79 research outputs found
Photoexcited transients in disordered semiconductors: Quantum coherence at very short to intermediate times
We study theoretically electron transients in semiconductor alloys excited by
light pulses shorter than 100 femtoseconds and tuned above the absorption edge
during and shortly after the pulse, when disorder scattering is dominant.
We use non-equilibrium Green functions employing the field-dependent
self-consistent Born approximation. The propagators and the particle
correlation function are obtained by a direct numerical solution of the Dyson
equations in differential form. For the purely elastic scattering in our model
system the solution procedures for the retarded propagator and for the
correlation function can be decoupled.The propagator is used as an input in
calculating the correlation function. Numerical results combined with a
cumulant expansion permit to separate in a consistent fashion the dark and the
induced parts of the self-energy. The dark behavior reduces to propagation of
strongly damped quasi-particles; the field induced self-energy leads to an
additional time non-local coherence. The particle correlation function is
formed by a coherent transient and an incoherent back-scattered component. The
particle number is conserved only if the field induced coherence is fully
incorporated. The transient polarization and the energy balance are also
obtained and interpreted.Comment: Accepted for publication in Phys. Rev. B; 37 pages,17 figure
Saddle Points and Stark Ladders: Exact Calculations of Exciton Spectra in Superlattices
A new, exact method for calculating excitonic absorption in superlattices is
described. It is used to obtain high resolution spectra showing the saddle
point exciton feature near the top of the miniband. The evolution of this
feature is followed through a series of structures with increasing miniband
width. The Stark ladder of peaks produced by an axial electric field is
investigated, and it is shown that for weak fields the line shapes are strongly
modified by coupling to continuum states, taking the form of Fano resonances.
The calculated spectra, when suitably broadened, are found to be in good
agreement with experimental results.Comment: 9 pages Revtex v3.0, followed by 4 uuencoded postscript figures,
SISSA-CM-94-00
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
Ward identities for disordered metals and superconductors
This article revisits Ward identities for disordered interacting normal
metals and superconductors. It offers a simple derivation based on gauge
invariance and recasts the identities in a new form that allows easy analysis
of the quasiparticle charge conservation (as e.g. in a normal metal) or
non-conservation (as e.g. in a d-wave superconductor).Comment: Discussion of decoherence at T=0 remove
Non-equilibrium Green's function approach to inhomogeneous quantum many-body systems using the Generalized Kadanoff Baym Ansatz
In non-equilibrium Green's function calculations the use of the Generalized
Kadanoff-Baym Ansatz (GKBA) allows for a simple approximate reconstruction of
the two-time Green's function from its time-diagonal value. With this a drastic
reduction of the computational needs is achieved in time-dependent
calculations, making longer time propagation possible and more complex systems
accessible. This paper gives credit to the GKBA that was introduced 25 years
ago. After a detailed derivation of the GKBA, we recall its application to
homogeneous systems and show how to extend it to strongly correlated,
inhomogeneous systems. As a proof of concept, we present results for a
2-electron quantum well, where the correct treatment of the correlated electron
dynamics is crucial for the correct description of the equilibrium and dynamic
properties
Approximation of excitonic absorption in disordered systems using a compositional component weighted CPA
Employing a recently developed technique of component weighted two particle
Green's functions in the CPA of a binary substitutional alloy we
extend the existing theory of excitons in such media using a contact potential
model for the interaction between electrons and holes to an approximation which
interpolates correctly between the limits of weak and strong disorder. With our
approach we are also able to treat the case where the contact interaction
between carriers varies between sites of different types, thus introducing
further disorder into the system. Based on this approach we study numerically
how the formation of exciton bound states changes as the strengths of the
contact potentials associated with either of the two site types are varied
through a large range of parameter values.Comment: 27 pages RevTeX (preprint format), 13 Postscript figure file
Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}
The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an
interesting phenomenon, as it is very similar to that found in perovskite
manganese oxides although the compound differs both in its crystalline
structure and electronic properties from the manganites. At the same time,
other pyrochlore compounds, though sharing the same structure with
Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and
transport properties found in this material. Mostly due to the absence of
evidence for significant doping into the Mn-O sublattice, and the tendency of
Tl to form conduction bands, the traditional double exchange mechanism
mentioned in connection with manganites does not seem suitable to explain the
experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7}
consisting of a lattice of intermediate valence ions fluctuating between two
magnetic configurations, representing Mn-3d orbitals, hybridized with a
conduction band, which we associate with Tl. This model had been proposed
originally for the analysis of intermediate valence Tm compounds. With a
simplified treatment of the model we obtain the electronic structure and
transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to
experiments. The presence of a hybridization gap in the density of states seems
important to understand the reported Hall data.Comment: 8 pages + 5 postscript fig
Pressure dependence of the sound velocity in a 2D lattice of Hertz-Mindlin balls: a mean field description
We study the dependence on the external pressure of the velocities
of long wavelength sound waves in a confined 2D h.c.p. lattice of 3D
elastic frictional balls interacting via one-sided Hertz-Mindlin contact
forces, whose diameters exhibit mild dispersion. The presence of an underlying
long range order enables us to build an effective medium description which
incorporates the radial fluctuations of the contact forces acting on a single
site. Due to the non linearity of Hertz elasticity, self-consistency results in
a highly non-linear differential equation for the "equation of state" linking
the effective stiffness of the array with the applied pressure, from which
sound velocities are then obtained. The results are in excellent agreement with
existing experimental results and simulations in the high and intermediate
pressure regimes. It emerges from the analysis that the departure of
from the ideal Hertz behavior must be attributed primarily to the
fluctuations of the stress field, rather than to the pressure dependence of the
number of contacts
Parquet approach to nonlocal vertex functions and electrical conductivity of disordered electrons
A diagrammatic technique for two-particle vertex functions is used to
describe systematically the influence of spatial quantum coherence and
backscattering effects on transport properties of noninteracting electrons in a
random potential. In analogy with many-body theory we construct parquet
equations for topologically distinct {\em nonlocal} irreducible vertex
functions into which the {\em local} one-particle propagator and two-particle
vertex of the coherent-potential approximation (CPA) enter as input. To
complete the two-particle parquet equations we use an integral form of the Ward
identity and determine the one-particle self-energy from the known irreducible
vertex. In this way a conserving approximation with (Herglotz) analytic
averaged Green functions is obtained. We use the limit of high spatial
dimensions to demonstrate how nonlocal corrections to the (CPA)
solution emerge. The general parquet construction is applied to the calculation
of vertex corrections to the electrical conductivity. With the aid of the
high-dimensional asymptotics of the nonlocal irreducible vertex in the
electron-hole scattering channel we derive a mean-field approximation for the
conductivity with vertex corrections. The impact of vertex corrections onto the
electronic transport is assessed quantitatively within the proposed mean-field
description on a binary alloy.Comment: REVTeX 19 pages, 9 EPS diagrams, 6 PS figure
On Metal-Insulator Transitions due to Self-Doping
We investigate the influence of an unoccupied band on the transport
properties of a strongly correlated electron system. For that purpose,
additional orbitals are coupled to a Hubbard model via hybridization. The
filling is one electron per site. Depending on the position of the additional
band, both, a metal--to--insulator and an insulator--to--metal transition occur
with increasing hybridization. The latter transition from a Mott insulator into
a metal via ``self--doping'' was recently proposed to explain the low carrier
concentration in . We suggest a restrictive parameter regime for
this transition making use of exact results in various limits. The predicted
absence of the self--doping transition for nested Fermi surfaces is confirmed
by means of an unrestricted Hartree--Fock approximation and an exact
diagonalization study in one dimension. In the general case metal--insulator
phase diagrams are obtained within the slave--boson mean--field and the
alloy--analog approximation.Comment: 9 pages, Revtex, 6 postscript figure
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