Hidetoshi Fukuyama

Phase Diagram of Two-Dimensional Organic Conductors: (BEDT-TTF) 2X

The effects of on-site Coulomb interaction in two-dimensional organic conductors, κ-(BEDT-TTF) 2 X, α-(BEDT-TTF) 2 I 3 and (BEDT-TTF) 2 MHg(SCN) 4 , have been studied within the Hartree-Fock approximation. If the strength of Coulomb interaction exceeds some critical value, various kinds of antiferromagnetic states appear and these phases have been classified on a phase diagram with two key parameters, which leads to a unified view on the ground states of this unique family.

Magnetoresistance in Two-Dimensional Disordered Systems: Effects of Zeeman Splitting and Spin-Orbit Scattering

Effects of Zeeman splitting and spin-orbit scattering on the resistance in two-dimensional disordered systems are theoretically studied. The field dependence of the magnetoresistance is shown to have the characteristic anisotropy. The present theory explains the qualitative features of the experimental observations by Komori et al. in Cu granular films.

Mean Field Theory of RVB and Superconductivity

The mean field theory for the coupled fermion (spinon)-boson (holon) system is developed by taking account self-consistently of coherent motions of both spinon and holon and singlet pair of spinon (RVB). The relative phase of RVB order parameters in x - and , y -directions, whose magnitudes are fixed to be the same, is treated as arbitrary. The phase, which minimizes the free energy, is found to be π for wide range of parameters. The bose condensation of holons is also examined in the case of weak interlayer electron transfer integrals. The specific heat, magnetic susceptibility and nuclear relaxation rate are calculated. It is found that their anomalies at the superconducting transition temperature depend markedly on the concentration of holons. The T -linear term in specific heat at low temperatures is shown to be vanishing, which is consistent with the recent experiments in Bi-Sr-Ca-Cu-O by Sera et al . and Kumagai et al .

Fermi surface and spin fluctuations in extended t-J model

The spin fluctuations in the extended t - J model have been investigated in the slave-boson mean-field theory by extending the early work by Suzumura et al. [J. Phys. Soc. Jpn. 57 (1988) 2768] based on both uniform and singlet RVB (Resonating Valence Bond). By choosing the transfer integrals to reproduce the Fermi surface of LSCO and YBCO, which is essentially rotated by 45°, it is found that incommensurate spin fluctuations can be expected for the Fermi surface of LSCO type, whereas only commensurate ones for that of YBCO type, in agreement with experiments. Moreover, the Knight shift is shown to decrease as the temperature is lowered in the presence of the singlet RVB, while it is essentially independent of temperature in the uniform RVB state. These two cases correspond to, in our classification, T c =60 K and T c =90 K YBCO systems, which also agrees with the qualitative difference observed experimentally.

Solitons in Spin-Peierls Systems and Applications to Polyacetylene

Analytical estimate of the formation energy, E s , of a soliton is performed for a system of strongly correlated one-dimensional π-electrons coupled to lattice distortions. The problem is first transformed into that of a spin-Peierls systems and then the spin degrees of freedom are represented by the phase Hamiltonian with the help of the boson representation of the spinless fermions. Following Cross and Fisher, parameters in this phase Hamiltonian are adjusted so that the spin wave velocity and the critical exponent of the correlation function agree with the exact results. The soliton solution of the resulting quantum sine-Gordon systems is constructed by the method of Dashen et al. The result is applied to polyacetylene, where we obtain E s ≈0.3 eV, which is close to the value estimated by Su et al. for the non-interacting systems.

Localization Effects in Two-Dimensional Superconductors

Effects of localization on two-dimensional superconductors have been investigated by taking into account quantum corrections to the theory of dirty superconductors in the order of ( e F τ 0 ) -1 where e F and τ 0 are the Fermi energy and the relaxation time of an electron, respectively. The mean field superconducting transition temperature is seen to be reduced by localization. This is because the Coulomb repulsive interaction is enhanced and the density of states of electrons is depressed in dirty systems. Recent experiments by Kobayashi et al. on granular films of tin are discussed in this context.

Effects of Interactions on Non-Metallic Behaviors in Two-Dimensional Disordered Systems

It is shown that the mutual interactions in the presence of impurity scattering leads to ln T term not only in the conductivity but also in the imaginary part of the self-energy function of two-dimensional electrons gas. Thus the prefactor of ln T term in the conductivity is not universal but depends on the strength of interactions.

Effect of Spin-Orbit Interaction on Magnetoresistance in the Weakly Localized Regime of Three-Dimensional Disordered Systems

Effects of Zeeman spin splitting and spin-orbit interaction on magnetoresistance in three-dimensional disordered systems are investigated in the weakly localized regime and the results are compared with recent experiments on InSb by Morita et al.

Antiferromagnetic Long Range Order in Disordered Spin-Peierls Systems

A theoretical possibility of the existence of disorder-induced antiferromagnetic long range order in spin-Peierls system has been presented, which will be in accordance with the recent experimental indication of the coexistence of dimerization and antiferromagnetism in CuGe 1- y Si y O 3 .

Pinning in Peierls-Frohlich State and Conductivity

The equation is derived which describes the dynamics of the phase of the charge density wave in the Peierls-Frohlich state in the presence of the impurity scattering. By use of the correlation function of the phase, the frequency dependence of the electrical conductivity, σ(ω), is calculated at T =0 K for the case of weak pinning forces. It is found that σ(ω) has a pronounced peak at a finite ω and that the theoretical results. If ω is replaced by ( T - T p ) where T p is the transition temperature the results are reminiscent of the conductivity in the region of fluctuations.