2,073 research outputs found
Variational solution of the T-matrix integral equation
We present a variational solution of the T-matrix integral equation within a
local approximation. This solution provides a simple form for the T matrix
similar to Hubbard models but with the local interaction depending on momentum
and frequency. By examining the ladder diagrams for irreducible polarizability,
a connection between this interaction and the local-field factor is
established. Based on the obtained solution, a form for the T-matrix
contribution to the electron self-energy in addition to the GW term is
proposed. In the case of the electron-hole multiple scattering, this form
allows one to avoid double counting.Comment: 7 pages, 7 figure
Theory of Polaron Resonance in Quantum Dots and Quantum-Dot Molecules
The theory of exciton coupling to photons and LO phonons in quantum dots
(QDs) and quantum-dot molecules (QDMs) is presented. Resonant-round trips of
the exciton between the ground (bright) and excited (dark or bright) states
mediated by the LO-phonon alter the decay time and yield the Rabi oscillation.
The initial distributions of the population in the ground and the excited
states dominate the oscillating amplitude and frequency. This property provides
a detectable signature to the information stored in a qubit made from QD or QDM
for a wide range of temperature T. Our results presented herein provide an
explanation to the anomaly on T-dependent decay in self-assembled InGaAs/GaAs
QDMs recently reported by experiment.Comment: 30 pages, 8 figure
Dynamics and Thermodynamics of the Glass Transition
The principal theme of this paper is that anomalously slow, super-Arrhenius
relaxations in glassy materials may be activated processes involving chains of
molecular displacements. As pointed out in a preceding paper with A. Lemaitre,
the entropy of critically long excitation chains can enable them to grow
without bound, thus activating stable thermal fluctuations in the local density
or molecular coordination of the material. I argue here that the intrinsic
molecular-scale disorder in a glass plays an essential role in determining the
activation rate for such chains, and show that a simple disorder-related
correction to the earlier theory recovers the Vogel-Fulcher law in three
dimensions. A key feature of this theory is that the spatial extent of
critically long excitation chains diverges at the Vogel-Fulcher temperature. I
speculate that this diverging length scale implies that, as the temperature
decreases, increasingly large regions of the system become frozen and do not
contribute to the configurational entropy, and thus ergodicity is partially
broken in the super-Arrhenius region above the Kauzmann temperature . This
partially broken ergodicity seems to explain the vanishing entropy at and
other observed relations between dynamics and thermodynamics at the glass
transition.Comment: 20 pages, no figures, some further revision
Path Integral of the Two Dimensional Su-Schrieffer-Heeger Model
The equilibrium thermodynamics of the two dimensional Su-Schrieffer-Heeger
Model is derived by means of a path integral method which accounts for the
variable range of the electronic hopping processes. While the lattice degrees
of freedom are classical functions of time and are integrated out exactly, the
electron particle paths are treated quantum mechanically. The free energy of
the system and its temperature derivatives are computed by summing at any
over the ensemble of relevant particle paths which mainly contribute to the
total partition function. In the low regime, the {\it heat capacity over T}
ratio shows un upturn peculiar of a glassy like behavior. This feature is more
sizeable in the square lattice than in the linear chain as the overall hopping
potential contribution to the total action is larger in higher dimensionality.Comment: Phys.Rev.B vol.71 (2005
A study of the thermal and optical characteristics of radiometric channels for Earth radiation budget applications
An improved dynamic electrothermal model for the Earth Radiation Budget Experiment (ERBE) total, nonscanning channels is formulated. This model is then used to accurately simulate two types of dynamic solar observation: the solar calibration and the so-called pitchover maneuver. Using a second model, the nonscanner active cavity radiometer (ACR) thermal noise is studied. This study reveals that radiative emission and scattering by the surrounding parts of the nonscanner cavity are acceptably small. The dynamic electrothermal model is also used to compute ACR instrument transfer function. Accurate in-flight measurement of this transfer function is shown to depend on the energy distribution over the frequency spectrum of the radiation input function. A new array-type field of view limiter, whose geometry controls the input function, is proposed for in-flight calibration of an ACR and other types of radiometers. The point spread function (PSF) of the ERBE and the Clouds and Earth's Radiant Energy System (CERES) scanning radiometers is computed. The PSF is useful in characterizing the channel optics. It also has potential for recovering the distribution of the radiative flux from Earth by deconvolution
Phase diagram for Coulomb-frustrated phase separation in systems with negative short-range compressibility
Using numerical techniques and asymptotic expansions we obtain the phase
diagram of a paradigmatic model of Coulomb frustrated phase separation in
systems with negative short-range compressibility. The transition from the
homogeneous phase to the inhomogeneous phase is generically first order in
isotropic three-dimensional systems except for a critical point. Close to the
critical point, inhomogeneities are predicted to form a BCC lattice with
subsequent transitions to a triangular lattice of rods and a layered structure.
Inclusion of a strong anisotropy allows for second- and first-order transition
lines joined by a tricritical point.Comment: 4 pages, 3 figures. Improved figures and presentatio
Concept study for a high-efficiency nanowire-based thermoelectric
Materials capable of highly efficient, direct thermal-to-electric energy
conversion would have substantial economic potential. Theory predicts that
thermoelectric efficiencies approaching the Carnot limit can be achieved at low
temperatures in one-dimensional conductors that contain an energy filter such
as a double-barrier resonant tunneling structure. The recent advances in growth
techniques suggest that such devices can now be realized in heterostructured,
semiconductor nanowires. Here we propose specific structural parameters for
InAs/InP nanowires that may allow the experimental observation of near-Carnot
efficient thermoelectric energy conversion in a single nanowire at low
temperature
Equilibration of integer quantum Hall edge states
We study equilibration of quantum Hall edge states at integer filling
factors, motivated by experiments involving point contacts at finite bias.
Idealising the experimental situation and extending the notion of a quantum
quench, we consider time evolution from an initial non-equilibrium state in a
translationally invariant system. We show that electron interactions bring the
system into a steady state at long times. Strikingly, this state is not a
thermal one: its properties depend on the full functional form of the initial
electron distribution, and not simply on the initial energy density. Further,
we demonstrate that measurements of the tunneling density of states at long
times can yield either an over-estimate or an under-estimate of the energy
density, depending on details of the analysis, and discuss this finding in
connection with an apparent energy loss observed experimentally. More
specifically, we treat several separate cases: for filling factor \nu=1 we
discuss relaxation due to finite-range or Coulomb interactions between
electrons in the same channel, and for filling factor \nu=2 we examine
relaxation due to contact interactions between electrons in different channels.
In both instances we calculate analytically the long-time asymptotics of the
single-particle correlation function. These results are supported by an exact
solution at arbitrary time for the problem of relaxation at \nu=2 from an
initial state in which the two channels have electron distributions that are
both thermal but with unequal temperatures, for which we also examine the
tunneling density of states.Comment: 12 pages, 5 figures, final version as publishe
Role of strong correlation in the recent ARPES experiments for cuprate superconductors
Motivated by recent photoemission experiments on cuprates, the low-lying
excitations of a strongly correlated superconducting state are studied
numerically. It is observed that along the nodal direction these low-lying
one-particle excitations show a linear momentum dependence for a wide range of
excitation energies and, thus, they do not present a kink-like structure. The
nodal Fermi velocity , as well as other observables, are
systematically evaluated directly from the calculated dispersions, and they are
found to compare well with experiments. It is argued that the parameter
dependence of is quantitatively explained by a simple picture of a
renormalized Fermi velocity.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let
Disorder-Induced First Order Transition and Curie Temperature Lowering in Ferromagnatic Manganites
We study the effect that size disorder in the cations surrounding manganese
ions has on the magnetic properties of manganites. This disorder is mimic with
a proper distribution of spatially disordered Manganese energies. Both, the
Curie temperature and the order of the transition are strongly affected by
disorder. For moderate disorder the Curie temperature decreases linearly with
the the variance of the distribution of the manganese site energies, and for a
disorder comparable to that present in real materials the transition becomes
first order. Our results provide a theoretical framework to understand disorder
effects on the magnetic behavior of manganites.Comment: 4 pages, three figures include
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