402 research outputs found
Electron-phonon relaxation and excited electron distribution in zinc oxide and anatase
We propose a first-principle method for evaluations of the time-dependent
electron distribution function of excited electrons in the conduction band of
semiconductors. The method takes into account the excitations of electrons by
external source and the relaxation to the bottom of conduction band via
electron-phonon coupling. The methods permits calculations of the
non-equilibrium electron distribution function, the quasi-stationary
distribution function with steady-in-time source of light, the time of setting
of the quasi-stationary distribution and the time of energy loss via relaxation
to the bottom of conduction band. The actual calculations have been performed
for titanium dioxide in the anatase structure and zinc oxide in the wurtzite
structure. We find that the quasi-stationary electron distribution function for
ZnO is a fermi-like curve that rises linearly with increasing excitation energy
whereas the analogous curve for anatase consists of a main peak and a shoulder.
The calculations demonstrate that the relaxation of excited electrons and the
setting of the quasi-stationary distribution occur within the time no more than
500 fsec for ZnO and 100 fsec for anatase.
We also discuss the applicability of the effective phonon model with
energy-independent electron-phonon transition probability. We find that the
model only reproduces the trends in changing of the characteristic times
whereas the precision of such calculations is not high. The rate of energy
transfer to phonons at the quasi-stationary electron distribution also have
been evaluated and the effect of this transfer on the photocatalyses has been
discussed. We found that for ZnO this rate is about 5 times less than in
anatase.Comment: 21 p., 9 figure
Linear-response theory and lattice dynamics: a muffin-tin orbital approach
A detailed description of a method for calculating static linear-response
functions in the problem of lattice dynamics is presented. The method is based
on density functional theory and it uses linear muffin-tin orbitals as a basis
for representing first-order corrections to the one-electron wave functions. As
an application we calculate phonon dispersions in Si and NbC and find good
agreement with experiments.Comment: 18 pages, Revtex, 2 ps figures, uuencoded, gzip'ed, tar'ed fil
Electronic and structural properties of superconducting MgB, CaSi and related compounds
We report a detailed study of the electronic and structural properties of the
39K superconductor \mgbtwo and of several related systems of the same family,
namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which
include zone-center phonon frequencies and transport properties, are performed
within the local density approximation to the density functional theory, using
the full-potential linearized augmented plane wave (FLAPW) and the
norm-conserving pseudopotential methods. Our results indicate essentially
three-dimensional properties for these compounds; however, strongly
two-dimensional -bonding bands contribute significantly at the Fermi
level. Similarities and differences between \mgbtwo and \bebtwo (whose
superconducting properties have not been yet investigated) are analyzed in
detail. Our calculations for \mgalbtwo show that metal substitution cannot be
fully described in a rigid band model. \casitwo is studied as a function of
pressure, and Be substitution in the Si planes leads to a stable compound
similar in many aspects to diborides.Comment: Revised version, Phys.Rev.B in pres
Lattice instabilities of cubic NiTi from first principles
The phonon dispersion relation of NiTi in the simple cubic B2 structure is
computed using first-principles density-functional perturbation theory with
pseudopotentials and a plane-wave basis set. Lattice instabilities are observed
to occur across nearly the entire Brillouin zone, excluding three
interpenetrating tubes of stability along the (001) directions and small
spheres of stability centered at R. The strongest instability is that of the
doubly degenerate M5' mode. The atomic displacements of one of the eigenvectors
of this mode generate a good approximation to the observed B19' ground-state
structure.Comment: 11 pages, 3 figure
Optical studies of gap, hopping energies and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO2
We have investigated the electronic structure of the zigzag ladder (chain)
compound SrCuO2 combining polarized optical absorption, reflection,
photoreflectance and pseudo-dielectric function measurements with the model
calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at
300 K along (perpendicular) to the Cu-O chains. We have found that the lowest
energy gap, the correlation gap, is temperature independent. The electronic
structure of this oxide is calculated using both the
local-spin-density-approximation with gradient correction method, and the
tight-binding theory for the correlated electrons. The calculated density of
electronic states for non-correlated and correlated electrons shows
quasi-one-dimensional character. The correlation gap values of 1.42 eV
(indirect transition) and 1.88 eV (direct transition) have been calculated with
the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV
(between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV.
We concluded that SrCuO_2 belongs to the correlated-gap insulators.Comment: 24 pages, 8 figures, to be published in Phys.Rev.
Phonons and related properties of extended systems from density-functional perturbation theory
This article reviews the current status of lattice-dynamical calculations in
crystals, using density-functional perturbation theory, with emphasis on the
plane-wave pseudo-potential method. Several specialized topics are treated,
including the implementation for metals, the calculation of the response to
macroscopic electric fields and their relevance to long wave-length vibrations
in polar materials, the response to strain deformations, and higher-order
responses. The success of this methodology is demonstrated with a number of
applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic
Accurate ab initio spin densities
We present an approach for the calculation of spin density distributions for
molecules that require very large active spaces for a qualitatively correct
description of their electronic structure. Our approach is based on the
density-matrix renormalization group (DMRG) algorithm to calculate the spin
density matrix elements as basic quantity for the spatially resolved spin
density distribution. The spin density matrix elements are directly determined
from the second-quantized elementary operators optimized by the DMRG algorithm.
As an analytic convergence criterion for the spin density distribution, we
employ our recently developed sampling-reconstruction scheme [J. Chem. Phys.
2011, 134, 224101] to build an accurate complete-active-space
configuration-interaction (CASCI) wave function from the optimized matrix
product states. The spin density matrix elements can then also be determined as
an expectation value employing the reconstructed wave function expansion.
Furthermore, the explicit reconstruction of a CASCI-type wave function provides
insights into chemically interesting features of the molecule under study such
as the distribution of - and -electrons in terms of Slater
determinants, CI coefficients, and natural orbitals. The methodology is applied
to an iron nitrosyl complex which we have identified as a challenging system
for standard approaches [J. Chem. Theory Comput. 2011, 7, 2740].Comment: 37 pages, 13 figure
Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems
Recent trends of ab initio studies and progress in methodologies for
electronic structure calculations of strongly correlated electron systems are
discussed. The interest for developing efficient methods is motivated by recent
discoveries and characterizations of strongly correlated electron materials and
by requirements for understanding mechanisms of intriguing phenomena beyond a
single-particle picture. A three-stage scheme is developed as renormalized
multi-scale solvers (RMS) utilizing the hierarchical electronic structure in
the energy space. It provides us with an ab initio downfolding of the global
band structure into low-energy effective models followed by low-energy solvers
for the models. The RMS method is illustrated with examples of several
materials. In particular, we overview cases such as dynamics of semiconductors,
transition metals and its compounds including iron-based superconductors and
perovskite oxides, as well as organic conductors of kappa-ET type.Comment: 44 pages including 38 figures, to appear in J. Phys. Soc. Jpn. as an
invited review pape
Antisymmetric Magnetic Interactions in Oxo-Bridged Copper(II) Bimetallic Systems
The antisymmetric magnetic interaction is studied using correlated wave-function-based calculations in oxo-bridged copper bimetallic complexes. All of the anisotropic multispin Hamiltonian parameters are extracted using spin-orbit state interaction and effective Hamiltonian theory. It is shown that the methodology is accurate enough to calculate the antisymmetric terms, while the small symmetric anisotropic interactions require more sophisticated calculations. The origin of the antisymmetric anisotropy is analyzed, and the effect of geometrical deformations is addressed.
Physics Opportunities with the 12 GeV Upgrade at Jefferson Lab
This white paper summarizes the scientific opportunities for utilization of
the upgraded 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) and
associated experimental equipment at Jefferson Lab. It is based on the 52
proposals recommended for approval by the Jefferson Lab Program Advisory
Committee.The upgraded facility will enable a new experimental program with
substantial discovery potential to address important topics in nuclear,
hadronic, and electroweak physics.Comment: 64 page
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