39 research outputs found
Ab initio two-dimensional multiband low-energy models of EtMe_3Sb[Pd(dmit)_2]_2 and \kappa-(BEDT-TTF)_2Cu(NCS)_2 with 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.Comment: 10 pages, 9 figures, 3 tables; submitted to Physical Review
Extended Ensemble Molecular Dynamics for Thermodynamics of Phases
The first-order phase transitions and related thermodynamics properties are
primary concerns of materials sciences and engineering. In traditional
atomistic simulations, the phase transitions and the estimation of their
thermodynamic properties are challenging tasks because the trajectories get
trapped in local minima close to the initial states. In this study, we
investigate various extended ensemble molecular dynamics (MD) methods based on
the multicanonical ensemble method using the Wang-Landau (WL) approach. We
performed multibaric-multithermal (MBMT) method to fluid phase, gas-liquid
transition, and liquid-solid transition of the Lennard-Jones (LJ) system. The
derived thermodynamic properties of the fluid phase and the gas-liquid
transition from the MBMT agree well with the previously reported equation of
states (EOSs). However, the MBMT cannot correctly predict the liquid-solid
transition. The multiorder-multithermal (MOMT) ensemble shows significantly
enhanced sampling between liquid and solid states with an accurate estimation
of transition temperatures. We further investigated the dynamics of each system
based on their free energy shapes, providing fundamental insights for their
sampling behaviors. This study guides the prediction of broader crystalline
materials, e.g., alloys, for their phases and thermodynamic properties from
atomistic modeling
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.Comment: 14 pages, 6 figures, and 1 tabl
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 Hückel results, especially much stronger onsite interaction U ∼0.8 eV (U / t ∼12–15) than the Hückel 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
Nonlinear Conductivity of Geometrically Frustrated Iridate Ca5Ir3O12
We report the discovery of nonlinear conductivity along the c-axis in a single crystal of Ca5Ir3O12, which indicates a semiconducting behavior with a narrow band gap of ∼0.2 eV. The resistivity decreases with increase in the applied current. This nonlinearity is reversible with the direction of current. We also show the ab initio density functional band structures and the Fermi surface. We found that the spin–orbit interactions result in an appreciable change in the low-energy electronic structure; the interaction splits the metallic bands and leads to a pocket-like band structure, thus reducing the metallic trend. The size of the spin–orbit interaction is estimated as ∼0.3 eV, which is large enough to be comparable to the valence bandwidth of ∼0.5 eV. The Fermi surface exhibits a sheet structure along the c*-axis, due to the 1D chain structure of edge-sharing IrO6