in previous theories based on the two-fluid model, enabling a consistent explanation for the experiments with the rf and dc fields.
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Hikaru Kawamura, Atsushi Yamamoto
2007 Volume 76 Issue 7 Pages
073704
Published: July 15, 2007
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The ordering of the classical Heisenberg antiferromagnet on the triangular lattice with the bilinear–biquadratic interaction is studied by Monte Carlo simulations. It is shown that the model exhibits a topological phase transition at a finite-temperature driven by topologically stable vortices, while the spin correlation length remains finite even at and below the transition point. The relevant vortices could be of three different types, depending on the value of the biquadratic coupling. Implications to recent experiments on the triangular antiferromagnet NiGa
2S
4 is discussed.
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Hideyuki Hara, Hiroshi Kontani
2007 Volume 76 Issue 7 Pages
073705
Published: July 15, 2007
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In hole-doped high-
Tc superconductors, thermal conductivity κ increases drastically just below
Tc, which has been considered as a hallmark of a nodal gap. In contrast, such a coherence peak in κ is not visible in electron-doped compounds, which may indicate a full-gap state such as a
d+
is-wave state. To settle this problem, we study κ in the Hubbard model using the fluctuation-exchange (FLEX) approximation, which predicts that the nodal
d-wave state is realized in both hole- and electron-doped compounds. The contrasting behavior of κ in both compounds originates from the differences in the hot/cold spot structure. In general, a prominent coherence peak in κ appears in line-node superconductors
only when the cold spot exists on the nodal line.
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Hiroyuki Yamase, Andrey A. Katanin
2007 Volume 76 Issue 7 Pages
073706
Published: July 15, 2007
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The most salient features observed around a metamagnetic transition in Sr
3Ru
2O
7 are well captured in a simple model for spontaneous Fermi surface symmetry breaking under a magnetic field, without invoking a putative quantum critical point. The Fermi surface symmetry breaking happens in both a majority and a minority spin band but with a different magnitude of the order parameter, when either band is tuned close to van Hove filling by the magnetic field. The transition is of second order for high temperature
T and changes to first order for low
T. The first order transition is accompanied by a metamagnetic transition. The uniform magnetic susceptibility and the specific heat coefficient show strong
T dependence, especially a log
T divergence at van Hove filling. The Fermi surface instability then cuts off such non-Fermi liquid behavior and gives rise to a cusp in the susceptibility and a specific heat jump at the transition temperature.
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Eiichi Matsuoka, Daichi Usui, Hiroshi Tanida, Shintaro Nakamura, Tsuto ...
2007 Volume 76 Issue 7 Pages
073707
Published: July 15, 2007
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Specific heat and magnetization measurements were performed on single crystalline PrPd
3S
4. A λ-type anomaly at
TM=1.56 K in the specific heat shifts to higher temperatures by magnetic fields, begins to shift to lower temperatures above 2 T, and disappears above 3.6 T. The 4
f ground state of Pr
3+ was deduced to be a magnetic triplet. Below
TM, the magnetic susceptibility increases while no spontaneous magnetization appears. These results strongly reveal that PrPd
3S
4 shows a multipolar ordering at
TM. The most probable ordering state for PrPd
3S
4 is an antiferroquadrupolar (AFQ) ordering since the
B–
T phase diagrams are of the “re-entrant” type, which is typical for AFQ ordering compounds.
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Takeshi Shibayama, Minoru Nohara, Hiroko Aruga Katori, Yoshihiko Okamo ...
2007 Volume 76 Issue 7 Pages
073708
Published: July 15, 2007
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Superconductivity with
Tc\\simeq2 K was discovered in the intermetallic binary compounds Rh
2Ga
9 and Ir
2Ga
9. This is the first observation of superconductivity in the Rh–Ga and Ir–Ga binary systems. Both compounds crystallize in a distorted Co
2Al
9-type structure (monoclinic, space group:
Pc), which lacks spatial inversion symmetry. Specific heat measurements revealed that both compounds are weak-coupling BCS superconductors having an isotropic superconducting gap. Measurements in magnetic fields indicated type-I superconductivity with a critical field
Hc(0)\\simeq130 Oe for Rh
2Ga
9 and type-II superconductivity with an upper critical field
Hc2(0)\\simeq250 Oe for Ir
2Ga
9.
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Masashige Matsumoto, Mikito Koga
2007 Volume 76 Issue 7 Pages
073709
Published: July 15, 2007
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A longitudinal excitation mode is important in spin-wave theory for three-dimensional interacting integer spin systems with uniaxial anisotropy of easy plane type. There is a critical value of the anisotropy above which the long-range order disappears. Owing to suppression of the magnetic moment near the quantum critical point, spin fluctuations parallel to the ordered moment appear as the longitudinal mode with a substantially strong inelastic neutron scattering intensity. We propose that the longitudinal mode is observable in the pressure-induced ordered phase of a candidate magnet NiCl
2–4SC(NH
2)
2.
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Takeshi Waki, Koichi Arai, Masashi Takigawa, Yuta Saiga, Yoshiya Uwato ...
2007 Volume 76 Issue 7 Pages
073710
Published: July 15, 2007
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We report results of
11B NMR and susceptibility measurements on the quasi two-dimensional frustrated dimer spin system SrCu
2(BO
3)
2 under high pressure. At 2.4 GPa and in a magnetic field of 7 T, NMR lines split with decreasing temperature in two steps. A gradual splitting below
T=30 K breaking the four-fold symmetry of magnetic response is followed by a further sudden splitting below 3.6 K. The latter indicates a magnetic phase transition, which is also marked by a kink in the susceptibility at 1.44 GPa. From the magnetic hyperfine shift data, we conclude that the low-
T phase has a doubled unit cell containing two types of dimers, one in a nearly singlet state and the other with a finite magnetization down to
T=0.
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Ryo Kawahara, Hiizu Nakanishi
2007 Volume 76 Issue 7 Pages
074001
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We study slow relaxation processes in a point vortex model for two-dimensional pure electron plasma in the strongly magnetized limit. By numerical simulations, we show that the system settles down to a final state via a slow relaxation after it relaxes into a quasi-stationary state via an initial fast relaxation. By analyzing simulation data, we demonstrate that (i) the system relaxes into the maximum one-body entropy state after the slow relaxation, (ii) the time scale of the slow relaxation in the unit of bulk rotation time increases linearly with the number of electrons, and (iii) each electron undergoes a superdiffusive motion during the slow relaxation process. However, the time scale in which each electron diffuses over the system size turns out to be much shorter than that of the slow relaxation; this suggests that the correlation among the superdiffusive trajectories is important in the slow relaxation process.
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Yasushi Kondo, Mikio Nakahara, Shogo Tanimura, Sachiko Kitajima, Chika ...
2007 Volume 76 Issue 7 Pages
074002
Published: July 15, 2007
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A quantum system with finite degrees of freedom can simulate a composite of a system and its environment if the state of the hypothetical environment is randomized by external manipulation. We propose to examine various techniques in quantum information processing in virtual noisy environment with two concrete examples. One simulates phase decoherence of a single qubit in a transmission line, and the other does in a quantum memory. In both cases, the bang-bang control, a typical example of useful techniques employed in quantum information processing, is observed to be effective to suppress decoherence.
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Kazuaki Nakayama
2007 Volume 76 Issue 7 Pages
074003
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The discretization of a vortex filament is studied. A discrete vortex model and a discrete version of the local induction approximation technique (LIA), which guarantees that the vortex filament obeys the discrete nonlinear Schrödinger equation, is proposed.
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Kwok Wing Chow, Lai Pan Yip, Roger Grimshaw
2007 Volume 76 Issue 7 Pages
074004
Published: July 15, 2007
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A derivative nonlinear Schrödinger equation with variable coefficient is considered. Special exact solutions in the form of a solitary pulse are obtained by the Hirota bilinear transformation. The essential ingredients are the identification of a special chirp factor and the use of wavenumbers dependent on time or space. The inclusion of damping or gain is necessary. The pulse may then undergo broadening or compression. Special cases, namely, exponential and algebraic dispersion coefficients, are discussed in detail. The case of exponential dispersion also permits the existence of a 2-soliton. This provides a strong hint for special properties, and suggests that further tests for integrability need to be performed. Finally, preliminary results on other types of exact solutions, e.g., periodic wave patterns, are reported.
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Masaru Uchiyama, Jun’ichi Ieda, Miki Wadati
2007 Volume 76 Issue 7 Pages
074005
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We study soliton solutions for the Gross–Pitaevskii equation of the spinor Bose–Einstein condensates with hyperfine spin
F=2 in one-dimension. Analyses are made in two ways: by assuming single-mode amplitudes and by generalizing Hirota’s direct method for multi-components. We obtain one-solitons of single-peak type in the ferromagnetic, polar and cyclic states, respectively. Moreover, twin-peak type solitons both in the ferromagnetic and the polar state are found.
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Shinichi Horata, Tetsuyuki Yukawa
2007 Volume 76 Issue 7 Pages
074101
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For understanding the origin of anisotropies in the cosmic microwave background, rules to construct a quantized universe are proposed based on the dynamical triangulation method of the simplicial quantum gravity. A
d-dimensional universe having the disk topology
Dd is created in terms of a simplicial manifold with
d-simplices as the building blocks. Numerical simulations made mainly for two-dimensional universes are supplemented in order to examine appropriateness of the construction rules through comparing to analytic results of the matrix model and the boundary Liouville theory. Time is defined along the radial direction perpendicular to the boundary surface, and the space coordinates are embeded on the surface with the geodesic distance. Furthermore, a simulation in four-dimension is performed to analyze the observed temperature fluctuation by comparing to the scalar curvature correlation on the
S2-surface formed as the last scattering surface of an
S3 universe.
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T. Mori, S. Uehara, Y. Watanabe, K. Abe, I. Adachi, H. Aihara, K. Arin ...
2007 Volume 76 Issue 7 Pages
074102
Published: July 15, 2007
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We report on a high statistics measurement of the total and differential cross sections of the process γγ→π
+π
− in the π
+π
− invariant mass range 0.8GeV⁄
c2<
W<1.5GeV⁄
c2 with 85.9 fb
−1 of data collected at \\sqrt
s=10.58 GeV and 10.52 GeV with the Belle detector. A clear signal of the
f0(980) resonance is observed in addition to the
f2(1270) resonance. An improved 90% confidence level upper limit
B(η′(958)→π
+π
−)<2.9×10
−3 is obtained for
P- and
CP-violating decay of the η′(958) meson using the most conservative assumption about the interference with the background.
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Yong Wu, Si-Liang Zeng, Bin Duan, Jun Yan, Jian-Guo Wang, Chen-Zhong D ...
2007 Volume 76 Issue 7 Pages
074301
Published: July 15, 2007
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Using the fully relativistic Dirac wave functions combining with the channel properties, the radiative recombination (RR) processes of electron into bare uranium (U
92+) are investigated in the relative energy approaching to zero, and the X-ray spectrum emitted from the direct RR and sequential cascade processes is simulated. It is found that the direct RR spectrum for capture to the ground and low excited states is in a good agreement with the experimental measurements, but there exist significant discrepancies in the cascade spectrum produced in captures to the high Rydberg states. We also find that the shorter the measurement duration is, the smaller the discrepancies. These results show that the enhancement of RR rate coefficients in the ultra-low energy (<1 meV) found in numerous experiments mainly comes from the processes for capture to the high Rydberg states. Qualitatively, this conclusion is in harmony with the field-induced recombination (FIR) model proposed recently by Hörndl
et al. [Phys. Rev. Lett.
95 (2005) 243201], but the theoretical intensity ratios between Ly-α
1 and Ly-α
2 and between Ly-β
1 and Ly-β
2, which are calculated with the FIR effects considered, are in conflict with the measured data. Further experimental and theoretical studies are expected.
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Hiromi Okamoto, Kenichi Kaneta, Andrew M. Sessler
2007 Volume 76 Issue 7 Pages
074501
Published: July 15, 2007
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In this study, we analyze a method of controlling the ratios of beam emittances, i.e., the projections of the six-dimensional phase space volume occupied by a charged-particle ensemble. We study difference resonances, induced between the three degrees of freedom, by coupling sources. A possible application of such an emittance manipulation is discussed with numerical examples. The control of beam emittance ratios is thought to be useful in various cases where the emittance of one direction is particularly important, or where the ratio of emittances plays an essential role.
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Kenji Maruyama, Hirohisa Endo, Hideoki Hoshino
2007 Volume 76 Issue 7 Pages
074601
Published: July 15, 2007
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The RMC and Voronoi–Delaunay analyses were applied to study the void structure in the network-forming liquids: Rb
20Se
80 and BiBr
3. The distribution function of the relative positions of atoms and void centers in space, the coordination number of atoms and voids around a void and the size distribution of voids were examined using the previous experimental data for
S(
Q) to characterize the connectivity of the bonding network supporting voids and the intermediate-range correlation as manifested in the pre-peak of
S(
Q). The voids in l-Rb
20Se
80 are formed by the 8-membered ring-like Se chains. A Rb cluster composed of 3–4 Rb atoms replaces one of five void sites and binds ∼4 Se rings involving voids around it through the interactions between Rb and Se and between Rb and Rb. In l-BiBr
3 the hexagonal cages involving voids are formed by the corner-sharing of the trigonal pyramidal BiBr
3 blocks. The strong preference of Bi ion for three-fold coordination helps to bind between the neighboring cages and to construct a honeycomb network in highly correlated fashion. The pre-peak of the observed
S(
Q) indicative of the intermediate-range order arises from the correlation between the neighboring Se rings in l-Rb
20Se
80 and between the neighboring hexagonal cages formed by BiBr
3 blocks in l-BiBr
3. In conclusion the pre-peak of
S(
Q) in l-Rb
20Se
80 and l-BiBr
3 is related to the pre-peak of the void-based concentration–concentration structure factor
S′
CC(
Q) due to an intermediate chemical order in the structure.
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Francois Beceau Pelap, Mansour Mouhamadou Faye
2007 Volume 76 Issue 7 Pages
074602
Published: July 15, 2007
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Dynamics of modulated waves in a nonlinear electrical bi-inductance line are theoretically examined by the means of a perturbation method. It has been recently established that in such lines, modulations of weakly nonlinear dispersive waves are described by the complex Ginzburg–Landau equation (CGLE) for wavenumbers
k larger than the critical values
kc. Presently, we assume that in the vicinity of
kc, the CGLE is invalid and the associated instability criterion is useless. Considering a semi discrete approximation using an appropriate decoupling ansatz for the voltage of the two different cells, we demonstrated that in such regime, wave evolutions are governed by a modified form of the CGLE that involves higher-orders nonlinearities. The associated generalized modulation instability criterion is obtained and corresponding higher order soliton solutions are constructed.
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Takaaki Hirano, Yasuhiro Hatsugai
2007 Volume 76 Issue 7 Pages
074603
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We investigate the entanglement entropy (EE) of gapped
S=1 and
S=1⁄2 spin chains with dimerization. We find that the effective boundary degrees of freedom as edge states contribute significantly to the EE. For the
S=1⁄2 dimerized Heisenberg chain, the EE of the sufficiently long chain is essentially explained by the localized
S=1⁄2 effective spins on the boundaries. As for
S=1, the effective spins are also
S=1⁄2 causing a Kennedy triplet that yields a lower bound for the EE. In this case, the residual entanglement reduces substantially by a continuous deformation of the Heisenberg model to that of the AKLT Hamiltonian.
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Yukio Saito, Ryo Kawasaki
2007 Volume 76 Issue 7 Pages
074604
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Effect of an anisotropic detachment on a heteroepitaxial island shape is studied by means of a kinetic Monte Carlo simulation of a square lattice gas model. Only with molecular deposition followed by surface diffusion, islands grow in a ramified dendritic shape, similar to diffusion-limited aggregate (DLA). Introduction of molecular detachment from edges makes islands compact. To understand an anisotropic island shape observed in the experiment of pentacene growth on a hydrogen-terminated Si(111) vicinal surface, asymmetry in detachment around the substrate step is assumed. Edge molecules detach more to the higher terrace than to the lower terrace. The island edge from which molecules are easy to detach is smooth and the one hard to detach is dendritic. If islands are close to each other, islands tend to align in a line, since detached molecules from the smooth edge of the right island are fed to the dendritic and fast growing edge of the left island.
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Yoichi Tanaka, Norio Kawakami, Akira Oguri
2007 Volume 76 Issue 7 Pages
074701
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We study transport through a quantum dot coupled to normal and superconducting leads using the numerical renormalization group method. We show that the low-energy properties of the system are described by the local Fermi liquid theory despite of the superconducting correlations penetrated into the dot due to a proximity effect. We calculate the linear conductance due to the Andreev reflection in the presence of the Coulomb interaction. It is demonstrated that the maximum structure appearing in the conductance clearly characterizes a crossover between two distinct spin-singlet ground states, i.e. the superconducting singlet state and the Kondo singlet state. It is further elucidated that the gate-voltage dependence of the conductance shows different behavior in the superconducting singlet region from that in the Kondo singlet region.
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Yoshiro Kakehashi, Peter Fulde
2007 Volume 76 Issue 7 Pages
074702
Published: July 15, 2007
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Single-particle excitation spectra of the two-dimensional Hubbard model on the square lattice near half filling and at zero temperature are investigated on the basis of the self-consistent projection operator method. The method guarantees a high accuracy of the spectra with high energy and high momentum resolutions. It takes into account long-range intersite correlations as well as the strong on-site correlations. Effects of nonlocal excitations are clarified by comparing the results with those of the single-site approximation. The calculated spectra verify the quantum Monte-Carlo results for finite temperatures. The spectra at the Fermi level yield a hole-like Fermi surface in the underdoped region and an electron-like Fermi surface in the overdoped region. From a numerical analysis of the momentum dependent effective mass and self-energy, it is concluded that a marginal Fermi-liquid like state persists even at finite doping concentrations in the strongly correlated region because a van Hove singularity is pinned to the Fermi surface. It is also found that a kink structure appears in the quasiparticle energy band in the same region. The kink is shown to be caused by a mixing between the quasiparticle band and an excitation band with strong short-range antiferromagnetic correlations. These results suggest an explanation for some of the unusual properties of the normal state in high-
Tc cuprates.
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Haruhiro Hiraka, Soichi Ohta, Shuich Wakimoto, Masaaki Matsuda, Kazuyo ...
2007 Volume 76 Issue 7 Pages
074703
Published: July 15, 2007
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Neutron scattering experiments were carried out to explore Ni-impurity effects on static spin correlations in La
2−xSr
xCuO
4 in the vicinity of the superconductor-insulator boundary where both parallel and diagonal spin-density modulations (SDMs) coexist at low temperature. Dilute Ni substitution completely destroys the bulk superconductivity and alters well-defined incommensurate peaks of Ni-free SDMs into a single broad peak at (π,π). As the Ni concentration (
y) reaches ∼
x, the magnetic diffuse scattering is transformed to an intense sharp Bragg peak, corresponding to a three-dimensional antiferromagnetic (AF) order in bulk.
TN of such Ni-induced AF order depends on
x and appears to disappear at
x∼0.1. A simplified cross section for the
Q spectra taking into account the orthorhombic domain structure reproduces the Ni-impurity effect on the static spin correlations well, which results in a decreasing incommensurability of both types of SDMs. These effects are approximately ascribed by a reduction in the number of mobile holes, or by the effective hole number
xeff=
x−
y, similarly to the case of diagonal SDMs in insulating spin-glasses.
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Takao Suzuki, Fumiko Yamada, Isao Watanabe, Takayuki Goto, Akira Oosaw ...
2007 Volume 76 Issue 7 Pages
074704
Published: July 15, 2007
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Zero-field muon-spin-relaxation (ZF-μSR) measurements were carried out down to 80 mK on the randomness bond system Tl
1−xK
xCuCl
3 with
x=0.44. Time spectra are well fitted by the stretched exponential function exp(−λ
t)
β. The muon spin relaxation rate λ increases rapidly with decreasing temperature, and β tends to 0.5 at 80 mK. The divergent increase of λ suggests the critical slowing down of the frequency of the Cu-3
d spin fluctuations toward a spin frozen state below 80 mK, and the root-exponential-like behavior of the time spectrum indicates that the origin of the relaxation is possibly the spatially-fixed fluctuating dilute moments.
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Hiroshi Tsukahara, Tsuneya Ando
2007 Volume 76 Issue 7 Pages
074705
Published: July 15, 2007
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The dynamical conductivity of quantum wire arrays with disorder in a corrugation period, direction, and height are calculated in a self-consistent Born approximation. Anisotropic scattering associated with diffuse Bragg peaks causes the formation of pseudo-band-structure and gives rise to inter-band optical absorption in the dynamical conductivity in the direction perpendicular to quantum wires.
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Hiroyuki Kimura, Satoru Kobayashi, Yoshikazu Fukuda, Toshihiro Osawa, ...
2007 Volume 76 Issue 7 Pages
074706
Published: July 15, 2007
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Crystal and magnetic structure analyses have been performed for single crystals of multiferroic materials
RMn
2O
5 (
R = Y, Ho, Er) using the neutron diffraction technique. For all the compounds, the magnetic structure in the commensurate magnetic phase, where spontaneous electric polarization occurs, was determined to be a transverse spiral spin structure propagating along the
c-axis. The results demonstrate that the spin configuration for Mn
4+ and Mn
3+ ions is essentially the same in all three materials, suggesting that the ferroelectricity of the commensurate magnetic phase originates from the spin configurations of Mn ions. By contrast, the alignment of the induced 4
f-moment of Ho
3+ ions is quite different from that of Er
3+ ions, which might give a rich variety of magnetic field response for magnetic and dielectric properties in the
RMn
2O
5 system.
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Hiroshi Kontani
2007 Volume 76 Issue 7 Pages
074707
Published: July 15, 2007
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We study optical Hall conductivity for high-
Tc superconductors on the basis of microscopic Fermi liquid theory. Current vertex corrections (CVCs) are correctly taken into account to satisfy the conservation laws, and are performed for the first time for optical conductivities based on the fluctuation-exchange (FLEX) approximation. We find that (I) the CVC emphasizes the ω-dependence of σ
xy(ω) significantly when the antiferromagnetic (AF) fluctuations are strong. For this reason, the relation σ
xy(ω)∼{σ
xx(ω)}
2, which is satisfied in the extended Drude model given by the relaxation time approximation (RTA), is completely violated for a wide range of frequencies. Consequently, (II) the optical Hall coefficient
RH(ω) strongly depends on ω below infrared frequencies, which is consistent with experimental observations. Interestingly, although σ
xx(ω) does not follow a simple Drude form due to the ω-dependence of the relaxation time τ, θ
H(ω) and σ
xy(ω) follow approximate simple Drude forms since the ω-dependences of τ and CVC almost cancel each other out. In conclusion, anomalous optical transport phenomena in high-
Tc superconductors, which had been frequently assumed to be as evidence of the breakdown of the Fermi liquid state, can be well explained in terms of the nearly AF Fermi liquid once the CVC is taken into account.
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Tatsuma D. Matsuda, Shugo Ikeda, Etsuji Yamamoto, Yoshinori Haga, Yosh ...
2007 Volume 76 Issue 7 Pages
074708
Published: July 15, 2007
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We have succeeded in growing a single crystal of UCu
2Ge
2 with the tetragonal structure by the Sn-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization and specific heat. This is the first single crystal growth of UCu
2Ge
2. The magnetic properties are highly anisotropic, reflecting the crystal structure. An easy-axis of magnetization is found to be the [001] direction (
c-axis) with a saturation moment 1.75 μ
B/U, while the [100] direction (
a-axis) is the hard-axis in magnetization, indicating an Ising-type ferromagnetism. The electronic specific heat coefficient is determined as 30 mJ/(K
2·mol).
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Rui Sakano, Tomoko Kita, Norio Kawakami
2007 Volume 76 Issue 7 Pages
074709
Published: July 15, 2007
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We study the thermopower and some related transport quantities due to the orbital Kondo effect in a single quantum dot system with a finite value of Coulomb repulsion by means of the noncrossing approximation applied to the multiorbital impurity Anderson model. It is elucidated how the asymmetry of the renormalized tunneling resonance due to the two-orbital Kondo effect causes characteristic behavior of the thermopower at finite temperatures under gate-voltage and magnetic-field control, which is compared with that of the ordinary spin Kondo effect.
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Mamoru Fukunaga, Keisuke Nishihata, Hiroyuki Kimura, Yukio Noda, Kay K ...
2007 Volume 76 Issue 7 Pages
074710
Published: July 15, 2007
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In order to determine an exact relationship between magnetic ordering and ferroelectricity in multiferroic rare-earth manganese oxides, we have performed simultaneous measurements of neutron diffraction, ferroelectric
D–
E hysteresis loops and the permittivity ε of ErMn
2O
5 down to 2.5 K. Special attention was paid to the area around the ferroelectric phase transition temperature. In contrast to previous expectations based on separate measurements, the peak of ε was found to coincide with a magnetic incommensurate–commensurate phase transition. However, it was confirmed that electrical polarization appears at the transition to a one-dimensionally incommensurate magnetic phase, which is higher than the peak of ε by 1.2 K. A previously unknown low-temperature incommensurate magnetic phase of ErMn
2O
5 below 8 K was found. The lowest-temperature phase appears to be paraelectric according to the
D–
E loops, while pyroelectric measurements show that the phase exhibits a small polarization.
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Hidetsugu Kitatani, Hidetoshi Nishimori, Akira Aoki
2007 Volume 76 Issue 7 Pages
074711
Published: July 15, 2007
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We derive a rigorous lower bound on the average local energy for the Ising model with quenched randomness. The result is that the lower bound is given by the average local energy calculated for the isolated case, i.e. in the absence of all other interactions. The only condition for this statement to hold is that the distribution function of the random interaction corresponding to the average local energy under consideration is an even function of randomness (symmetric). All other interactions can be arbitrarily distributed including non-random cases. A non-trivial fact is that any introduction of other interactions to the isolated case always leads to an increase of the average local energy, which is opposite to ferromagnetic systems where the Griffiths inequality holds. Another inequality is proved for asymmetrically distributed interactions: When we consider the value of the thermal average of local energy in the configurational space of randomness, the probability for this value to be lower than that for the isolated case takes a maximum value on the Nishimori line as a function of the temperature. In this sense the system is most stable on the Nishimori line.
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Hisashi Takado, Yoshiaki Hashimoto, Akira Endo, Shingo Katsumoto, Yasu ...
2007 Volume 76 Issue 7 Pages
074712
Published: July 15, 2007
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We have observed anisotropic states of two-dimensional hole systems (2DHS) in higher Landau levels which are analogous to those found in 2D electron systems (2DES), the so-called stripe phase. Application of in-plane magnetic field along the hard (high resistance) direction stabilizes the anisotropic state, while that along the easy axis hardly affect the state. This is in contrast to the corresponding 2DES case where the stripe is easily reoriented by an in-plane magnetic field, and suggests that the built-in symmetry-breaking cause which favors a particular stripe orientation acts more strongly in 2DHS than in 2DES. We discuss the possible origin of the orientational pinning.
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Nobuya Maeshima, Kenji Yonemitsu
2007 Volume 76 Issue 7 Pages
074713
Published: July 15, 2007
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We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.
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Mikito Koga, Masashige Matsumoto
2007 Volume 76 Issue 7 Pages
074714
Published: July 15, 2007
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A new feature of Kondo effect due to an orbitally degenerate impurity is presented for a crystal-field singlet–triplet case in the
f2 configuration. There are two stable states at low temperatures: one is a Fermi-liquid dominated by the
f2 singlet; the other is a non-Fermi liquid stabilized by a quadrupolar type of exchange interaction between the
f2 triplet and conduction electrons. The numerical renormalization group analysis shows that the competition of both states strongly enhances a coupling of conduction electrons with a scalar type of hexadecapolar moment of the impurity state, indicating that the charge degrees of freedom is relevant for the multiorbital case. This result supports the superiority of high-rank multipoles in
f-electron systems such as the Pr-based filled skutterudite compounds.
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Yoshiya Homma, Dai Aoki, Yoshinori Haga, Hironori Sakai, Shugo Ikeda, ...
2007 Volume 76 Issue 7 Pages
074715
Published: July 15, 2007
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We succeeded in growing a single crystal of NpSb
2 with the SmSb
2-type orthorhombic structure by the Sb-flux method and measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization. NpSb
2 is found to be an Ising-type ferromagnet with the Curie temperature
TC=47 K, where the ordered moment of 1.6 μ
B/Np is oriented along the [001] direction. The electronic specific heat coefficient is estimated to be 22 mJ/(K
2·mol). The electrical resistivity for the current
J||[001] is by two orders of magnitude larger than that for
J⊥[001], suggesting the quasi-two-dimensional electronic state.
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Naomi Kawamura, Hiroshi Maruyama, Motohiro Suzuki, Tetsuya Ishikawa
2007 Volume 76 Issue 7 Pages
074716
Published: July 15, 2007
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To study the Zn and Ga 4
p electronic states in relation to the magnetic phase transition in Mn
3MC (
M=Zn or Ga) perovskites, the temperature dependence of the X-ray absorption spectrum and X-ray magnetic circular dichroism was recorded at the Zn and Ga
K-edges. As a result, it is found that the non-magnetic Zn and Ga atoms exhibit 4
p orbital polarization in the ferromagnetic state. For the second-order phase transition in Mn
3ZnC, the canted-ferromagnetic component is separated from the ferromagnetic one, and the temperature behavior is compared with the case of the first-order transition in Mn
3GaC. The Zn and Ga 4
p polarizations are strongly correlated with the Mn 4
p polarized states.
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Masaaki Geshi, Koichi Kusakabe, Hitose Nagara, Naoshi Suzuki
2007 Volume 76 Issue 7 Pages
074717
Published: July 15, 2007
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We investigated the magnetism in Ca and Sr pnictides by using the first-principles calculations. These compounds are half-metallic and ferromagnetic (FM) when they assume the zinc-blende structure at the equilibrium lattice constant. Ferromagnetism is induced by the spin polarization of the
p-orbitals of the pnictogen atoms; Ca and Sr atoms have no magnetic moments, which is different from that of CrAs or CrSb with a zinc-blende structure. To confirm the mechanism of the ferromagnetism, we have calculated a hypothetical crystal — fcc-As with two additional electrons — and have shown that fcc-As has the same magnetic moment as CaAs with a zinc-blende structure. This means that the role of Ca or Sr atoms is to provide electrons with As atoms at the fcc site and to sustain the distances between the As atoms and crystal symmetry. The FM exchange interactions between the pnictogen atoms are considered to exist in these lattices, which is briefly discussed.
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Bunju Shinozaki, Kazumasa Makise, Yukio Shimane, Hiroaki Nakamura, Kaz ...
2007 Volume 76 Issue 7 Pages
074718
Published: July 15, 2007
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Electron weak localization (WL), electron–electron (el–el) interaction and electron–phonon (el–ph) scattering effects on the transport properties have been investigated for two to three-dimensional indium zinc oxide films. The temperature
T dependence of the electron inelastic scattering rate 1⁄τ
in of two-dimensional films has been deduced from magnetoconductance data. The rate 1⁄τ
in well fits the sum of el–el and el–ph inelastic scattering rates, 1⁄τ
in=1⁄τ
in,el–el+1⁄τ
in,el–ph. Using the free electron model to determine the diffusion constant, we have obtained the effective electron mass
m*≈0.55
m0 by fitting the theory for 1⁄τ
in,el–el to data at sufficiently low temperatures. Under a high magnetic field applied to suppress WL, the Hall coefficient varies, independently of the dimensionality, with temperature by about a two times larger rate than that of resistance. This result agrees well with the theory of Al’tshuler, Aronov, and Lee. At temperatures above ≈10 K, the
T dependence of the inverse of mobility 1⁄μ is well described by the Grüneisen–Bloch formula with the use of the Debye temperature Θ
D≈750–760 K. The el–ph coupling constant λ
el–ph≈1.1–1.3 has been obtained from the combination of data on τ
in(
T), 1⁄μ(
T),
m*, and Θ
D.
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Takashi Koretsune, Yukitoshi Motome, Akira Furusaki
2007 Volume 76 Issue 7 Pages
074719
Published: July 15, 2007
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We study Mott transition in the two-dimensional Hubbard model on an anisotropic triangular lattice. We use the Lanczos exact diagonalization of finite-size clusters up to eighteen sites, and calculate Drude weight, charge gap, double occupancy and spin structure factor. We average these physical quantities over twisted boundary conditions in order to reduce finite-size effects. We find a signature of the Mott transition in the dependence of the Drude weight and/or charge gap on the system size. We also examine the possibility of antiferromagnetic order from the spin structure factor. Combining these information, we propose a ground-state phase diagram which has a nonmagnetic insulating phase between a metallic phase and an insulating phase with antiferromagnetic order. Finally, we compare our results with those reported in the previous theoretical studies, and discuss the possibility of an unconventional insulating state.
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Masashi Arita, Hitoshi Sato, Masayuki Higashi, Kunta Yoshikawa, Kenya ...
2007 Volume 76 Issue 7 Pages
074720
Published: July 15, 2007
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We have performed the O 1
s x-ray absorption (XAS) and resonant inverse photoemission spectroscopy (RIPES) in Mott insulator YTiO
3, band insulator CaTiO
3 and Y
0.61Ca
0.39TiO
3 with a metal–insulator transition. In CaTiO
3, the spectrum has strong π
* and σ
* peaks derived from the Ti 3
d–O 2
p hybridization bands near the Fermi level. On the other hand the peaks almost disappear in the XAS spectrum of YTiO
3. In Y
0.61Ca
0.39TiO
3, the intensities of the π
* and σ
* structures increase with decreasing temperature, showing that degree of the
p–
d hybridization increases from the insulator to metal phases.
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Kenji Sasaoka, Chikara Ishii
2007 Volume 76 Issue 7 Pages
074721
Published: July 15, 2007
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Magnon current through mesoscopic ferro- or antiferromagnetic channel, sandwiched by bulk magnets, is investigated with a special attention to the effects of spin diluted sites introduced in the central scattering region of the channel. A scattering theory is developed to derive Landauer-type formula for magnon current which is applicable to both ferro- and anti-ferromagnetic channels. Making use of this formula, a series of numerical calculations of magnon transmission coefficient has been performed in the case of several typical configurations of spin diluted sites. It is shown that magnon transmission coefficient shows a series of sharp dips in the vicinity of bottom of magnon energy bands, reflecting the formation of resonance states in this energy region.
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