Kunihiko Yamaji

On the Angle Dependence of the Magnetoresistance in Quasi-Two-Dimensional Organic Superconductors

The dependence of the title is shown to originate in a singular angle dependence of the distribution width of the area of the semiclassical closed orbits under the magnetic field in the reciprocal space. In quasi-two-dimensional metals with a cylindrical Fermi surface, the distribution width of the area of the orbits on the Fermi surface vanishes at certain inclination angles of the magnetic field, entailing the appearance of remarkable effects. The observed peaks of the coarse-grained magnetoresistance are found to correspond to these special angles.

Origin of the Field-Induced SDW in the Bechgaard Salts

The energetic gain giving rise to the magnetic-field-induced SDW state in the Bechgaard salts is clarified to originate in the effect of the field to the electronic orbital motion. A variational total energy of the SDW state under the field is obtained on the basis of the anisotropic two-dimensional Hubbard model. Its semi-classical and fully quantum evaluations for the case of the most basic set of the SDW wave vector and the magnetization clearly show a gain of the total energy under the field, which is in fair agreement with the observed increase of the perpendicular magnetic susceptibility. The total energy proves to be lowered by quantization of the closed orbits, since the zero-field state density in the energy region of closed-orbits is a decreasing function of energy. The nature of the successive SDW phase transitions under the field is briefly discussed.

On the High-Temperature Susceptibilities of the Two-Dimensional Ferromagnetic Heisenberg Spin Systems

With the honeycomb, square and triangular lattices the high-temperature susceptibilities and specific heats have been extended up to the eighth and seventh order, respectively, for general spin by means of the method of Rushbrooke and Wood with the aid of high-speed computers. Analyses of these coefficients by means of the ratio and Pade approximant methods seem to give evidences for non-divergence at finite temperatures of the high-temperature susceptibilities of the two-dimensional ferromagnetic Heisenberg spin systems contrary to the assertion of Stanley and Kaplan.

Superconducting transition of the two-chain Hubbard model indicated by diagonalization calculations

Abstract In the two-chain Hubbard system the on-site Coulumb interaction at each site generates a pair-transfer interaction between the two bands consisting of bonding and antibonding orbitals for the two sites located on the opposite chains and linked by the transverse transfer energy. It is shown to be as strong as the intraband repulsive interaction generated in the same way so that it can give rise to superconductivity due to the two-band effect. Verifying this reasoning, exact-and also variational-diagonalization calculations clearly indicate that a superconducting state arises in this system in an appropriate region of band and filling parameters for a wide range of Coulomb strengths. A scaling analysis with systems of electron number equal to 6, 10 and 14 supports the assertion that the superconductivity survives in the limit of infinite size. The essence of this mechanism looks relevant to a wide variety of systems including D[M(dmit)2]2 superconductors and high-Tc oxides.

Variational Monte Carlo Indications of d-Wave Superconductivity in the Two-Dimensional Hubbard Model

The possibility of superconductivity in the ground state of the two-dimensional Hubbard model was investigated by means of the variational Monte Carlo method. With slight hole doping on the 6 ×6 and 10 ×10 lattices, a clear minimum was obtained on the curve of the total energy versus the amplitude of the d -wave gap function for large on-site Coulomb energies. Superconducting pair correlations were observed to be well developed. The-next-nearest-neighbor transfer was found to appreciably affect the minimum depth.

Density Correlation of Classical 1-d Electron Gas with Reference to the 4kFAnomaly in TTF-TCNQ

The intensity of the X-ray diffuse scattering from an electron-phonon system is calculated correct up to the second order of the electron-phonon interaction. The result consists of two terms: one is described as arising from softening of the phonon frequency due to the interaction, and the other as the shift of the center of the phonon coordinate. The former dominates when the electronic motion is faster than the phonon motion, and vice versa. It is argued that for a strongly Coulomb-correlated electron system, such as TTF-TCNQ, the electronic motion is slow enough to make the latter contribution important. A simple model for TTF-TCNQ is presented, which neglects the electron transfer entirely and for which the latter term is expressed in terms of the density correlation function of the electron system. This is calculated by applying the Monte Carlo method to a finite linear chain and also by using the hypernetted chain equation. We find a peak in the correlation function at q =4 k F , which explains the ...

Ground State of the Three-Band Hubbard Model

The ground state of the two-dimensional three-band Hubbard model in oxide superconductors is investigated by using the variational Monte Carlo method. The Gutzwiller-projected BCS and spin density wave (SDW) functions are employed in the search for a possible ground state with respect to dependences on electron density. Antiferromagnetic correlations are considerably strong near half-filling. It is shown that the d-wave state may exist away from half-filling for both the hole and electron doping cases. The overall structure of the phase diagram obtained by our calculations qualitatively agrees with experimental indications. The superconducting condensation energy is in reasonable agreement with the experimental value obtained from specific heat and critical magnetic field measurements for optimally doped samples. The inhomogeneous SDW state is also examined near 1/8 doping. Incommensurate magnetic structures become stable due to hole doping in the underdoped region, where the transfer

Stripe formation in high-Tc superconductors

{t}_{\mathrm{pp}}

Pressure- and Anion-Dependences of the SDW Phase Transition in the Bechgaard Salts

between oxygen orbitals plays an important role in determining a stable stripe structure.

Possible coexistence of superconductivity and static SDW stripes in the two-dimensional Hubbard model

The non-uniform ground state of the two-dimensional three-band Hubbard model for the oxide high-Tc superconductors is investigated using a variational Monte Carlo method. We examine the effect produced by holes doped into the antiferromagnetic (AF) background in the underdoped region. It is shown that the AF state with spin modulations and stripes is stabilized due to holes travelling in the CuO plane. The structures of the modulated AF spins are dependent upon the parameters used in the model. The effect of the boundary conditions is reduced for large systems. We show that there is a region where incommensurability is proportional to the hole density. Our results give a consistent description of stripes observed by the neutron-scattering experiments based on the three-band model for the CuO plane.