Yoshihide Yoshimoto

Data fitting with a spline using a real-coded genetic algorithm

To obtain a good approximation for data fitting with a spline, frequently we have to deal with knots as variables. The problem to be solved then becomes a continuous nonlinear and multivariate optimization problem with many local optima. Therefore, it is difficult to obtain the global optimum. In this paper, we propose a method for solving this problem by using a real-coded genetic algorithm. Our method can treat not only data with a smooth underlying function, but also data with an underlying function having discontinuous points and/or cusps. We search for the best model among candidate models by using the Bayes Information Criterion (BIC). With this, we can appropriately determine the number and locations of knots automatically and simultaneously. Five examples of data fitting are given to show the performance of our method.

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

We derive effective Hubbard-type Hamiltonians of κ-(BEDT-TTF) 2 X , using an ab initio downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer t ∼0.067 eV for a metal X =Cu(NCS) 2 and 0.055 eV for a Mott insulator X =Cu 2 (CN) 3 , as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended Huckel results, especially much stronger onsite interaction U ∼0.8 eV ( U / t ∼12–15) than the Huckel estimates ( U / t ∼7–8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis.

Molecular dynamics and Monte Carlo hybrid simulation for fuzzy tungsten nanostructure formation

For the purposes of long-term use of tungsten divertor walls, the formation process of the fuzzy tungsten nanostructure induced by exposure to the helium plasma was studied. In the present paper, the fuzzy nanostructures formation has been successfully reproduced by the new hybrid simulation method in which the deformation of the tungsten material due to pressure of the helium bubbles was simulated by the molecular dynamics and the diffusion of the helium atoms was simulated by the random walk based on the Monte Carlo method. By the simulation results, the surface height of the fuzzy nanostructure increased only when helium retention was under the steady state. It was proven that the growth of the fuzzy nanostructure was brought about by bursting of the helium bubbles. Moreover, we suggest the following key formation mechanisms of the fuzzy nanostructure: (1) lifting in which the surface lifted up by the helium bubble changes into a convexity, (2) bursting by which the region of the helium bubble changes into a concavity, and (3) the difference of the probability of helium retention by which the helium bubbles tend to appear under the concavity. Consequently, the convex-concave surface structure was enhanced and grew to create the fuzzy nanostructure.

Local and Reversible Change of the Reconstruction on Ge(001) Surface between c(4×2) and p(2×2) by Scanning Tunneling Microscopy

The reconstruction on a Ge(001) surface is locally and reversibly changed between c (4×2) and p (2×2) by controlling the bias voltage of a scanning tunneling microscope (STM) at 80 K. It is c (4×2)...

First-principles calculation of effective onsite Coulomb interactions of 3 d transition metals: Constrained local density functional approach with maximally localized Wannier functions

We present a new ab initio method for calculating effective onsite Coulomb interactions of itinerant and strongly correlated electron systems. The method is based on constrained local density functional theory formulated in terms of maximally localized Wannier functions. This scheme can be implemented with any basis, and thus allows us to perform the constrained calculation with plane-wave-based electronic-structure codes. We apply the developed method to the evaluation of the onsite interaction of 3d transition-matal series. The results are discussed using a heuristic formula for screened Coulomb interactions.

Time-Fluctuation of the Dimer Structure on a Ge(001) Surface Studied by a Monte Carlo Simulation and a First-Principles Calculation

The time-resolving dynamical Monte Carlo simulations (TDMCS) on Ge(001) surface at 200 K for the orientational arrangement at finite temperature are performed. The model potential for the continuou...

Ab initiotwo-dimensional multiband low-energy models of EtMe3Sb[Pd(dmit)2]2andκ-(BEDT-TTF)2Cu(NCS)2with comparisons to single-band models

We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe_3Sb[Pd(dmit)_2]_2 and \kappa-(BEDT-TTF)_2Cu(NCS)_2, after a downfolding scheme based on the constrained random phase approximation (cRPA) and maximally-localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit)_2 salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered as the orbital degrees of freedom, while, in the \kappa-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a three-band model for the Pd(dmit)_2 salt and a two-band model for the \kappa-(BEDT-TTF) salt are derived. We derive single band models for the HOMO-antibonding state for both of the compounds as well.

THE Ge(001) SURFACE RECONSTRUCTION: DFT AND MCS

The problem of relative energetic stabilities of the high order reconstructions of the Ge(001) surface is revisited by a more refined first-principles calculation based on density functional theory. Using this result, we performed a Monte Carlo simulation of the phase transition, and obtained 315 K as the transition temperature of p(2× 1) → c(4× 2). This reproduces fairly well the transient temperature (250–350 K) observed by an X-ray diffraction experiment. The obtained geometry of the c(4× 2) structure compares well with an X-ray diffraction experiment. The potential energy curves of flip-flop motions of both single dimer and dimer in type-P defect are also obtained.

Extended multicanonical method combined with thermodynamically optimized potential: Application to the liquid-crystal transition of silicon

A novel method is proposed to study first-order phase transition in real materials. It is applied to the liquid-crystal transition of silicon successfully. It consists of two parts: a direct simulation of the transition by an extended multicanonical ensemble with an order parameter defined with structure factors that characterize the transition, and optimization of a model interatomic potential in terms of the ensemble from an accurate one. These provide a principle to project a first-principles approach on a model-based approach conserving thermodynamic properties of multiple phases.

Optical Absorption Study by Ab initio Downfolding Approach: Application to GaAs

We examine whether essence and quantitative aspects of electronic excitation spectra are correctly captured by an effective low-energy model constructed from an {\em ab initio} downfolding scheme. A global electronic structure is first calculated by {\em ab initio} density-functional calculations with the generalized gradient approximation. With the help of constrained density functional theory, the low-energy effective Hamiltonian for bands near the Fermi level is constructed by the downfolding procedure in the basis of maximally localized Wannier functions. The excited states of this low-energy effective Hamiltonian ascribed to an extended Hubbard model are calculated by using a low-energy solver. As the solver, we employ the Hartree-Fock approximation supplemented by the single-excitation configuration-interaction method considering electron-hole interactions. The present three-stage method is applied to GaAs, where eight bands are retained in the effective model after the downfolding. The resulting spectra well reproduce the experimental results, indicating that our downfolding scheme offers a satisfactory framework of the electronic structure calculation, particularly for the excitations and dynamics as well as for the ground state.