We investigate spin-charge mixing effect on resonant tunneling in spin-polarized Tomonaga-Luttinger liquid with double impurities. The mixing arises from Fermi velocity difference between two spin species due to Zeeman effect. Zero bias conductance is calculated as a function of gate voltage V-g, gate magnetic field B-g, temperature, and magnetic field applied to the system. Mixing effect is shown to cause rotation of the lattice pattern of the conductance peaks in (V-g,B-g) plane, which can be observed in experiments. At low temperatures, the contour shapes are classified into three types, reflecting the fact that effective barrier potential is renormalized towards "perfect reflection," "perfect transmission," and magnetic field induced "spin-filtering," respectively.

We consider the tunnelling of phonon excitations across a potential barrier spatially separating two condensates with different macroscopic phases. We analyse the relation between the phase difference φ of the two condensates and the transmission coefficient T by solving the Bogoliubov equations. It is found that T strongly depends on φ, and that the perfect transmission of low-energy excitations disappears when the phase difference reaches the critical value which gives the maximum supercurrent of the condensate. We also discuss the feasibility of observing the phase differences in experiments. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

The effects of collective modes on the thermoelectric properties of a charge density system is studied. We derive the temperature dependences of thermoelectric power and thermal conductivity by applying the linear response theory to the Frohlich Hamiltonian. Energy dissipation has been attributed to the nonlinear interaction between the phase mode and the amplitude mode, ignoring disorder effects. We have found that the temperature dependence of the correlation function between electrical and heat currents is the same as that of the correlation function between electrical currents. This implies that thermoelectric power is inversely proportional to temperature. We have also found that the temperature dependences of all correlation functions are essentially determined by the same mechanism-nonlinear amplitude-phase interaction. The thermal conductivity is nearly constant at a temperature above the amplitude mode gap, and is exponentially low at a temperature sufficiently below it.

We investigate collective excitations of Bose-Einstein condensates at absolute zero in a double-well trap. We solve the Bogoliubov equations with a double-well trap, and show that the crossover from the dipole mode to the Josephson plasma mode occurs in the lowest energy excitation. It is found that the anomalous tunneling property of low energy excitations is crucial to the crossover. ©2005 The Physical Society of Japan.

We consider edge reconstruction of electrons in a two-dimensional harmonic trap under a strong magnetic field. In this system the edge reconstruction occurs as a result of competition between electron-electron interaction and confining potential. We develop a bosonization scheme for two-dimensional electron systems. With this method we obtain the excitation spectrum and demonstrate that the edge reconstruction occurs when magnetic field reaches a critical value. We also show that the edge reconstruction depends on the number of electrons. We calculate the third-order terms of bosons in Hamiltonian and examine the effect of those terms with a perturbation theory.

With use of the Kronig-Penney model, we study the excitation spectrum of a Bose-Einstein condensate in a one-dimensional periodic potential. We solve the Bogoliubov equations analytically and obtain the band structure of the excitation spectrum for arbitrary values of the lattice depth. We find that the excitation spectrum is gapless and linear at low energies, and that it is due to the anomalous tunneling of low-energy excitations, predicted by Kagan et al.

We investigate spin-charge mixing effect on the resonant tunneling in a spin-polarized Tomonaga-Luttinger liquid, which arises from Fermi velocity differences between two spin species due to Zeeman effect. Zero bias conductance is calculated as a function of gate voltage V-g gate magnetic field B-g, temperature and magnetic field applied to the system. Mixing effect is found to cause deformations of contour shapes of the conductance in (V-g,B-g) plane. At low temperatures, contour shapes are divided into three types; the conductance peaks (1) vanish, (2) form lattices, and (3) form lines. Each corresponds to the phase impurity potential become (1) irrelevant for two spins, (2) relevant for two spins, and (3) relevant for one selective spin. (c) 2005 Elsevier B.V. All rights reserved.

We consider the edge reconstruction of electrons in a two-dimensional harmonic trap under a strong magnetic field. In this system the edge reconstruction occurs as a result of competition between electron-electron interaction and confining potential. To describe it, we develop a bosonization scheme for two-dimensional electron systems. With this method we obtain the excitation spectrum and confirm that the edge reconstruction occurs when the value of the magnetic field reaches the critical value. We also show that the edge reconstruction depends on the number of electrons too. (c) 2005 Elsevier B.V. All rights reserved.

We study the superfluid-Mott-insulator transition of antiferromagnetic spin-1 bosons in an optical lattice described by a Bose-Hubbard model. Our variational study with the Gutzwiller variational wave function determines that the superfluid-Mott-insulator transition is a first-order one at a part of the phase boundary curve, contrary to the spinless case.

The antiferromagnetic spin-1 bosons in an optical lattice are studied by the Gutzwiller approximation. We find that the superfluid-Mott insulator transition is of a first-order on a part of the phase boundary.

Entropy and specific heat are calculated using the spin polaron formulation at finite temperature. Our theoretical approach makes use of the Matsubara Green's function method where the interaction term in the S-matrix is the spin polaron Hamiltonian, which is constructed in a representation where holes are described as spinless fermions (holons) and spins as normal bosons. In the absence of this interaction term, the normal entropy and specific heat are obtained from the free holon thermodynamic potential and are found to resemble the BCS expressions in the low temperature regime. A second cumulant expansion of the thermodynamic potential with the spin polaron interaction yields an expression for the specific heat whose dominant term in the low temperature limit and small quasiparticle energy difference, resembles the superconducting-state electronic specific heat of the BCS theory.

We consider the tunneling of phonon excitations across a potential barrier spatially separating two condensates, which have different phases. We analyze the relation between the phase difference of two condensates and the coefficient of tunneling transmission by solving the Bogoliubov-de Gennes equations. As a result, the transmission coefficient T depends strongly on because of the -dependent quasi-bound state. © 2004 Plenum Publishing Corporation.

We generalize the perturbation theory of weakly interacting bosons to Bose-Einstein condensates with hyperfine spin F=1. Analytical expressions of second-order self-energies beyond the mean-field approximation are derived at zero temperature. We further extend the perturbation theory to the finite-temperature case, and derive the corresponding self-energies. We evaluate these self-energies near the poles of the first-order (mean-field) Green's functions within the on-shell approximation, and expand the results in powers of wave number in the long-wavelength limit. Excitation spectra for each mode are obtained analytically at low temperatures. We also derive the ground-state energy and thermodynamic quantities such as free energy of the system.

We discuss the phase diagrams obtained by calculating temperature dependences of superconducting and density-wave correlation functions for a single (5, 0) carbon nanotube (CN). We use one-loop renormalization group method within logarithmic accuracy. In this system, we must specify scattering channels in terms of momentum along the circumferential direction as well as the axis direction, because (5,0) CN has two degenerate bands crossing the Fermi energy with circumferential momenta. We find that the most divergent order is singlet superconducting or charge-density wave with 0 or 10pih/2piR circumferential momentum, where R is the tube radius. (C) 2002 Elsevier Science B.V. All rights reserved.

Using the finite temperature (Matsubara) Green's function method, the hole spectral function is calculated in a representation where holes are described as spinless fermions (holons) and spins as normal bosons. The holon self-energy term for multiple spin-wave processes is analytically determined with the spin polaron Hamiltonian as the interaction term in the S-matrix. The usual prescription of analytic continuation then enables us to obtain the retarded expression from the Matsubara self-energy term yielding the holon spectral function.

We study the excitation spectrum of a spatially homogeneous Bose-condensed gas. Using finite-temperature field theory, we derive the dependence of the excitation spectrum on both the gaseous parameter and the temperature. In particular, we derive the asymptotic forms of the spectral weight at finite temperature. We then discuss the effects of the interaction between the excitations on the dispersion relation, in particular on its curvature for small momentum. From the gaseous parameter dependence of the curvature at zero temperature, we find that there exists a threshold for the gaseous parameter, above which the spectrum becomes stable and no damping processes occur. The excitation stability is analyzed at finite temperature, and it is found that the effect of the finite temperature is to increase the stability of the excitation. We numerically calculated the ratio of the critical velocity to the sound velocity, finding that the critical velocity is small compared to the sound velocity for specific gaseous parametcrs and temperatures.

We study the influence of damping on quantum interference of a particle in free space and in a harmonic potential. Using the influence-functional method, it is shown that the suppression of quantum interference is described by the ratio of the thermal diffusion induced by the environment to the quantum-mechanical diffusion. We describe the interference pattern as a function of the friction constant and the temperature, and discuss the difference between the strong damping limit and the classical limit.

general formula for the supercurrent between different internal structures in a wide class of hybrid junctions is derived on the basis of the Andreev-reflection picture. The formula extends existing formulas and also enables us to analyze novel B-phase/A-phase/B-phase junctions in super fluid He-3 systems. We propose a mechanism for pi states due to the (l) over cap texture in the A phase of the junction, which could elucidate major features of the pi states with higher critical current (H states) discovered in super fluid 3 He weak links. The bistability of the pi states is also discussed.

We investigate the properties of skyrmion in the ferromagnetic state of spin-1 Bose-Einstein condensates by means of the mean-field theory and show that the size of a skyrmion is fixed to the order of the healing length. It is shown that the interaction between two skyrmions with oppositely rotating spin textures is attractive when their separation is large, following a unique power-law behavior with a power of -7/2.

We have performed a variational analysis on the evolution of superconductivity from weak to strong coupling regime. We have improved the Bardeen Cooper Schrieffer (BCS) wave function, and show that the electronic correlations are important in obtaining the correct behavior of superconducting condensation energy In contrast to a crossover without thermodynamic anomaly discussed in a dilute system. we show the existence of a quantum phase transition near half filling. which suggests that an evolution is not always continuous. The transition is driven by charge density waves instabilities. We have found that superconductivity and charge density waves coexist in the presence of a weak intersite repulsion. The ground state phase diagram is determined by varying both interactions and filling.

We calculate the d.c. Josephson current through charge density wave (CDW) region connected to two superconductors (S-1, S-2) by tunnel junctions. At T = 0 and without impurities, we discover infrared divergence in the d.c. Josephson current caused by the phase mode in the long-junction limit: d/nu (F) >> (h) over bar/Delta (s), (h) over bar/Delta (CDW) where T is temperature. These limits are expressed by using the energy gap in S region Delta (S), the length d, the Fermi velocity nu (F), and the energy gap Delta (CDW) in CDW region. The critical current is proportional to (Delta (CDW)(h) over bar nu (F)/d)(1/2) proportional to Delta (CDW) (xi (CDW)/d)(1/2). At beta Delta (S)(T), beta Delta (CDW)(T) >> 1 and B (h) over bar nu (F)/d << 1, the T-dependence of the correction caused by the phase mode is T-linear where beta = 1/k(B)T and k(B) is Boltzmann constant, At finite temperatures and with impurities, the correction caused by the phase mode becomes proportional to ln T. At finite temperatures and without impurities, the critical current is similar to the Ambegaokar-Baratoff formula which is proportional to Deltas(T) in the short-junction limit: d/nu (F) << (h) over bar/Delta (S), (h) over bar/Delta (CDW). (C) 2001 Elsevier Science B.V. All rights reserved.

We calculate de Josephson current through charge density wave (CDW) region connected to two superconductors (S-1, S-2) by tunnel junctions at zero temperature. We discover infrared anomaly of the de Josephson current caused by phase mode in the long junction limit. In this limit, the Josephson current is proportional to the characteristic energy consisting of physical quantities in CDW region, while in the short junction limit, the Josephson current is similar to the Ambrgaokar-Baratoff formula which is proportional to the energy gap of superconductor. (C) 2001 Elsevier Science B.V. All rights reserved.

We calculated electronic states and Josephson currents for superconducting multilayers with one-dimensional SISIS and SNSNS junctions, where S is a superconductor, I, an insulator, and N, a normal metal. We obtained numerical solutions of the Bogoliubov-de Gennes equations for the case where the Fermi wavelength of quasi-electrons and quasiholes is comparable to the coherence length, barrier width, and thickness of the intermediate superconductor. It was found that the maximum Josephson current has a peak structure as a function of barrier height slightly above the Fermi energy, because of resonant tunneling of quasi-electrons and quasi-holes. Though the maximum Josephson current of the SNSNS double junction is smaller than that of the SNS single junction, the maximum Josephson current of the SISIS junction is larger than that of the SIS junction for a range of barrier heights above the Fermi energy. The Josephson current is a superposition of sin(phi /2) and sin phi terms, where rp is a phase difference of the superconductors in both ends. The sin(phi /2) term is dominant for the SISIS junction, while the sin phi, term dominates the SNSNS junction, and their coefficients vary continuously as a function of the barrier height. (C) 2001 Elsevier Science B.V. All rights reserved.

We propose a simple phenomenological model for dissipative dynamics of a superfluid. Qualitative agreement between the results of our numerical calculations and of the experiment on a superfluid helium three weak link system gives strong support for our model. (C) 2000 Elsevier Science B.V. All rights reserved.

We obtain full, exact numerical solutions for eigenvalue problems of the Bogoliubov-de Gennes (BdG) equations for the case of superconducting multilayers. Furthermore, the transport current was calculated. This powerful numerical calculation enables us to get solutions for systems with arbitrary barrier and pair potentials where the Fermi-wavelength for quasi-electrons and quasi-holes is comparable to the size of the Cooper pair and coherence length. We apply this new method to one-dimensional SIS and SNS junctions, where S is a superconductor, I is an insulator, and N is a normal metal. It was found that even in this simple system the Josephson current differs from the perfect sinusoidal function and its components through continuous and quasi-bound states have a variety of directions and magnitudes owing to the wave nature of quasi-electrons and quasi-holes. (C) 2000 Elsevier Science B.V. All rights reserved.

我々は、無限大次元のHubbard模型をGutzwiller波動関数に反強磁性相関を取り入れることで以下のことを示した。Brinkman-Rice理論で記述されるFermi液体から反強磁性体ヘモット転移する際、有効質量は増大するが発散しない。ハーフ・フィリングでは、ハバード・ギャップが開くことで絶縁体化する。キャリアをドープすると反強磁性金属相が現れるが、ランダウのフェルミ液体論を反強磁性領域に拡張して解析した結果、この領域では準粒子間に引力が働く。従って、超伝導相を含む豊富な相が実現し、相互作用の大きさや粒子数を変化させることで量子相転移することが判明した。