187 research outputs found
Resonant Transport in Nb/GaAs/AlGaAs/GaAs Microstructures
Resonant transport in a hybrid semiconductor-superconductor microstructure
grown by MBE on GaAs is presented. This structure experimentally realizes the
prototype system originally proposed by de Gennes and Saint-James in 1963 in
\emph{all}-metal structures. A low temperature single peak superimposed to the
characteristic Andreev-dominated subgap conductance represents the mark of such
resonant behavior. Random matrix theory of quantum transport was employed in
order to analyze the observed magnetotransport properties and ballistic effects
were included by directly solving the Bogoliubov-de Gennes equations.Comment: 7 pages REVTeX, 4 figures, to be published by World Scientific in
Proceedings of International Symposium on Mesoscopic Superconductivity and
Spintronics (NTT R&D Center Atsugi, Japan, March 2002
Excitonic Effects in Quantum Wires
We review the effects of Coulomb correlation on the linear and non-linear
optical properties of semiconductor quantum wires, with emphasis on recent
results for the bound excitonic states. Our theoretical approach is based on
generalized semiconductor Bloch equations, and allows full three-dimensional
multisubband description of electron-hole correlation for arbitrary confinement
profiles. In particular, we consider V- and T-shaped structures for which
significant experimental advances were obtained recently. Above band gap, a
very general result obtained by this approach is that electron-hole Coulomb
correlation removes the inverse-square-root single-particle singularity in the
optical spectra at band edge, in agreement with previous reports from purely
one-dimensional models. Strong correlation effects on transitions in the
continuum are found to persist also at high densities of photoexcited carriers.
Below bandgap, we find that the same potential- (Coulomb) to kinetic-energy
ratio holds for quite different wire cross sections and compositions. As a
consequence, we identify a shape- and barrier-independent parameter that
governs a universal scaling law for exciton 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: Proc. OECS-5 Conference, G\"ottingen, 1997 (To appear in Phys. Stat.
Sol. (b)
Reflectionless tunneling in planar Nb/GaAs hybrid junctions
Reflectionless-tunneling was observed in Nb/GaAs superconductor/semiconductor
junctions fabricated through a two-step procedure. First, periodic
-doped layers were grown by molecular beam epitaxy near the GaAs
surface, followed by an As cap layer to protect the surface during {\it
ex-situ} transfer. Second, Nb was deposited by dc-magnetron sputtering onto the
GaAs(001) 2 4 surface {\it in-situ} after thermal desorption of the
cap layer. The magnetotransport behavior of the resulting hybrid junctions was
successfully analyzed within the random matrix theory of phase-coherent Andreev
transport. The impact of junction morphology on reflectionless tunneling and
the applicability of the fabrication technique to the realization of complex
superconductor/semiconductor mesoscopic systems are discussed.Comment: 10 pages, 3 figures, to be published in Appl. Phys. Let
Robust reconstruction of the discrete state for a class of nonlinear uncertain switched systems
n/
General analytic predictions of two-field inflation and perturbative reheating
16 pages, 5 figures, Version 2 includes a revised comparison with Meyers and Tarrant [41
Non-Gaussianity from the hybrid potential
We study the hybrid inflationary potential in a regime where the defect field
is light, and more than 60 e-folds of accelerated expansion occur after the
symmetry breaking transition. Using analytic and numerical techniques, we then
identify parameter values within this regime for which the statistics of the
primordial curvature perturbation are significantly non-Gaussian. Focusing on
this range of parameters, we provide a specific example which leads to an
observationally consistent power spectrum, and a level of non-Gaussianity
within current WMAP bounds and in reach of the Planck satellite. An interesting
feature of this example is that the initial conditions at horizon crossing
appear quite natural.Comment: 9 pages, 7 figure
Effectiveness Of Aminosidine (Paromomycin) Sulfate In Chronic Cryptosporidium Diarrhea In Aids Patients: An Open, Uncontrolled, Prospective Clinical Trial
Well-width dependence of exciton-phonon scattering in InxGa1 - xAs/GaAs single quantum wells
The temperature and density dependencies of the exciton dephasing time in In0.18Ga0.82As/GaAs single quantum wells with different thicknesses have been measured by degenerate four-wave mixing. The exciton-phonon scattering contribution to the dephasing is isolated by extrapolating the dephasing rate to zero-exciton density. From the temperature dependence of this rate we have deduced the linewidth broadening coefficients for acoustic and optical phonons. We find acoustic-phonon coefficients that increase from 1.6 to 3 μeV/K when increasing the well width from 1 to 4 nm. This is in quantitative agreement with theoretical predictions when the spatial extension of the exciton wave function, strongly penetrating into the GaAs barrier in thin InxGa1-xAs quantum wells, is taken into account. The optical-phonon coefficient does not show a systematic dependence on well thickness, and is comparable with the value for bulk GaAs
A Prognostic Model for Estimating the Time to Virologic Failure in HIV-1 Infected Patients Undergoing a New Combination Antiretroviral Therapy Regimen
<p>Abstract</p> <p>Background</p> <p>HIV-1 genotypic susceptibility scores (GSSs) were proven to be significant prognostic factors of fixed time-point virologic outcomes after combination antiretroviral therapy (cART) switch/initiation. However, their relative-hazard for the time to virologic failure has not been thoroughly investigated, and an expert system that is able to predict how long a new cART regimen will remain effective has never been designed.</p> <p>Methods</p> <p>We analyzed patients of the Italian ARCA cohort starting a new cART from 1999 onwards either after virologic failure or as treatment-naïve. The time to virologic failure was the endpoint, from the 90<sup>th </sup>day after treatment start, defined as the first HIV-1 RNA > 400 copies/ml, censoring at last available HIV-1 RNA before treatment discontinuation. We assessed the relative hazard/importance of GSSs according to distinct interpretation systems (Rega, ANRS and HIVdb) and other covariates by means of Cox regression and random survival forests (RSF). Prediction models were validated via the bootstrap and c-index measure.</p> <p>Results</p> <p>The dataset included 2337 regimens from 2182 patients, of which 733 were previously treatment-naïve. We observed 1067 virologic failures over 2820 persons-years. Multivariable analysis revealed that low GSSs of cART were independently associated with the hazard of a virologic failure, along with several other covariates. Evaluation of predictive performance yielded a modest ability of the Cox regression to predict the virologic endpoint (c-index≈0.70), while RSF showed a better performance (c-index≈0.73, p < 0.0001 vs. Cox regression). Variable importance according to RSF was concordant with the Cox hazards.</p> <p>Conclusions</p> <p>GSSs of cART and several other covariates were investigated using linear and non-linear survival analysis. RSF models are a promising approach for the development of a reliable system that predicts time to virologic failure better than Cox regression. Such models might represent a significant improvement over the current methods for monitoring and optimization of cART.</p
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