89 research outputs found
Structural and superconducting transition in selenium under high pressures
First-principles calculations are performed for electronic structures of two
high pressure phases of solid selenium, -Po and bcc.
Our calculation reproduces well the pressure-induced phase transition from
-Po to bcc observed in selenium.
The calculated transition pressure is 30 GPa lower than the observed one, but
the calculated pressure dependence of the lattice parameters agrees fairly well
with the observations in a wide range of pressure.
We estimate the superconducting transition temperature of both
the -Po and the bcc phases by calculating the phonon dispersion and the
electron-phonon interaction on the basis of density-functional perturbation
theory.
The calculated shows a characteristic pressure dependence, i.e.
it is rather pressure independent in the -Po phase, shows a
discontinuous jump at the transition from -Po to bcc, and then decreases
rapidly with increasing pressure in the bcc phase.Comment: 8 pages, 11 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
Linear Response Calculations of Lattice Dynamics in Strongly Correlated Systems
We introduce a new linear response method to study the lattice dynamics of
materials with strong correlations. It is based on a combination of dynamical
mean field theory of strongly correlated electrons and the local density
functional theory of electronic structure of solids. We apply the method to
study the phonon dispersions of a prototype Mott insulator NiO. Our results
show overall much better agreement with experiment than the corresponding local
density predictions.Comment: 4 pages, 2 figure
Acoustical-Mode-Driven Electron-Phonon Coupling in Transition-Metal Diborides
We show that the electron-phonon coupling in the transition-metal diborides
NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in
contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio
results, described in terms of phonon dispersion, linewidth, and partial
electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2
have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is
an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g}
frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA
frequency at A for TaB2 is almost half of that of MgB2 or NbB2.Comment: 4 pages, 4 figures, and 1 tabl
Extracting the electron--boson spectral function F() from infrared and photoemission data using inverse theory
We present a new method of extracting electron-boson spectral function
F() from infrared and photoemission data. This procedure is
based on inverse theory and will be shown to be superior to previous
techniques. Numerical implementation of the algorithm is presented in detail
and then used to accurately determine the doping and temperature dependence of
the spectral function in several families of high-T superconductors.
Principal limitations of extracting F() from experimental
data will be pointed out. We directly compare the IR and ARPES
F() and discuss the resonance structure in the spectra in
terms of existing theoretical models
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
Role of Boron p-Electrons and Holes in Superconducting MgB2, and other Diborides: A Fully-Relaxed, Full-Potential Electronic Structure Study
We present the results of fully-relaxed, full-potential electronic structure
calculations for the new superconductor MgB2, and BeB2, NaB2, and AlB2, using
density-functional-based methods. Our results described in terms of (i) density
of states (DOS), (ii) band-structure, and (iii) the DOS and the charge density
around the Fermi energy EF, clearly show the importance of B p-band for
superconductivity. In particular, we show that around EF, the charge density in
MgB2, BeB2 and NaB2 is planar and is associated with the B plane. For BeB2 and
NaB2, our results indicate qualitative similarities but significant
quantitative differences in their electronic structure due to different lattice
constants a and c.Comment: 4 pages, 4 figures, Submitted to Phys Rev. Lett. on March 6, 2001;
resubmission on April 2
Reevaluating electron-phonon coupling strengths: Indium as a test case for ab initio and many-body-theory methods
Using indium as a test case, we investigate the accuracy of the
electron-phonon coupling calculated with state-of-the-art ab initio and
many-body theory methods. The ab initio calculations -- where electrons are
treated in the local-density approximation, and phonons and the electron-phonon
interaction are treated within linear response -- predict an electron-phonon
spectral function alpha^2 F(omega) which translates into a relative tunneling
conductance that agrees with experiment to within one part in 1000. The
many-body theory calculations -- where alpha^2 F(omega) is extracted from
tunneling data by means of the McMillan-Rowell tunneling inversion method --
provide spectral functions that depend strongly on details of the inversion
process. For the the most important moment of alpha^2 F(omega), the
mass-renormalization parameter lambda, we report 0.9 +/- 0.1, in contrast to
the value 0.805 quoted for nearly three decades in the literature. The ab
initio calculations also provide the transport electron-phonon spectral
function alpha_{tr}^2 F(omega), from which we calculate the resistivity as a
function of temperature in good agreement with experiment.Comment: 16 pages, 5 figure
Continuous Time Quantum Monte Carlo Method for Fermions: Beyond Auxiliary Field Framework
Numerically exact continuous-time Quantum Monte Carlo algorithm for finite
fermionic systems with non-local interactions is proposed. The scheme is
particularly applicable for general multi-band time-dependent correlations
since it does not invoke Hubbard-Stratonovich transformation. The present
determinantal grand-canonical method is based on a stochastic series expansion
for the partition function in the interaction representation. The results for
the Green function and for the time-dependent susceptibility of multi-orbital
super-symmetric impurity model with a spin-flip interaction are presented
Magnons in real materials from density-functional theory
We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.Comment: 1 LaTeX file and 1 postscript figur
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