67 research outputs found
On X-ray-singularities in the f-electron spectral function of the Falicov-Kimball model
The f-electron spectral function of the Falicov-Kimball model is calculated
within the dynamical mean-field theory using the numerical renormalization
group method as the impurity solver. Both the Bethe lattice and the hypercubic
lattice are considered at half filling. For small U we obtain a single-peaked
f-electron spectral function, which --for zero temperature-- exhibits an
algebraic (X-ray) singularity () for . The
characteristic exponent depends on the Coulomb (Hubbard) correlation
U. This X-ray singularity cannot be observed when using alternative
(Keldysh-based) many-body approaches. With increasing U, decreases and
vanishes for sufficiently large U when the f-electron spectral function
develops a gap and a two-peak structure (metal-insulator transition).Comment: 8 pages, 8 figures, revte
Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdZnSe
Compound semiconductor alloys of the type ABC find widespread applications as
their electronic bulk band gap varies continuously with x, and therefore a
tayloring of the energy gap is possible by variation of the concentration. We
model the electronic properties of such semiconductor alloys by a multiband
tight-binding model on a finite ensemble of supercells and determine the band
gap of the alloy. This treatment allows for an intrinsic reproduction of band
bowing effects as a function of the concentration x and is exact in the
alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as
a well-defined model system and give a careful analysis on the proper choice of
the basis set and supercell size, as well as on the necessary number of
realizations. The results are compared to experimental results obtained from
ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy
(MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe
nanowires reported in the literature.Comment: 7 pages, 6 figure
Localization properties of driven disordered one-dimensional systems
We generalize the definition of localization length to disordered systems
driven by a time-periodic potential using a Floquet-Green function formalism.
We study its dependence on the amplitude and frequency of the driving field in
a one-dimensional tight-binding model with different amounts of disorder in the
lattice. As compared to the autonomous system, the localization length for the
driven system can increase or decrease depending on the frequency of the
driving. We investigate the dependence of the localization length with the
particle's energy and prove that it is always periodic. Its maximum is not
necessarily at the band center as in the non-driven case. We study the
adiabatic limit by introducing a phenomenological inelastic scattering rate
which limits the delocalizing effect of low-frequency fields.Comment: Accepted for publication in European Physical Journal
Extended DFT+U+V method with on-site and inter-site electronic interactions
In this article we introduce a generalization of the popular DFT+U method
based on the extended Hubbard model that includes on-site and inter-site
electronic interactions. The novel corrective Hamiltonian is designed to study
systems for which electrons are not completely localized on atomic states
(according to the general scheme of Mott localization) and hybridization
between orbitals from different sites plays an important role. The application
of the extended functional to archetypal Mott - charge-transfer (NiO) and
covalently bonded insulators (Si and GaAs) demonstrates its accuracy and
versatility and the possibility to obtain a unifying and equally accurate
description for a broad range of very diverse systems
Metal-insulator transition in EuO
It is shown that the spectacular metal-insulator transition in Eu-rich EuO
can be simulated within an extended Kondo lattice model. The different orders
of magnitude of the jump in resistivity in dependence on the concentration of
oxygen vacancies as well as the low-temperature resistance minimum in
high-resistivity samples are reproduced quantitatively. The huge colossal
magnetoresistance (CMR) is calculated and discussed
Fluctuation-driven insulator-to-metal transition in an external magnetic field
We consider a model for a metal-insulator transition of correlated electrons
in an external magnetic field. We find a broad region in interaction and
magnetic field where metallic and insulating (fully magnetized) solutions
coexist and the system undergoes a first-order metal-insulator transition. A
global instability of the magnetically saturated solution precedes the local
ones and is caused by collective fluctuations due to poles in electron-hole
vertex functions.Comment: REVTeX 4 pages, 3 PS figure
Linked Cluster Expansion Around Mean-Field Theories of Interacting Electrons
A general expansion scheme based on the concept of linked cluster expansion
from the theory of classical spin systems is constructed for models of
interacting electrons. It is shown that with a suitable variational formulation
of mean-field theories at weak (Hartree-Fock) and strong (Hubbard-III) coupling
the expansion represents a universal and comprehensive tool for systematic
improvements of static mean-field theories. As an example of the general
formalism we investigate in detail an analytically tractable series of ring
diagrams that correctly capture dynamical fluctuations at weak coupling. We
introduce renormalizations of the diagrammatic expansion at various levels and
show how the resultant theories are related to other approximations of similar
origin. We demonstrate that only fully self-consistent approximations produce
global and thermodynamically consistent extensions of static mean field
theories. A fully self-consistent theory for the ring diagrams is reached by
summing the so-called noncrossing diagrams.Comment: 17 pages, REVTEX, 13 uuencoded postscript figures in 2 separate file
Spectral functions of the Falicov-Kimball model with electronic ferroelectricity
We calculate the angular resolved photoemission spectrum of the
Falicov-Kimball model with electronic ferroelectricity where - and
-electrons have different hoppings. In mix-valence regimes, the presence of
strong scattering processes between - excitons and a hole, created by
emission of an electron, leads to the formation of pseudospin polarons and
novel electronic structures with bandwidth scaling with that of -
excitons. Especially, in the two-dimensional case, we find that flat regions
exist near the bottom of the quasiparticle band in a wide range of the - and
-level energy difference.Comment: 5 pages, 5 figure
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