Showing papers in "Journal of the Physical Society of Japan in 1998"

Bose-Einstein Condensation with Internal Degrees of Freedom in Alkali Atom Gases

Abstract: The Bogoliubov theory is extended to a Bose-Einstein condensation with internal degrees of freedom, realized recently in 23 Na gases where several hyperfine states are simultaneously cooled optically. Starting with a Hamiltonian constructed from general gauge and spin rotation symmetry principles, fundamental equations for condensate are derived. The ground state where time reversal symmetry is broken in some cases and low-lying collective modes, e.g. spin and density wave modes, are discussed. Novel vortex as a topological defect can be created experimentally.

Berry's Phase and Absence of Back Scattering in Carbon Nanotubes.

Abstract: The absence of back scattering in carbon nanotubes is shown to be ascribed to Berry's phase which corresponds to a sign change of the wave function under a spin rotation of a neutrino-like particle in a two-dimensional graphite. Effects of trigonal warping of the bands appearing in a higher order k · p approximation are shown to give rise to a small probability of back scattering.

Quantum Transport in Two-Dimensional Graphite System

Abstract: In a self-consistent Born approximation, the density of states and the conductivity are calculated in a two-dimensional graphite sheet in magnetic fields. Two different cases of scatterers are considered, the short-range case where the range is smaller than the lattice constant and the long-range case where it is comparable or slightly larger. The quantum theory provides results quite different from the results of Boltzmann transport theory even in the absence of a magnetic field. In high magnetic fields, the conductivity exhibits a series of peaks, whose values depend only on the natural constants and the Landau level index. The conductivity of undoped systems is always given by a universal conductivity \(e^{2}/\pi^{2}\hbar\) independent of a magnetic field.

Impurity Scattering in Carbon Nanotubes Absence of Back Scattering

Abstract: The effective potential of an impurity in a k · p scheme is derived in two-dimensional graphite sheet. When the potential range is smaller than the lattice constant, it has an off-diagonal matrix element between K and K ′ points comparable to the diagonal element. With the increase of the range, this off-diagonal element decreases rapidly and the diagonal element for envelopes at A and B sites becomes identical. The crossover between these two regimes occurs around the range smaller than the lattice constant. In the latter regime, back scattering between states with + k and - k vanishes identically for the bands crossing the Fermi level in the absence of a magnetic field, leading to an extremely large conductivity. The absence of the back scattering disappears in magnetic fields, giving rise to a huge positive magnetoresistance.

Two Spin-State Transitions in LaCoO 3

Abstract: Neutron-diffraction measurements have revealed a new anomalous thermal lattice expansion of LaCoO 3 near 500 K that indicates the existence of a second spin-state transition, in addition to the one previously established near 100 K. The anomalous expansion and the temperature dependence of the Co magnetic moments are successfully interpreted in a wide temperature range based on a simple model assuming low-spin (LS, S =0), intermediate-spin (IS, S =1), and high-spin (HS, S =2) states of Co atoms. The first spin transition, near 100 K, is from LS to IS, and the second, near 500 K, is from IS to a mixed state of IS and HS. The fitted model parameters indicate that the initially-large energy difference between IS and HS states decreases towards zero as the second transition proceeds. The large drop in resistivity associated with the latter transition appears to be correlated with the population of the HS state.

Spin Wave Mode of Edge-Localized Magnetic States in Nanographite Zigzag Ribbons

Abstract: We consider the low-energy magnetic excitations of nanographite ribbons with zigzag edges. The zigzag ribbons possess almost flat bands at the Fermi level which cause a ferrimagnetic spin polarization localized at the edge sites. The spin wave mode of this magnetic state is investigated by a random phase approximation of the corresponding Hubbard model. This result is used to derive an effective Heisenberg model with ladder structure. Although this system has a spin gap (Haldane type), our analysis shows that the gap is small and the tendency towards ferrimagnetic correlation at the edges is strong.

The Coupled Modified Korteweg-de Vries Equations

Abstract: Generalization of the modified KdV equation to a multi-component system, that is expressed by \(\frac{\partial u_i}{\partial t} + 6 \bigl( \sum_{j,k=0}^{M-1} C_{jk} u_j u_k \bigr) \frac{\partial u_i}{\partial x} + \frac{\partial^3 u_{i}}{\partial x^3} =0\), i =0, 1, …, M -1, is studied. We apply a new extended version of the inverse scattering method to this system. It is shown that this system has an infinite number of conservation laws and multi-soliton solutions. Further, the initial value problem of the model is solved.

Phase diagram of superconductivity on the anisotropic triangular lattice Hubbard model : An effective model of κ-(BEDT-TTF) salts

Abstract: We study the electronic states of the anisotropic triangular lattice Hubbard model at half-filling, which is a simple effective model for the organic superconducting κ-BEDT-TTF compounds. We treat the effect of the Coulomb interaction by the fluctuation exchange (FLEX) method, and obtain the phase diagram of this model for various sets of parameters. It is shown that d-wave superconductivity is realized in a wide region of the phase diagram, next to the antiferromagnetic states. The obtained phase diagram explains well the characteristics of the experimental results.

Spin Fluctuation-Induced Superconductivity in Organic Compounds

Abstract: Spin fluctuation-induced superconductivity in two-dimensional organic compounds such as κ-( ET) 2 X is investigated using a simplified dimer Hubbard model with a right-angled isosceles triangular lattice (transfer matrices -τ, -τ ′ ). The dynamical susceptibility and the self-energy are calculated self-consistently within the fluctuation exchange approximation, and the value for T c obtained by solving the linearized Eliashberg-type equations is in good agreement with the experimental results. The pairing symmetry is of the d x 2 - y 2 type. The calculated ( U /τ )-dependence of T c compares qualitatively well with the observed pressure dependence of T c . Varying the value for τ ′ /τ from 0 to 1, we interpolate between the square lattice and the regular triangular lattice and find first that relative values of T c for κ-( ET) 2 X and cuprates are reasonably explained and second that T c tends to decrease with increasing τ ′ /τ. No superconductivity is found for τ ′ /τ = 1, the regular triangular lattice.

Large-Scale Electronic-Structure Calculations Based on the Adaptive Finite-Element Method

Abstract: The adaptive finite-element method proposed in our previous work [Phys. Rev. B 54 (1996) 7602] is extended to fully self-consistent calculations of realistic materials. Our method is highly adaptive, sparse, parallel, and suited for the O(N) methods, thanks to the localized finite-element basis functions. Accurate ionic forces can also be calculated within practical time usage. Applications to the structural properties of diamond, c-BN, and the C 60 molecule, and molecular dynamics within O(N 3 ) scaling are shown first, followed by detailed error analyses. Then the O(N) method based on the orbital formulation is realized within our approach.

Interplay of Field-Induced Multipoles in CeB6

Abstract: We propose a theory of CeB 6 in a magnetic field by extending and slightly modifying a previous paper. The antiferro-quadrupolar ordering induces dipoles and octupoles when the magnetic field is applied. The nature of those field-induced multipoles is studied by changing the direction of the field. It is shown that the contradiction between NMR and neutron diffraction is resolved by taking into account the influence of octupoles. Thus it is concluded that CeB 6 is the first example in which the octupoles play a crucial role on the anomalous f -electron ordered phase.

Magnetization Plateaus in NH4CuCl3

Abstract: The magnetic susceptibility and high-field magnetization process of NH 4 CuCl 3 with double chains of CuCl 3 have been measured using single crystals. No anomaly indicative of the three-dimensional ordering is observed in susceptibility data above 1.7 K. It is found, by magnetization measurement down to 0.5 K, that in contrast to KCuCl 3 and TlCuCl 3 , NH 4 CuCl 3 has a gapless magnetic ground state at zero field. It is observed that the magnetization curve has two plateaus at one-quarter and three-quarters of the saturation magnetization, irrespective of the external field direction. The origin of the plateaus is attributed not to the magnetic anisotropy, but to the quantum effect. The relation between the plateaus and the period of the spin state is discussed in terms of a recent theory presented by Oshikawa et al . [Phys. Rev. Lett. 78 (1997) 1984].

Charge ordering and spin gap in NaV2O5

Abstract: A possible ground state of NaV 2 O 5 is proposed based on the Hartree approximation for both on-site and intersite Coulomb interactions. The results indicate that the intersite Coulomb interaction induces a zigzag type of charge disproportionation (i.e., charge ordering) along the ladders of V ions, resulting in localized spins between neighboring ladders to form a spin gap. This new state, which is different from the spin-Peierls state so far believed, seems to be consistent with the existing experimental results.

Correlation between the Doping Dependences of Superconducting Gap Magnitude 2Δ 0 and Pseudogap Temperature T* in High-T c Cuprates

Abstract: We report that in La 2- x Sr x CuO 4 (La214) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), the resistivity ρ and magnetic susceptibility χ decrease more sharply below T * , corresponding to the onset temperature of a spin gap (pseudogap) at least for Bi2212. In each system, T * scales with the superconducting gap magnitude 2Δ 0 ( T * ∼2Δ 0 /4.3 k B ), which increases as the doping level is lowered. We also report that 2Δ 0 is almost proportional to T max (> T * ), where T max is the temperature exhibiting a broad peak of χ and k B T max has been considered to give the measure of effective exchange energy J eff ; the α factors in 2Δ 0 ∼α k B T max ∼α J eff are nearly one and two for La214 and Bi2212, respectively.

Contrasting Kondo-Lattice Behavior in CeTSi 3 and CeTGe 3 (T=Rh and Ir)

Abstract: Specific-heat, magnetic-susceptibility and electrical-resistivity measurements were carried out on four Ce compounds of tetragonal BaNiSn 3 -type, CeTSi 3 and CeTGe 3 ( T= Rh and Ir). All these compounds show magnetic transitions at low temperatures. CeRhSi 3 and CeIrSi 3 are a Kondo-lattice compound with a relatively high Kondo temperature of the order of 100 K, while CeRhGe 3 and CeIrGe 3 exhibit a very weak Kondo effect, and three magnetic phase transitions below 14.6 K and 8.7 K, respectively. These contrasting behaviors in the silicides and germanides are discussed by comparing with properties of other BaNiSn 3 -type Ce compounds, like CeFeGe 3 and CeCoGe 3 .

Magnetic phase diagrams of Kondo compounds Ce0.75La0.25B6 and Ce0.6La0.4B6

Abstract: We measured the elastic and thermal properties to examine the competing effect between the intersite interactions and the Kondo coupling in Ce 0.75 La 0.25 B 6 and Ce 0.6 La 0.4 B 6 with the Γ 8 ground state. The huge softening of the transverse C 44 mode of both compounds at low temperatures indicates a remarkable fluctuation of the quadrupolar moments O y z , O z x and O x y with Γ 5 symmetry. In the magnetic phase diagram of Ce 0.75 La 0.25 B 6 , we found a new ordered phase IV in addition to the antiferroquadrupolar phase II and the antiferromagnetic phase III. The compound Ce 0.6 La 0.4 B 6 , however, shows the Fermi liquid state without the long-range order at low fields and transits into the ordered phase II or III in the magnetic field above 16 kOe.

Transformations for the Camassa-Holm Equation, Its High-Frequency Limit and the Sinh-Gordon Equation

Abstract: In this article we present a new transformation between two integrable hierarchies of the Camassa-Holm equation and the Hunter-Saxton equation. For instance we present a transformation between the Harry-Dym equation and the extended Harry-Dym equation. Moreover, we describe a relationship between the Hunter-Saxton equation, which is the high-frequency limit of the Camassa-Holm equation, and the Sinh-Gordon equation by a reciprocal transformation.

Structure of the Fulde-Ferrell-Larkin-Ovchinnikov State in Two-Dimensional Superconductors

Abstract: Nonuniform superconducting state due to strong spin magnetism is studied in two-dimensional (2D) type-II superconductors near the second order phase transition line between the normal and superconducting states. The optimum spatial structure of the order parameter is examined in systems with cylindrically symmetric Fermi surfaces. It is found that states with 2D structures have lower free energies than the traditional one-dimensional solutions, at low temperatures and high magnetic fields. For s -wave pairing, triangular, square, and hexagonal states are favored depending on the temperature, while square states are favored at low temperatures for d -wave pairing. In these states, the order parameters have 2D structures such as square and triangular lattices.

Neutron-Diffraction Studies on the Magnetic Ordering Process in the Layered Mn Perovskite La2-2xSr1+2xMn2O7 (x = 0.40, 0.45 and 0.48).

Abstract: Comprehensive neutron-diffraction studies on La 2-2 x Sr 1+2 x Mn 2 O 7 ( x =0.40,0.45 and 0.48) single crystals have revealed that the low-temperature magnetic phase consists of planar ferromagnetic (FM) and A-type antiferromagnetic (AFM) components, indicating a canted AFM ordering. Upon increasing the hole concentration x , the canting angle between planes changes from 6.3° (nearly planar FM) at x =0.40 to 180° (A-type AFM) at x =0.48, while the ordering temperature of the FM component, T C , decreases from 120 K to 0 K, correspondingly. We have also discovered that the A-type AFM ordering remains above T C and shows an anomalous exponential decrease to T N ∼200 K. This newly found intermediate A-type AFM phase may play a significant role in the enhancement of CMR effects in this layered Mn perovskite system.

Simulation of Lattice Polymers with Multi-Self-Overlap Ensemble

Abstract: A novel family of dynamical Monte Carlo algorithms for lattice polymers is proposed. Our central idea is to simulate an extended ensemble in which the self-avoiding condition is systematically weakened. The degree of self-overlap is controlled in a similar manner as the multicanonical ensemble. As a consequence, the ensemble – the multi-self-overlap ensemble – contains adequate portions of self-overlapping conformations as well as higher energy ones. It is shown that the multi-self-overlap ensemble algorithm correctly reproduces the canonical averages at finite temperatures of the HP model of lattice proteins. Moreover, it is superior in performance to the standard multicanonical algorithm when applied to a complicated problem of a polymer with eight-stickers. An alternative algorithm based on the exchange Monte Carlo method is also discussed.

Magnetic properties of REMe0.5Mn0.5O3 (RE = Rare Earth Element, Me = Ni, Co)

Abstract: Magnetic properties of REMe 0.5 Mn 0.5 O 3 with Me = Ni and Co are investigated for a series of rare earth (RE) elements. In both systems with Me = Ni and Co, the Curie temperature ( T C ), below which the magnetic moments of Me 2+ and Mn 4+ allign ferromagnetically, decreases remarkably as RE goes from La to heavier elements; from 295 K for RE = La to 50 K for Lu with Me = Ni, and from 250 K for La to 95 K for Ho with Me = Co. The decrease of T C is interpreted as due to the bond angle (θ) dependence of the superexchange interactions between Me 2+ and Mn 4+ through O 2- ; T C is roughly linear to cos 2 θ as predicted by a simple theory [C. Boekema et al. : Int. J. Mag. 3 (1972) 341], but a distinct deviation from the linear relation is observed for the late RE elements. The magnetic moments of RE ions appears only in a low temperature region, typically below 30 K. Qualitative discussion on the exchange interactions involving RE is presented.

Coexistence of SDW and Purely-Electronic CDW in Quarter-Filled Organic Conductors

Abstract: We have performed a mean field calculation in order to determine the ground state of a modified Hubbard model of a quarter-filled one-dimensional chain. It is found that the coexistence of 2 k F spin density wave (SDW) and purely-electronic 2 k F charge density wave (CDW) is realized due to the effect of next-nearest-neighbor Coulomb repulsion. Phase diagrams are obtained as the values of nearest- and next-nearest-neighbor Coulomb interaction energies are varied. Competition between the two coexistent states with 2 k F CDW and 4 k F CDW is revealed, which gives a first order transition. Our result supports the X-ray result confirming the presence of a coexisting phase of purely-electronic 2 k F CDW and 2 k F SDW in (TMTSF) 2 PF 6 , a quarter-filled one-dimensional chain in terms of holes.

Kondo Effect in Single Quantum Dot Systems — Study with Numerical Renormalization Group Method —

Abstract: The tunneling conductance is calculated as a function of the gate voltage in wide temperature range for the single quantum dot systems with Coulomb interaction. We assume that two orbitals are active for the tunneling process. We show that the Kondo temperature for each orbital channel can be largely different. The tunneling through the Kondo resonance almost fully develops in the region \(T \lesssim 0.1 T_{\rm K}^{*} \sim 0.2 T_{\rm K}^{*}\), where T K * is the lowest Kondo temperature when the gate voltage is varied. At high temperatures the conductance changes to the usual Coulomb oscillations type. In the intermediate temperature region, the degree of the coherency of each orbital channel is different, so strange behaviors of the conductance can appear. For example, the conductance once increases and then decreases with temperature decreasing when it is suppressed at T =0 by the interference cancellation between different channels. The interaction effects in the quantum dot systems lead the sensitivit...

Superconducting Properties of CeRu2

Abstract: We have measured the specific heat and the magnetization in the superconducting mixed state of CeRu 2 at low temperatures and in high magnetic fields. We have clarified a rich variety of superconducting properties such as the temperature dependence of the electronic specific heat and Maki parameters κ 1 , κ 2 and κ 3 . From these experimental results, CeRu 2 is concluded to be of the BCS-type in superconductivity. Nevertheless, interesting is the magnetic field dependence of the electronic specific heat at 0.5 K, indicating a \(\sqrt{H}\) like behavior in low fields and a H -linear dependence in high fields up to the upper critical field of 50 kOe. This characteristic specific heat is discussed on the basis of a microscopic theory by Brandt et al.

Low Temperature Properties of the Magnetic Semiconductor TmTe

Abstract: The magnetic susceptibility, elastic constant, specific heat and magnetization of the magnetic semiconductor TmTe have been measured in detail. A phase transition which is likely to be an antiferro quadrupolar ordering was found to occur at 1.8 K. The specific heat measurements under magnetic fields along the three main crystal axes revealed the unusual characters of this phase transition. The ( H - T ) phase diagram below 5 T is very similar to that of the antiferro quadrupolar ordering in CeB 6 . Above 5 T, however, the phase line for H ∥[100] begins to close toward T =0 K. The physical properties in the paramagnetic region at high temperatures are discussed in the mean field approximation. The theoretical fitting of the elastic softening indicates the antiferro inter-ionic quadrupolar interactions. However, the mean field theory can not explain the specific heat results.

Evidence for unconventional superconductivity of Sr2RuO4 from specific-heat measurements

Abstract: We measured the specific heat of single crystals of non-cuprate layered perovskite superconductor Sr 2 RuO 4 . The crystals with different T c up to 1.2 K exhibit a large residual electronic coefficient γ 0 , which decreases systematically with increasing T c . This behavior is consistent with the presence of nodes in the superconducting gap and with the variations of T c due to pair breaking by impurities and defects. To quantitatively account for the observed large γ 0 , however, we need to introduce additional mechanism.

Partially Disordered Phase in Frustrated Triangular Lattice Antiferromagnet CuFeO 2

Abstract: We reinvestigated successive magnetic phase transitions ( T N1 ∼14.0 K, T N2 ∼10.5 K) in a frustrated triangular lattice antiferromagnet (TLA) CuFeO 2 by neutron diffraction measurements using single crystals. The magnetic structure of the intermediate-temperature phase between T N1 and T N2 is found to be a quasi -long range ordered sinusoidally amplitude-modulated structure with a temperature dependent propagation wave vector ( q q 0). These features of successive phase transitions are well explained by reinvestigated Monte-Carlo simulation of a 2D Ising TLA with competing exchange interactions up to 3rd neighbors, in spite of the Heisenberg spin character of orbital singlet Fe 3+ magnetic ions.

Orbital Frustration and Resonating Valence Bond State in the Spin-1/2 Triangular Lattice LiNiO2.

Abstract: From the 7 Li nuclear magnetic resonance (NMR) and specific heat measurements of LiNiO 2 , consisting of a spin-1/2 triangular lattice, it is demonstrated that the previously suggested spin-glass-like freezing below T s g =8 K, which is caused by defects of Li ions, is not the true ground state, but rather a quantum disordered state is realized without a spin gap. This state is characterized by power-law behavior of the temperature dependence of the specific heat below 0.5 K. A spin-liquid state with short-range ferromagnetic correlations is suggested from new insight into the frustration of orbital ordering for doubly degenerate e g shells in the triangular lattice.

Charge ordering and sound velocity anomaly in La1-xSrxMnO3 (x ≥ 0.5)

Abstract: The electrical resistivity, magnetization, dilatation and sound velocity have been measured for La 1- X Sr X MnO 3 (0.48≤ X ≤0.90) polycrystals. Observed anomalies in the sound velocity and the dilatation strongly suggest the occurrence of the charge ordering within the Sr concentration range 0.48≤ X ≤0.82. A phase diagram of the charge-ordered state in La 1- X Sr X MnO 3 as a function of temperature T and the Sr concentration X is proposed.

A Density Matrix Algorithm for 3D Classical Models

Abstract: We generalize the corner transfer matrix renormalization group, which consists of White's density matrix algorithm and Baxter's method of the corner transfer matrix, to three dimensional (3D) classical models. The renormalization group transformation is obtained through the diagonalization of density matrices for a cubic cluster. A trial application to 3D Ising model with m = 2 is shown as the simplest case.