Masatoshi Imada

Monte Carlo Calculation of Quantum Systems. II. Higher Order Correction

The higher order correction term is obtained for the Monte Carlo calculation of the path integral. The correction is expressed only by a modification of the potential term: \(V(\textbf{\itshape r}_{1}, \cdots , \textbf{\itshape r}_{N}) \rightarrow V+(\hbar ^{2}/24m)(\beta /M)^{2} \sum _{i=1}^{N} (\partial V/\partial \textbf{\itshape r}_{i})^{2}\), where r i s are coordinates of particles, N is number of particles with mass m , β is ( k T ) -1 and M is number of partitions. By this method one can reduce the computation time remarkably. A rapid convergence of energy is obtained for the case of harmonic oscillator.

Frequency-dependent local interactions and low-energy effective models from electronic structure calculations

We propose a systematic procedure for constructing effective models of strongly correlated materials. The parameters, in particular the on-site screened Coulomb interaction

Two-Dimensional Hubbard Model –Metal Insulator Transition Studied by Monte Carlo Calculation–

U

Comparison of Ab initio Low-Energy Models for LaFePO, LaFeAsO, BaFe2As2, LiFeAs, FeSe, and FeTe: Electron Correlation and Covalency

, are calculated from first principles, using the random-phase approximation. We derive an expression for the frequency-dependent

Numerical Studies on the Hubbard Model and the t-J Model in One- and Two-Dimensions

U(\ensuremath{\omega})

Nonmagnetic Insulating States near the Mott Transitions on Lattices with Geometrical Frustration and Implications for κ-(ET)2Cu2(CN)3

and show, for the case of nickel, that its high-frequency part has significant influence on the spectral functions. We propose a scheme for taking into account the energy dependence of

Theoretical evidence for strong correlations and incoherent metallic state in FeSe

U(\ensuremath{\omega})

Ab initio Derivation of Low-Energy Model for Iron-Based Superconductors LaFeAsO and LaFePO

, so that a model with an energy-independent local interaction can still be used for low-energy properties.

Two Types of Mott Transitions

The Hubbard model on a square lattice in the ground state is investigated. Various aspects of Mott transition at half-filling are clarified from the quantum Monte Carlo study. Critical exponents of the transition for charge and spin correlations are estimated. For the doping concentration δ, the charge susceptibility is proportional to δ -1 indicating the divergence of charge mass for δ→0, while the spin susceptibility is always finite and not singular near δ=0. Several theoretical consequences of the above results are discussed. Incommensurate peak amplitude of spin correlation scales as (δ-δ c ) -1 with δ c <0.01 in disagreement with RPA results ln (δ-δ c ). The antiferromagnetic order at the half-filling also shows strong correlation character and is not sensitive to the shape of the fermi surface, while the incommensurate peak observed away from the half-filling is sensitively suppressed by the loss of partial nesting consistently with the weak coupling picture. Comparisons with experimental indicatio...

Ab initio Derivation of Low-Energy Model for κ-ET Type Organic Conductors

Effective low-energy Hamiltonians for several different families of iron-based superconductors are compared after deriving them from the downfolding scheme based on first-principles calculations. Systematic dependences of the derived model parameters on the families are elucidated, many of which are understood from the systematic variation of the covalency between Fe-3 d and pnictogen-/chalcogen- p orbitals. First, LaFePO, LaFeAsO (1111), BaFe 2 As 2 (122), LiFeAs (111), FeSe, and FeTe (11) have overall similar band structures near the Fermi level, where the total widths of 10-fold Fe-3 d bands are mostly around 4.5 eV. However, the derived effective models of the 10-fold Fe-3 d bands ( d model) for FeSe and FeTe have substantially larger effective onsite Coulomb interactions U ∼4.2 and 3.4 eV, respectively, after the screening by electrons on other bands and after averaging over orbitals, as compared to ∼2.5 eV for LaFeAsO. The difference is similar in the effective models containing p orbitals of As, Se...