Kazuyoshi Yoshimi

PHYSBO (optimization tools for PHYSics based on Bayesian Optimization) is a Python library for fast and scalable Bayesian optimization. It has been developed mainly for application in the basic sciences such as physics and materials science. Bayesian optimization is used to select an appropriate input for experiments/simulations from candidate inputs listed in advance in order to obtain better output values with the help of machine learning prediction. PHYSBO can be used to find better solutions

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors TMTTF(2)X, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for

Based on the non-skeleton diagrammatic expansion satisfying the compressibility and spin-susceptibility sum rules, we investigate static charge and spin responses in a two-dimensional extended Hubbard model with the nearest-neighbor Coulomb repulsion in the vicinity of its charge-ordering transition point. In this expansion, we can calculate approximate charge and spin response functions by systematic inclusion of vertex corrections, from which we obtain the uniform susceptibility equal to the s

The molecular solids ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\text{X}{[\mathrm{Pd}{(\mathrm{dmit})}_{2}]}_{2}$ (where X represents a cation) are typical compounds whose electronic structures are described by single-orbital Hubbard-type Hamiltonians with geometrical frustration. Using the ab initio downfolding method, we derive the low-energy effective Hamiltonians for ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}\text{X}{[\mathrm{Pd}{(\mathrm{dmit})}_{2}]}_{2}$ w

We study theoretically competition between the charge ordering and the superconductivity in two-dimensional organic conductor beta-(meso-DMBEDT-TTF)_2PF_6. We analyze the extended Hubbard model on a weakly-dimerized lattice based on the random phase approximation and Eliashberg equations. We found reentrant behavior of the checkerbord-type charge-ordered phase in the phase diagram, and the triplet superconductivity due to the charge fluctuation in the neighboring region. In the low temperature a

An ab initio investigation of the family of molecular compounds TM_{2}X is conducted, where TM is either TMTSF or TMTTF and X takes centrosymmetric monovalent anions. By deriving the extended Hubbard-type Hamiltonians from first-principles band calculations and evaluating not only the intermolecular transfer integrals but also the Coulomb parameters, we discuss their material dependence in the unified phase diagram. Furthermore, we apply the many-variable variational Monte Carlo method to accura

We study the deformation of a Fermi surface (FS) near charge-ordering (CO) transition. By applying a fluctuation-exchange approximation to the two-dimensional extended Hubbard model, we show that the FS is largely modified by strong charge fluctuations when the wave number of the CO pattern does not match the nesting vector of the FS in a noninteracting system. We also discuss the enhanced anisotropy in quasiparticle properties in the resultant metallic state.