3,054 research outputs found
Thermodynamics and Excitations of Condensed Polaritons in Disordered Microcavities
We study the thermodynamic condensation of microcavity polaritons using a
realistic model of disorder in semiconductor quantum wells. This approach
correctly describes the polariton inhomogeneous broadening in the low density
limit, and treats scattering by disorder to all orders in the condensed regime.
While the weak disorder changes the thermodynamic properties of the transition
little, the effects of disorder in the condensed state are prominent in the
excitations and can be seen in resonant Rayleigh scattering.Comment: 5 pages, 3 eps figures (published version
Alloy disorder effects on the room temperature optical properties of GaInNAs quantum wells
The effect of alloy disorder on the optical density of states and the average
room temperature carrier statistics in GaInNAs quantum wells is discussed. A
red shift between the peak of the room temperature photoluminescence and the
surface photovoltage spectra, that systematically increases with the nitrogen
content within the quantum wells is observed. The relationship between this
Stokes' shift and the absorption linewidth in different samples suggests that
the photoexcited carriers undergo a continuous transition, from being in
quasi-thermal equilibrium with the lattice to being completely trapped by the
quantum dot-like potential fluctuations, as the nitrogen fraction in the alloy
is increased. The values of the 'electron temperature' inferred from the
photoluminescence spectra are found to be consistent with this interpretation.Comment: 3 figure
Coordination Dependence of Hyperfine Fields of 5sp Impurities on Ni Surfaces
We present first-principles calculations of the magnetic hyperfine fields H
of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the
dependence of H on the coordination number by placing the impurity in the
surfaces, on top of them at the adatom positions, and in the bulk. We find a
strong coordination dependence of H, different and characteristic for each
impurity. The behavior is explained in terms of the on-site s-p hybridization
as the symmetry is reduced at the surface. Our results are in agreement with
recent experimental findings.Comment: 4 pages, 3 figure
Bilinear Quantum Monte Carlo: Expectations and Energy Differences
We propose a bilinear sampling algorithm in Green's function Monte Carlo for
expectation values of operators that do not commute with the Hamiltonian and
for differences between eigenvalues of different Hamiltonians. The integral
representations of the Schroedinger equations are transformed into two
equations whose solution has the form , where
and are the wavefunctions for the two related systems and
is a kernel chosen to couple and . The Monte Carlo process,
with random walkers on the enlarged configuration space , solves
these equations by generating densities whose asymptotic form is the above
bilinear distribution. With such a distribution, exact Monte Carlo estimators
can be obtained for the expectation values of quantum operators and for energy
differences. We present results of these methods applied to several test
problems, including a model integral equation, and the hydrogen atom.Comment: 27 page
Many-body diagrammatic expansion in a Kohn-Sham basis: implications for Time-Dependent Density Functional Theory of excited states
We formulate diagrammatic rules for many-body perturbation theory which uses
Kohn-Sham (KS) Green's functions as basic propagators. The diagram technique
allows to study the properties of the dynamic nonlocal exchange-correlation
(xc) kernel . We show that the spatial non-locality of is
strongly frequency-dependent. In particular, in extended systems the
non-locality range diverges at the excitation energies. This divergency is
related to the discontinuity of the xc potential.Comment: 4 RevTeX pages including 3 eps figures, submitted to Phys. Rev. Lett;
revised version with new reference
Unusual thermoelectric behavior of packed crystalline granular metals
Loosely packed granular materials are intensively studied nowadays.
Electrical and thermal transport properties should reflect the granular
structure as well as intrinsic properties. We have compacted crystalline
based metallic grains and studied the electrical resistivity and the
thermoelectric power as a function of temperature () from 15 to 300K. Both
properties show three regimes as a function of temperature. It should be
pointed out : (i) The electrical resistivity continuously decreases between 15
and 235 K (ii) with various dependences, e.g. at low ,
while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near
60K, and (v) presents a rather unusual square root of temperature dependence at
low temperature. It is argued that these three regimes indicate a competition
between geometric and thermal processes, - for which a theory seems to be
missing in the case of TEP. The microchemical analysis results are also
reported indicating a complex microstructure inherent to the phase diagram
peritectic intricacies of this binary alloy.Comment: to be published in J. Appl. Phys.22 pages, 8 figure
Center of mass and relative motion in time dependent density functional theory
It is shown that the exchange-correlation part of the action functional
in time-dependent density functional theory , where
is the time-dependent density, is invariant under the
transformation to an accelerated frame of reference , where is an arbitrary
function of time. This invariance implies that the exchange-correlation
potential in the Kohn-Sham equation transforms in the following manner:
. Some of the
approximate formulas that have been proposed for satisfy this exact
transformation property, others do not. Those which transform in the correct
manner automatically satisfy the ``harmonic potential theorem", i.e. the
separation of the center of mass motion for a system of interacting particles
in the presence of a harmonic external potential. A general method to generate
functionals which possess the correct symmetry is proposed
The density dependence of the transition temperature in a homogenous Bose flui
Transition temperature data obtained as a function of particle density in the
He-Vycor system are compared with recent theoretical calculations for 3D
Bose condensed systems. In the low density dilute Bose gas regime we find, in
agreement with theory, a positive shift in the transition temperature of the
form . At higher densities a maximum is
found in the ratio of for a value of the interaction parameter,
na, that is in agreement with path-integral Monte Carlo calculations.Comment: 4 pages, 3 figure
The design features of the body of the portable electrocardiograph "ECG-EXPRESS"
The sequence of the industrial product shaping is specified, the main modeling characteristics of the configuration and some of its parts are designated subject to the electrocardiograph design and the technologies used for the production of its body
A two-dimensional, two-electron model atom in a laser pulse: exact treatment, single active electron-analysis, time-dependent density functional theory, classical calculations, and non-sequential ionization
Owing to its numerical simplicity, a two-dimensional two-electron model atom,
with each electron moving in one direction, is an ideal system to study
non-perturbatively a fully correlated atom exposed to a laser field. Frequently
made assumptions, such as the ``single active electron''- approach and
calculational approximations, e.g. time dependent density functional theory or
(semi-) classical techniques, can be tested. In this paper we examine the
multiphoton short pulse-regime. We observe ``non-sequential'' ionization, i.e.\
double ionization at lower field strengths as expected from a sequential,
single active electron-point of view. Since we find non-sequential ionization
also in purely classical simulations, we are able to clarify the mechanism
behind this effect in terms of single particle trajectories. PACS Number(s):
32.80.RmComment: 10 pages, 16 figures (gzipped postscript), see also
http://www.physik.tu-darmstadt.de/tqe
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