Mitake Miyazaki

Diagonal Stripe States in the Light-doping Region in the Two-dimensional Hubbard Model

We studied the ground state of the two-dimensional Hubbard model near half filling on the square lattice. Using Variational Monte Carlo method, we show that the diagonal stripe state of bond-centered type, in which the domain wall is located between two sites, becomes more stable than the vertical stripe state when the doping hole density x is as low as x ≃0.06. The inverse of stripe periodicity δ and the hole density x are observed to have the relationship δ∼ x (δ< x ) for the bond-centered diagonal stripe state (the vertical stripe state) in the light-doping region. These results are in good agreement with elastic neutron scattering experiments in La 2- x Sr x CuO 4 .

Incommensurate Antiferromagnetism Coexisting with Superconductivity in Two-Dimensional d–p Model

Numerical studies of the two-dimensional d–p model using the Gutzwiller ansatz have exhibited the incommensurate antiferromagnetic state coexisting with superconductivity in the under- and lightly doped regions. Our results are based on the variational Monte Carlo method for the three-band Hubbard model with d and p orbitals. We obtained the finite superconducting condensation energy for the coexistent sate at the doping rate x =1/8, 1/12, and 1/16, up to the systems of 256 unit cells with 768 atoms (oxygen and copper atoms). The phase diagram for the hole-doped case is consistent with recent results reported for layered high temperature cuprates.

Possible spin-singlet superconductivity in (TMTSF)2X: Superconducting transition temperature in a magnetic field

We study the transition temperature T c ( H ) of a quasi-one-dimensional (Q1D) superconductivity which is derived from the quantum effect of electron motion in a strong magnetic field. We calculate T c ( H ) of both isotropic and anisotropic superconductivity by taking account of the optimal momentum of the Cooper pairs and the effect of higher harmonic terms along the second conducting axis in the tight-binding model. We find that, although T c ( H ) of the spin-singlet superconductivity is strongly suppressed by the Zeeman effect, the suppression of T c ( H ) is not very severe if we take the optimal pair-momentum and the higher harmonic terms into account. The obtained T c ( H ) for the spin-singlet superconductivity is consistent with the experimental results in TMTSF salts.

Correlated-Electron Systems and High-Temperature Superconductivity

We present recent theoretical results on superconductivity in correlated-electron systems, especially in the two-dimensional Hubbard model and the three-band d-p model. The mechanism of superconductivity in high-temperature superconductors has been extensively studied on the basis of various electronic models and also electron-phonon models. In this study, we investigate the properties of superconductivity in correlated-electron systems by using numerical methods such as the variational Monte Carlo method and the quantum Monte Carlomethod. The Hubbard model is one of basic models for strongly correlated electron systems, and is regarded as the model of cuprate high temperature superconductors. The d-p model is more realistic model for cuprates. The superconducting condensation energy obtained by adopting the Gutzwiller ansatz is in reasonable agreement with the condensation energy estimated for YBa2Cu3O7. We show the phase diagram of the ground state using this method. We have further investigated the stability of striped and checkerboard states in the under-doped region. Holes doped in a half-filled square lattice lead to an incommensurate spin and charge density wave. The relationship of the hole density x and incommensurability δ, δ~x, is satisfied in the lower doping region, as indicated by the variationalMonte Carlocalculations for the two-dimensional Hubbard model. A checkerboard-like charge-density modulation with a roughly period has also been observed by scanning tunneling microscopy experiments in Bi2212 and Na-CCOC compounds. We have performed a variational Monte Carlo simulation on a two-dimensional t-t′-t″- U Hubbard model with a Bi-2212 type band structure and found that the period checkerboard spin modulation, that is characterized by multi Q vectors, is indeed stabilized. We have further performed an investigation by using a quantumMonte Carlomethod, which is a numerical method that can be used to simulate the behavior of correlated electron systems. We present a new algorithm of the quantum Monte Carlo diagonalization that is a method for the evaluation of expectation value without the negative sign problem. We compute pair correlation functions and show that pair correlation is indeed enhanced with hole doping.

Mott transition in cuprate high-temperature superconductors

In this study, we investigate the metal-insulator transition of charge transfer type in high-temperature cuprates. We first show that we must introduce a new band parameter in the three-band d-p model to reproduce the Fermi surface of high temperature cuprates such as BSCCO, YBCO and Hg1201. We present a new wave function of a Mott insulator based on the improved Gutzwiller function, and show that there is a transition from a metal to a charge-transfer insulator for such parameters by using the variational Monte Carlo method. This transition occurs when the level difference

Spin-Density-Wave Transition in Quasi-One-Dimensional System Under Uniaxial Pressure

\Delta_{dp}\equiv \epsilon_p-\epsilon_d

Superconductivity of Quasi-One-Dimensional Electrons in Strong Magnetic Field

between d and p orbitals reaches a critical value

Field-Induced Spin Density Wave in Periodic Potential

(\Delta_{dp})_c

Enhancement of de Haas-van Alphen Oscillation due to Spin in the Magnetic Breakdown System

. The energy gain

Superconductivity of Quasi-One- and Quasi-Two-Dimensional Tight-Binding Electrons in a Magnetic Field

\Delta E