23,884 research outputs found
Coherent phenomena in semiconductors
A review of coherent phenomena in photoexcited semiconductors is presented.
In particular, two classes of phenomena are considered: On the one hand the
role played by optically-induced phase coherence in the ultrafast spectroscopy
of semiconductors; On the other hand the Coulomb-induced effects on the
coherent optical response of low-dimensional structures.
All the phenomena discussed in the paper are analyzed in terms of a
theoretical framework based on the density-matrix formalism. Due to its
generality, this quantum-kinetic approach allows a realistic description of
coherent as well as incoherent, i.e. phase-breaking, processes, thus providing
quantitative information on the coupled ---coherent vs. incoherent--- carrier
dynamics in photoexcited semiconductors.
The primary goal of the paper is to discuss the concept of quantum-mechanical
phase coherence as well as its relevance and implications on semiconductor
physics and technology. In particular, we will discuss the dominant role played
by optically induced phase coherence on the process of carrier photogeneration
and relaxation in bulk systems. We will then review typical field-induced
coherent phenomena in semiconductor superlattices such as Bloch oscillations
and Wannier-Stark localization. Finally, we will discuss the dominant role
played by Coulomb correlation on the linear and non-linear optical spectra of
realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press)
(Some of the figures are not available in electronic form
Resonant Conversion of Massless Neutrinos in Supernovae
It has been noted for a long time that, in some circumstances, {\sl massless}
neutrinos may be {\sl mixed} in the leptonic charged current. Conventional
neutrino oscillation searches in vacuum are insensitive to this mixing. We
discuss the effects of resonant massless-neutrino conversions in the dense
medium of a supernova. In particular, we show how the detected
energy spectra from SN1987a and the supernova -process nucleosynthesis may
be used to provide very stringent constraints on the mixing of {\sl massless}
neutrinos.Comment: latex file, 20 pages, including 3 postscript figure
Bose Einstein Condensation in solid 4He
We have computed the one--body density matrix rho_1 in solid 4He at T=0 K
using the Shadow Wave Function (SWF) variational technique. The accuracy of the
SWF has been tested with an exact projector method. We find that off-diagonal
long range order is present in rho_1 for a perfect hcp and bcc solid 4He for a
range of densities above the melting one, at least up to 54 bars. This is the
first microscopic indication that Bose Einstein Condensation (BEC) is present
in perfect solid 4He. At melting the condensate fraction in the hcp solid is
5*10^{-6} and it decreases by increasing the density. The key process giving
rise to BEC is the formation of vacancy--interstitial pairs. We also present
values for Leggett's upper bound on the superfluid fraction deduced from the
exact local density.Comment: 4 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
Improving predictive power of physically based rainfall-induced shallow landslide models: a probabilistic approach
Distributed models to forecast the spatial and temporal occurrence of
rainfall-induced shallow landslides are based on deterministic laws. These
models extend spatially the static stability models adopted in geotechnical
engineering, and adopt an infinite-slope geometry to balance the resisting and
the driving forces acting on the sliding mass. An infiltration model is used to
determine how rainfall changes pore-water conditions, modulating the local
stability/instability conditions. A problem with the operation of the existing
models lays in the difficulty in obtaining accurate values for the several
variables that describe the material properties of the slopes. The problem is
particularly severe when the models are applied over large areas, for which
sufficient information on the geotechnical and hydrological conditions of the
slopes is not generally available. To help solve the problem, we propose a
probabilistic Monte Carlo approach to the distributed modeling of
rainfall-induced shallow landslides. For the purpose, we have modified the
Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability
Analysis (TRIGRS) code. The new code (TRIGRS-P) adopts a probabilistic approach
to compute, on a cell-by-cell basis, transient pore-pressure changes and
related changes in the factor of safety due to rainfall infiltration.
Infiltration is modeled using analytical solutions of partial differential
equations describing one-dimensional vertical flow in isotropic, homogeneous
materials. Both saturated and unsaturated soil conditions can be considered.
TRIGRS-P copes with the natural variability inherent to the mechanical and
hydrological properties of the slope materials by allowing values of the TRIGRS
model input parameters to be sampled randomly from a given probability
distribution. [..]Comment: 25 pages, 14 figures, 9 tables. Revised version; accepted for
publication in Geoscientific Model Development on 13 February 201
Local Optical Spectroscopy in Quantum Confined Systems: A Theoretical Description
A theoretical description of local absorption is proposed in order to
investigate spectral variations on a length scale comparable with the extension
of the relevant quantum states. A general formulation is derived within the
density-matrix formalism including Coulomb correlation, and applied to the
prototypical case of coupled quantum wires. The results show that excitonic
effects may have a crucial impact on the local absorption with implications for
the spatial resolution and the interpretation of near-field optical spectra.Comment: To appear in Phys. Rev. Lett. - 11 pages, 3 PostScript figures (1
figure in colors) embedded. Uses RevTex, and psfig style
Interminiband Rabi oscillations in biased semiconductor superlattices
Carrier dynamics at energy level anticrossings in biased semiconductor
superlattices, was studied in the time domain by solving the time-dependent
Schroedinger equation. The resonant nature of interminiband Rabi oscillations
has been explicitly demonstrated to arise from interference of intrawell and
Bloch oscillations. We also report a simulation of direct Rabi oscillations
across three minibands, in the high field regime, due to interaction between
three strongly coupled minibands.Comment: 13 pages, 16 figure
Shape-independent scaling of excitonic confinement in realistic quantum wires
The scaling of exciton binding energy in semiconductor quantum wires is
investigated theoretically through a non-variational, fully three-dimensional
approach for a wide set of realistic state-of-the-art structures. We find that
in the strong confinement limit the same potential-to-kinetic energy ratio
holds for quite different wire cross-sections and compositions. As a
consequence, a universal (shape- and composition-independent) parameter can be
identified that governs the scaling of the binding energy with size. Previous
indications that the shape of the wire cross-section may have important effects
on exciton binding are discussed in the light of the present results.Comment: To appear in Phys. Rev. Lett. (12 pages + 2 figures in postscript
The Changing Landscape of Blockbuster Punitive Damages Awards
This article investigates the determinants of the blockbuster punitive damages awards of at least 3.0 billion. The U.S. Supreme Court decision in State Farm v. Campbell suggested a single digit upper bound on the punitive damages/compensatory damages ratio, which reduced the annual number of blockbuster awards, the total annual value of blockbuster awards, and the punitive damages/compensatory damages ratio. Applying the 1:1 ratio from Exxon Shipping Co. et al. v. Baker et al. broadly would eliminate most of the blockbuster awards.
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation.Comment: to appear in Phys. Rev. Let
- …