We examine the stiffness of the ground-state of the Heisenberg spin-glass (SG) model in two (d = 2) and three (d = 3) dimensions. We calculate the excess energy Δ E_L(ω) which is gained by twisting the system of size L^d-1×(L+1) by a twist angle ω. We find that the energy can be scaled as Δ E_L(ω) ∼ A ω^α L^θ_s with α ∼ 1.9, and θ_s ∼ 0 for d = 2 and θ_s ∼ 0.8 for d = 3. These results are significantly different from those of the conventional defect energy method but rather similar to those of the pure ferromagnetic Heisenberg model. Thus, it is suggested that, in d = 3, the SG phase might occur at finite temperatures.

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The nonequilibrium relaxation (NER) process of fluctuations of physical quantities is studied simulationally. It is shown that the NER method, which is a convenient technique for studying the phase and transition, is useful not only for identifying the phase and locating the transition point, but also for estimating both static and dynamical exponents. As an example, the cubic-lattice ferromagnetic Ising model is analyzed. The transition inverse-temperature is estimated to be K_c=0.2216595(15). The exponents of correlation length, magnetization, and specific heat are estimated to be ν =0.635(5), β = 0.325(5) and α = 0.14(2), respectively. The dynamical exponent z is estimated to be 2.055(10).

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The phase transformation of α '-NaV₂O₅ has been studied by electron diffraction. It has been observed for the first time that two different types of diffuse scatterings exist; one appears slightly above T_c and the other appears already at room temperature. The former type of diffuse scattering locks into superlattice reflections at T_c. The superlattice reflections and the former type of diffuse scattering are explained by a long-period anti-phase-domain structure (LPAPDS) model and the fluctuation of its period.

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The space group of the room-temperature phase of α '-NaV₂O₅ has been determined to be centrosymmetric Pmmn by convergent-beam electron diffraction, which confirms the recent X-ray results. This result indicates that there exists only one type of vanadium site assigned to V^4.5+ at room temperature.

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The Josephson effect in cyclically coupled Bose-Einstein condensates is studied theoretically. We analyze the simultaneous Gross-Pitaevskii equations with coupling terms between adjacent condensates. Depending on the initial relative phases between condensates, Josephson current flows cyclically to create a quantized vortex. Reducing the coupling between condensates changes the motion from periodic to chaotic, thus suppressing the cyclic current. The relation to the Kibble-Zurek mechanism is discussed; the density of the generated vortices is less than that predicted by this mechanism.

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The doping of a two-orbital chain with mobile S=1/2 fermions and strong Hund's rule couplings stabilizing the S=1 spins strongly depends on the presence of a level difference among these orbitals. Using density matrix renormalization group (DMRG) methods, we find a finite spin gap upon doping and dominant pairing correlations without level difference, whereas the presence of a level difference leads to dominant charge density wave (CDW) correlations with gapless spin-excitations. The string correlation function also shows qualitative differences between the two models.

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We analyze the two-dimensional d-p model, considering both antiferromagnetic spin fluctuation and d_x²-y²-wave superconducting fluctuation. We adopt the fluctuation-exchange approximation in order to derive both normal and anomalous vertices composed only of the renormalized d-electron Green function and the on-site repulsive interaction among the d-electrons. Using these vertices, we derive a t-matrix as a superconducting fluctuation propagator. Then, we obtain self-consistent solutions in which the system is close to antiferromagnetic instability. In our solutions, the superconducting fluctuation couples strongly with the quasiparticle state and this causes the anomalous behavior in the temperature dependences of spin susceptibility and specific heat.

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Low-temperature resistivity of half-metals is investigated. To date it has been discussed that the one-magnon scattering process in half-metals is irrelevant for low-temperature resistivity, due to the fully spin-polarized electronic structure at the ground state. If one takes into account the non-rigid-band behavior of the minority band due to spin fluctuations at finite temperatures, however, the unconventional one-magnon scattering process is shown to be most relevant and gives T³ dependence in resistivity. This behavior may be used as a crucial test in the search for half-metallic materials which are potentially important for applications. Comparison with resistivity data of La_1-xSr_xMnO₃ as candidates for half-metals shows good agreement.

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Effects of the superconducting pair potential on the impurity scattering processes in metallic carbon nanotubes are studied theoretically. The backward scatterings of electron- and hole-like quasiparticles disappear. The impurity gives rise to backward scatterings of holes for incident electrons, and it also induces backward scatterings of electrons for incident holes. Negative and positive currents induced by such scatterings between electrons and holes cancel each other. Therefore, the nonmagnetic impurity does not hinder the supercurrent in the regions where the superconducting proximity effects occur, and thus the metallic nanotube is a good conductor for Cooper pairs. Relations with experiments are discussed.

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Inspired by Kondo et al.'s experiment,[Kondo] which reports that the p(2× 1) symmetric dimer phase is stable at low temperatures on a Si(001) surface, a two-leg ladder Hubbard model with three Fermi surfaces, which is an effective model of the Si(001) surface, is studied theoretically. The fluctuation-exchange (FLEX) method is adopted to take the effects of on-site Coulomb interactions into account. The electronic structures are discussed and an unconventional even-parity superconductivity through large antiferromagnetic spin fluctuations is found for a wide range of doping. Other possible electronic states with large superconducting fluctuations are also discussed.

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We point out that pairing interactions mediated by antiferromagnetic fluctuations necessarily include both singlet channels and triplet channels as attractive interactions. By taking into account this feature, a phase diagram of quasi-low-dimensional type II superconductors in parallel magnetic fields is proposed in a system where antiferromagnetic fluctuations contribute to the pairing interactions. A singlet pairing is favored at zero field, while a triplet pairing occurs at high fields where the singlet pairing is suppressed by the Pauli paramagnetic pair-breaking effect. As a result, the critical field increases divergently at low temperatures. A possible relation to experimental phase diagrams of a quasi-one-dimensional organic superconductor is briefly discussed. We also discuss the possibility that a triplet superconductivity is observed even at zero field.

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Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements have been performed on an intermetallic compound CeRu₂ down to 2.2 K. Vacuum tunneling spectra have been measured on the surface prepared by cracking the sample in helium gas at 4.2,K. The shape and temperature dependence of the quasiparticle density of states, N_s(E,r), have been well interpreted in terms of s-wave BCS superconductors. In magnetic fields, the hexagonal vortex lattice has been imaged by measurements of N_s(E=0,r) up to 2,T. This is the first observation of the vortex lattice by STS in heavy-fermion superconductors.

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The staggered local field at boron nuclei in the anti-ferromagnetic phase of NdB₆ has been measured by ¹¹B-NMR. The angular dependence of the local field is explained as a type I anti-ferromagnetic structure by taking the domain distribution into account, which is consistent with that obtained by the neutron scattering measurement. The origin of the local field is explained as mainly the dipolar field from the magnetic moment at the Nd site. The saturated magnetic moment is estimated to be 1.6 μ _B /Nd.

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¹⁶¹Dy Mössbauer effects were measured on the tetragonal antiferroquadrupolar ordered compound DyB₂C₂. In the magnetic phase below T_C=15.3 ,K, the Mössbauer spectra are interpreted well by a single set of hyperfine parameters which gives the Dy magnetic moment of 9.0 ,μ_B. The hyperfine field anomalously decreases below T_C as temperature decreases, which can be interpreted as symmetry frustration between antiferromagnetic and antiferroquadrupolar interactions acting on Dy³⁺ ions. Reflecting the 4f electronic state in which two Kramers doublets locate with a low-energy separation, the spectra in the antiferroquadrupolar ordered phase and paramagnetic phase are complicated and cannot be well analyzed by the two-level relaxation model with a single set of hyperfine parameters, although it is supposed that the Dy electronic state is one type.

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The properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically using the effective spin and pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state is hardly changed so that the spin-exchange coupling along the c-axis changes nearly continuously from positive to negative and is approximately zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes rapid suppression of the critical temperature, as observed experimentally.

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Charge ordering in the bilayer manganite system La_2-2xSr_1+2xMn₂O₇ with 0.30 ≤ x ≤ 0.50 is studied by neutron diffraction. The charge order is characterized by the propagation vector parallel to the [1 0 0] direction (MnO₂ direction), but the correlation length is short-ranged and extremely anisotropic, being ∼0.02a^* and ∼0.2a^* parallel and perpendicular to the modulation direction, respectively. The observed charge order can be viewed as a quasi-bistripe order, and accounts well for the x dependence of the resistivity. The quasi-bistripe order is stable within the ferromagnetic (FM) MnO₂ layers in the A-type antiferromagnetic order, but is destabilized by the three dimensional FM order.

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This paper presents a numerically integrated solution of moving boundary problems involving mass diffusion in polymer-penetrant system with a swelling controlled release of drug. Dividing the diffusion process into a finite number of fixed boundary problems, we have assumed the local similarity assumption to valid during the short period. Unlike the existing parameter expansion methods, the present solution is expected to be valid over a large range of control parameter representing the difference between solvent concentration at interface and the equilibrium value within the polymer. At small times, our numerical result shows that a larger values of the control parameter leads to a shorter interval for zero order drug release providing a higher release rate.

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Dynamic evolution of a small-scale structure to a large-scale structure is studied by a nonlinear dissipative LC ladder circuit. The circuit is described by a Burgers equation in the lowest order. We confirmed that the observed propagation of a shock wave and the confluence of shock waves are well described by the theory. A single hump wave is used to model a small-scale structure. When two single-hump waves with different amplitudes are applied to the circuit, they evolve into a large-scale structure in the course of propagation. Such a formation of a large-scale structure is unique in a nonlinear dissipative system, while the nonlinear dispersive system is known to preserve the nature of small-scale structures even after their interaction as known for solitons.

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Dynamics and decoherence are studied in a theoretical model in which an incident particle interacts with an array of harmonic oscillators. This system is analogous to the generalized Coleman-Hepp model treated in the previous paper and can also be formulated systematically and solved exactly. In spite of the similarities there exist certain differences between the two. We give detailed numerical calculations and discussions on the decoherence processes.

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By numerical means we investigate moments for a free path length in 2D Sinai's billiard systems ranging from ergodic systems to non-ergodic ones individually. The starting point is a formula of a mean free path length < l> expressed as a phase space average over an entire Birkhoff space. On the basis of the formula, we investigate the moments < lⁿ> for 0<n<2 numerically, and search the relation between < lⁿ> and < l> . Investigating rectangular, triangular and honeycomb lattices in which either an ellipse-shaped scatterer or a segment-shaped scatterer is located, for 0<n<1.2, we find almost universal two scaling indices representing the behavior of the moments irrespective of the details of the systems, when we adopt the mean free path length as a scaling variable. These results imply that the mean free path length can be interpreted as a relevant scaling variable in all cases. We also discuss some theoretical reasoning about the universality.

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In this paper we examine the effects of electron Coulomb distortion on the second, fourth, and fifth structure functions and the left-right asymmetry (A_LT) for ¹⁶O(e,e'p) reactions. For large incident electron energy the effects of the Coulomb distortion on the cross section are very small but have larger contribution on the structure functions. We obtain that the Coulomb effects are about 5%∼10% on the fourth and fifth structure functions and the left-right asymmetry in small missing momentum region. However, on the second structure functions, the effects are relatively small comparing with the fourth and fifth structure functions while they are bigger than the corresponding cross sections. In addition we compare our theoretical results with recently measured data from CEBAF.

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The photoelectron photoion coincidence (PEPICO) spectra have been measured for Se₂ and larger cluster, ``Se<sub>5</sub>'', at photon energies of 12.63keV and 12.68keV (i.e., on the both sides of the Se K-edge) by utilizing synchrotron radiation at the BL-12C station in Photon Factory, KEK. In the PEPICO spectra of Se<sub>2</sub>, broad peaks corresponding to z/n = 1 to 7 are observed and no clear peaks are seen at the half-integer of z/n. Most of the peaks are split into asymmetric doublets. These facts indicate that multiply charged ions are formed by the vacancy cascade as a consequence of X-ray absorption and exploded due to the Coulomb repulsion (Coulomb explosion). The experimentally observed peak width in the PEPICO spectrum can be interpreted quantitatively by assuming that the maximum initial charge before the Coulomb explosion is equal to 8. Although there is large apparent difference between the branching ratios of Se<sub>2</sub> and ``Se₅'', they are well reproduced on the both sides of the K-edge, by considering that the charges are randomly distributed within the clusters before the Coulomb explosion.

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Numerical estimation shows that an upper energy limit of electrons confined in the Nagoya Bumpy Torus, NBT-1M, exists under electron-cyclotron heating (ECH). This energy limit is determined from the power balance between the microwave input and the losses due to synchrotron radiation and Coulomb drag by bulk electrons, when the maximum harmonic number of ECH corresponding to the energy limit plays an important role. The calculation result of the orbit of the particle reveals us that mirror-trapped electrons can have energies of 10 MeV, while electrons passing through linked mirrors cannot be confined if their energies are above 3 MeV for the magnetic current of 4.2 kA. Successive ECH to a higher harmonic resonance with an injected microwave accelerates electrons, especially mirror-trapped ones, to the energy limit. However, the resonant heating efficiency estimated from the wave damping decreases as the harmonic number n _H increases, and also the total power loss increases as heated electrons become more energetic. The resultant power balance occurs at the electron energy of 4 MeV, to which resonances up to n_H≤ 10 contribute. This numerical result agrees with the observed maximum electron energy of 4 MeV.

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Plasma potential formation with electron cyclotron heating is investigated in a fully-ionized collisionless plasma flow along converging magnetic-field lines (monotonously increasing magnetic field) in the presence of a single electron cyclotron resonance (ECR) point. When the ECR point is located in a region of good curvature of the magnetic line, a potential hump (plug potential) with a potential dip (thermal barrier) is generated around the ECR point. This potential structure persists in the steady state, working as a plasma-flow dike potential. In case that the ECR point is located in a region of bad curvature, on the other hand, the dike potential is only transiently formed, and collapses gradually as low frequency curvature-driven instabilities grow.

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The moment method in statistical dynamics is used to study the thermodynamic properties of metals taking into account the anharmonicity effects of the lattice vibrations and hydrostatic pressures. The explicit expressions of the lattice constant, thermal expansion coefficient, and the specific heats C_v and C_p of cubic (fcc) metals are derived within the fourth order moment approximation. The thermodynamic quantities of Al, Au, Ag, Cu, and Pt metals are calculated as a function of the pressure, and they are in good agreement with the corresponding experimental results. The effective pair potentials work well for the calculations of transition and noble metals, compared to those of the sp-valence metals. For obtaining better agreement of the thermodynamic quantities of metals like Al, it is required at least to use the more sophisticated electronic many body potentials. In general, it has been shown that the anharmonicity effects of lattice vibration play a dominant role in determining the thermodynamic properties of metals under high pressures and at the finite (high) temperatures.

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Dielectric and thermal measurements were carried out for the RbH₂PO₄ crystal belonging to the tetragonal system at room temperature. The RbH₂PO₄ crystal, which could be obtained by heating the tetragonal RbH₂PO₄ crystal above the tetragonal-monoclinic transformation temperature, undergoes successive phase transitions at 390.1K and 248.8K. The tetragonal-monoclinic transformation is accompanied by a rearrangement of hydrogen bonds, namely, a change from a three-dimensional network of hydrogen bonds in tetragonal RbH₂PO₄ to a two-dimensional network in monoclinic one. The tetragonal space group {I\bar{4}2d} changes to the monoclinic P2₁/c by passing through the transformation temperature.

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The thermal conductivity κ(T), thermal diffusivity α(T) and magnetization M(T) have been measured for sintered CaMnO₃ and La_1-xCa_xMnO₃ in lightly electron-doped region (0.98≤ x≤ 1.0). κ(T) and α(T) of CaMnO₃ show an anomalous enhancement below the weak ferromagnetic transition temperature T_N. The enhancement is rapidly reduced by introduction of a small amount of Mn³⁺ ions in La_1-xCa_xMnO₃. The analyses suggest the possibility that the enhancement comes from the contribution of the heat transport by magnetic excitations.

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The pressure dependence of ionic conductivity was carried out in a single crystal of KTiOPO₄ (KTP) to 2GPa at room temperature in the frequency range from 100Hz to 10MHz. The samples were pressed by a piston-cylinder-type high-pressure vessel, and the pressure was monitored by using a manganin gage. The ionic conductivity decreased nonlinearly and monotonously with increasing pressure. The decrease of ionic conductivity would be mainly due to the increase of an activation energy for the ionic conduction. The increase of the activation energy was estimated to be 0.01eV at 2GPa. It is considered that the increase of the activation energy is due to the decrease of the size of ``bottle necks'' which are formed by oxygen atoms in the framework.

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We report an attempt to calculate energy eigenvalues of large quantum systems by the diagonalization of an effectively truncated Hamiltonian matrix. For this purpose we employ a specific way to systematically make a set of orthogonal states from a trial wavefunction and the Hamiltonian. In comparison with the Lanczos method, which is quite powerful if the size of the system is within the memory capacity of computers, our method requires much less memory resources at the cost of the extreme accuracy. In this paper we demonstrate that our method works well in the systems of one-dimensional frustrated spins up to 48 sites, of bosons on a chain up to 32 sites and of fermions on a ladder up to 28 sites. We will see this method enables us to study eigenvalues of these quantum systems within reasonable accuracy.

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The Kleinman's dielectric function with appropriately screened particle-hole interactions is numerically studied to investigate its effect on the spectral shape of the imaginary part, particularly for q larger than twice the Fermi wavenumber p_f. It is found that the spectral shape is remarkably enhanced on the low energy side under the influence of the proper inclusion of particle-hole interactions for q=2.28p_f, the smallest reciprocal lattice vector of sodium, in contrast with Hubbard's dielectric function. The interband optical absorption strength of sodium calculated from the Hopfield formula using this dielectric function is therefore enhanced enough to be in good agreement with experiment.

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We study dimensionality effects on the charge gap in the dimerized Hubbard model at quarter filling, with two approaches. First, we examine three chains coupled via the interchain one-particle hopping integral t_b, by the density-matrix renormalization-group (DMRG) method. Next, we consider the d=1+ε dimensional model, using the perturbative renormalization-group (PRG) method. The dimensionality is controlled through t_b and ε, respectively. Both approaches lead to the conclusion that, for a finite dimerization ratio, the charge gap decreases as the dimensionality increases.

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Quasiparticle energies of AlN, GaN, ZnO, ZnS and ZnSe in the wurtzite structure have been calculated by the GW approximation with a full random phase approximation dielectric matrix instead of using plasmon pole approximation. The linear muffin tin orbital basis was used for this work and the d orbitals of the Zn and Ga atoms were treated as valence state explicitly in every case. The calculated quasiparticle band gaps and the semicore levels by the GW method are in good agreements with the experimental values. The electronic energy levels of these five systems calculated using the local density, GW and Hartree-Fock approximations were compared systematically.

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We address the question as to why the ordering of O_xy-type quadrupole moment predominates that of T_xyz octupole moment at no applied field in CeB₆, in spite of the same coupling constant predicted. In the mean field approximation (MFA), the same results are obtained for the quadrupolar and the octupolar interactions. If we go beyond the MFA to apply the constant coupling approximation, it is shown that the ordering of O_xy is stabilized compared with that of T_xyz, as far as each interaction is treated separately.

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Dynamical process of the formation of nonlinear excitations such as charged or neutral solitons from a photogenerated electron-hole pair in polyacetylene is studied numerically, based on Su-Schrieffer-Heeger's model extended to involve on-site as well as nearest-neighbor electron-electron Coulomb repulsion terms. The electron-electron interactions are treated within the time-dependent unrestricted Hartree-Fock approximation. Lattice fluctuations and a weak local disorder are introduced in order to trigger the formation and to represent many different initial conditions. Starting from an initial configuration with an electron at the bottom of the conduction band and a hole at the top of the valence band, separated by the Peierls gap, the time dependent Schrödinger equation for the electron wave functions and the equation of motion for the lattice displacements are solved numerically. It is not systematic but rather stochastic whether nonlinear excitations are created or not. We have obtained branching ratios and distributions of formation time for different types of nonlinear excitations.

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We theoretically study how the coexistent state of the charge density wave and the spin density wave in the one-dimensional quarter filled band is affected by magnetic fields. We found that when the correlation between electrons is strong the spin density wave state is suppressed under high magnetic fields, whereas the charge density wave state still remains. This will be observed in experiments such as the X-ray scattering measurement.

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We show within the slave-boson mean field approximation that the two-dimensional t-J model has an intrinsic instability toward forming a quasi-one-dimensional (q-1d) Fermi surface. This q-1d state competes with, and is overcome by, the d-wave pairing state for a realistic parameter choice. However, we find that a small spatial anisotropy in t and J exposes the q-1d instability which has been hidden behind the d-wave pairing state, and brings about the coexistence with the d-wave pairing. We argue that this coexistence can be realized in La_2-xSr_xCuO₄ systems.

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The Landau parameters F₀^s , F₀^a ,F₁^s and F₁^a in the two-dimensional Hubbard model are calculated by the second order perturbation method of Abrikosov-Khalatnikov. The following points are revealed as the half-filling is approached: (1) F₀^a changes its sign from negative to positive, if the Coulomb interaction U is strong enough, i.e., U/t\gtrsim 2.0 . This means that the spin density oscillations, zero spin-sound, can propagate there; (2) F₁^s changes its sign from positive to negative, so that we can understand, on the basis of the anisotropic Fermi liquid theory, the behavior of the Drude weight whose magnitude decreases as the half-filling is approached even if there were no mass enhancement; (3) From the results of F₀^s and F₀^a , it is shown that both the charge and spin susceptibility decrease rapidly close to the half-filling. The latter is regarded as the so-called spin-gap behavior.

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The effect of uniaxial strain to spin-density-wave (SDW) transition in the organic conductor (TMTSF)₂PF₆ was studied. Strain was applied along the a-, b'- and c^*-axes separately. It was found that the SDW transition was most strongly suppressed by the strain along the one-dimensional (1-D) a-axis, and most weakly along the c^*-axis, whereas our first expectation was that the most effective strain to the SDW suppression was along the b'-axis, since the interaction along b or b' is directly associated with the denesting of the quasi-one-dimensional Fermi surface. We discuss why the strain along the axes other than the b'-axis suppress the 1-D nature and therefore SDW.

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A new type of photon-assisted tunneling (PAT) which will occur in a Mach-Zender electron interferometer with a δ-function time-varying potential is analytically predicted. In contrast to conventional PAT in Fabry-Perot electron interferometers, quite acute and cusp-like PAT signals appear in the conductance of the system. The width of the cusp is proportional to u₁²/ω, where u₁ and ω are the strength and the frequency of the time-varying potential. Numerical calculations using a transfer matrix technique show that this type of PAT will be observed in actual experiments where the time-varying potential has a broader spatial profile and the temperature is finite.

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Effects of a contact between a carbon nanotube and metallic electrode are studied in a tight-binding model. A model of dirty contact is introduced and discussed for electrode weakly coupled to a carbon nanotube. Measurements of a perfect transmission in two-terminal measurement may be possible by the using of the contact with both weak coupling and large area.

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Effect of interfacial roughness on Giant Magnetoresistance (GMR) in Fe/Cr multilayers has been studied by simultaneously depositing multilayers on a set of float glass substrates prepared with varying rms surface roughness. Morphological and other microstructural features of different multilayers are similar except for the interfacial roughness, thus allowing one to separate out the effect of interface roughness. GMR measurements on these multilayers show that increasing interfacial roughness causes GMR to decrease nonlinearly. GMR tends to saturate to a constant value for higher interfacial roughnesses because of the fact that different multilayers differ mainly in the correlated part of interfacial roughness.

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Although extensive studies have been carried out concerning the effective visible photoluminescence (PL) from porous and nanostructure Si, no conclusive argument about the mechanism of PL has been reported so far. The well-known quantum confinement model is appealing, but is an oversimplified model. As a first step towards a detailed analysis of the problem, we report in this series of papers local electronic properties of low-dimensional Si nanostructures, calculated in the non-orthogonal tight-binding (NTB) scheme. In the present paper (part1), we propose the existence of characteristic ``layer states'' for the electrons in a quantum wire, which is important in a detailed analysis of PL from Si. A succeeding paper (part2) will be devoted to the analysis of the local density of states.

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Stability of Li/Al(001)-c(2×2) structure in which Li atoms are adsorbed substitutionally on Al(001) surface is studied by performing first-principles total-energy calculations. Our results for optimized structural parameters of the system are in reasonably good agreement with experiment. It is found that the c(2×2) structure is stabilized mainly due to a binding-energy gain of Li with Al atoms. Density of states of the c(2×2) structure of Li/Al(001) is compared with that of Na/Al(001) system to clarify bonding nature of alkali-atom adsorption.

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We have studied on the possibility of superconductivity in the two-chain Hubbard model by using the variational Monte Carlo method changing the interchain transfer energy t_d. The energy gain per site Δ E in the variational BCS state in reference to the normal state energy, which is nothing but the superconducting condensation energy, was computed. We have fitted Δ E as a function of the inverse of the lattice size by a power-law function and found that in the bulk limit Δ E remains finite in a certain narrow range of t_d. Calculated momentum distribution functions reveal that the superconductivity is most stable in the case where the electron distribution in k-space is in a one-band-like situation, on the brink of transition to a two-band-like one. The SDW-type trial wave function was also examined but we did not obtain any energy gain with it in the above-mentioned superconducting region.

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We theoretically investigate the effect of magnetic field on the pseudogap phenomena in High-T_c cuprates. The obtained results well explain the experimental results including their doping dependences. In our previous paper [J. Phys. Soc. Jpn. 68 (1999) 2999], we have shown that the pseudogap phenomena observed in High-T_c cuprates are naturally understood as a precursor of the strong coupling superconductivity. On the other hand, there is an interpretation for the recent high field NMR measurements to be an evidence denying the pairing scenarios for the pseudogap. In this paper, we investigate the magnetic field dependence of NMR 1/T₁T on the basis of our formalism and show the interpretation to be inappropriate. We consider the Landau quantization for the superconducting fluctuations as a main effect of the magnetic field. The results indicate that the value of the characteristic magnetic field (B_ch) is remarkably large in case of the strong coupling superconductivity, especially near the pseudogap onset temperature (T^*). Therefore, the magnetic field dependences can not be observed and T^* does not vary when the strong pseudogap anomaly is observed. On the other hand, B_ch is small in the comparatively weak coupling case and T^* varies when the weak pseudogap phenomena are observed. These results properly explain the high magnetic field NMR experiments continuously from under-doped to over-doped cuprates. Moreover, we discuss the transport phenomena in the pseudogap phase. The behaviors of the in-plane resistivity, the Hall coefficient and the c-axis resistivity in the pseudogap phase are naturally understood by considering the d_x²-y²-wave pseudogap.

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Effects of the 2D superconducting fluctuation in a weak coupling case on the Cooper pairing susceptibility χ(q, ω) and the single particle Green's function are examined by taking account of the most divergent contributions to those. It is found that the most divergent terms due to vertex corrections to the irreducible parts of χ(0, 0) are renormalized to be comparable to less divergent terms due to the self-enegy corrections, which play main roles eventually. Then, χ(0, 0) diverges as T^(-T_F/2T)/log|T_F/T| at T → 0, where T and T_F are temperature and the Fermi temperature, respectively, while the pseudogap is developed in the single particle spectra below T_c⁽⁰⁾ to reach its maximum value $\Delta^{(0)} e^{-\frac1{12}}$ at T=0, where T_c⁽⁰⁾ and Δ⁽⁰⁾ are the transition temperature and the energy gap at T=0 obtained in the mean field approximation, respectively.

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We investigate effects of double scattering by antiferromagnetic spin fluctuation (magnon), denoted as DMS, on both the singlet and triplet particle-particle (p-p) interaction in the two-dimensional Hubbard model. Different band structures with different next-nearest-neighbor hopping parameter but with a fixed electron occupation number of around 0.85 are considered. It is first indicated that due to the retardation of magnon contribution from the p-p and hole-hole (h-h) pair in the intermediate states also arises in the vertex correction, which has sign opposite to that of the usual particle-hole (p-h) pair. The contributions of the p-h pair and of the p-p and h-h pair to the DMS correction are enhanced for large and small momentum transfer (MT), respectively. A model calculation is performed to show that when the Fermi level is near the Van Hove level, the DMS correction becomes comparable to the single magnon results. For singlet pairing state the DMS p-p interaction is repulsive at both large and small MT, while for triplet pairing state it is attractive at both MT. This indicates that the odd frequency anisotropic s wave triplet state is a possible candidate for superconducting states of the model, in addition to the x²-y² wave singlet one.

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The relation between the pseudogap state and the superconducting phase in strong coupling superconductors (like cuprates and organics) is investigated by extending the self-consistent T-matrix approximation below the transition temperature (T_c). The superconducting order parameter is obtained by solving the gap equation which includes the superconducting fluctuation in a self-consistent way; the fluctuation causes the pseudogap behavior above T_c. The rapid growth of the order parameter below T_c is found to be a characteristic feature of the strong coupling superconductors. As a result of the rapid growth of the order parameter, the spectral density at the Fermi level in the one-particle spectrum rapidly decreases and that at the peak position sharply grows as soon as the system goes into the superconducting state. These observations are consistent with angle-resolved photoemission spectroscopy experiments. The properties of the one-particle spectrum above and below T_c are found to be well explained by considering the self-energy correction originating from the superconducting fluctuation and the development of the order parameter.

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Ising-like Heisenberg model with a degree of freedom of degenerate orbits at each site is investigated. The orbital variable is two-fold and coupling constants between spin variables depend on the orbital configuration. The interactions between spin variables, however, are always fully frustrated. The model was originally proposed to explain magnetic properties of LiNiO₂ (S=1/2) by Kitaoka. We investigate thermodynamic properties of a generalized model with orbital degeneracy in the classical spin model mainly by a Monte Carlo method. This model is found to have various kinds of orders such as the sublattice ordering of spins, the orbital ordering and in particular, a new type of ordering where a product of the orbital variable and the spin variable becomes ordered while each of the variables is not ordered. Changing the dependence of the coupling constant of the spin variables on the orbital configuration, a rich structure of phase diagram is found.

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We performed a detailed study of the double exchange interaction. Having rearranged Kubo and Ohata's Hamiltonian [J. Phys. Soc. Jpn. 33 (1972) 21], we found that it can be treated perturbatively for the temperaure lower than T_C. The spin wave propagator was calculated and the spin stiffness was computed. It agrees reasonablly well with experimental results. We believe that the double exchange interaction is the fundamental mechanism in the perovskite manganites.

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