44,704 research outputs found
Coherent transport in Nb/delta-doped-GaAs hybrid microstructures
Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is
presented. The structures fabrication procedure is based on delta-doped layers
grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap
layer to protect the active semiconductor layers during ex situ transfer. The
superconductor is then sputter deposited in situ after thermal desorption of
the protective layer. Two types of structures in particular will be discussed,
i.e., a reference junction and the engineered one that contains an additional
insulating AlGaAs barrier inserted during the growth in the semiconductor. This
latter configuration may give rise to controlled interference effects and
realizes the model introduced by de Gennes and Saint-James in 1963. While both
structures show reflectionless tunneling-dominated transport, only the
engineered junction shows additionally a low-temperature single marked
resonance peaks superimposed to the characteristic Andreev-dominated subgap
conductance. The analysis of coherent magnetotransport in both microstructures
is successfully performed within the random matrix theory of Andreev transport
and ballistic effects are included by directly solving the Bogoliubov-de Gennes
equations. The impact of junction morphology on reflectionless tunneling and
the application of the employed fabrication technique to the realization of
complex semiconductor-superconductor systems are furthermore discussed.Comment: 9 pages, 8 figures, invited review paper, to be published in Mod.
Phys. Lett.
Melt-growth dynamics in CdTe crystals
We use a new, quantum-mechanics-based bond-order potential (BOP) to reveal
melt-growth dynamics and fine-scale defect formation mechanisms in CdTe
crystals. Previous molecular dynamics simulations of semiconductors have shown
qualitatively incorrect behavior due to the lack of an interatomic potential
capable of predicting both crystalline growth and property trends of many
transitional structures encountered during the melt crystal
transformation. Here we demonstrate successful molecular dynamics simulations
of melt-growth in CdTe using a BOP that significantly improves over other
potentials on property trends of different phases. Our simulations result in a
detailed understanding of defect formation during the melt-growth process.
Equally important, we show that the new BOP enables defect formation mechanisms
to be studied at a scale level comparable to empirical molecular dynamics
simulation methods with a fidelity level approaching quantum-mechanical method
Heavy flavor kinetics at the hadronization transition
We investigate the in-medium modification of the charmonium breakup processes
due to the Mott effect for light (pi, rho) and open-charm (D, D*)
quark-antiquark bound states at the chiral/deconfinement phase transition. The
Mott effect for the D-mesons effectively reduces the threshold for charmonium
breakup cross sections, which is suggested as an explanation of the anomalous
J/psi suppression phenomenon in the NA50 experiment. Further implications of
finite-temperature mesonic correlations for the hadronization of heavy flavors
in heavy-ion collisions are discussed.Comment: 4 pages, 2 figures, Contribution to SQM2001 Conference, submitted to
J. Phys.
Elliptic Flow from a Transversally Thermalized Fireball
The agreement of elliptic flow data at RHIC at central rapidity with the
hydrodynamic model has led to the conclusion of very rapid thermalization. This
conclusion is based on the intuitive argument that hydrodynamics, which assumes
instantaneous local thermalization, produces the largest possible elliptic flow
values and that the data seem to saturate this limit. We here investigate the
question whether incompletely thermalized viscous systems may actually produce
more elliptic flow than ideal hydrodynamics. Motivated by the extremely fast
primordial longitudinal expansion of the reaction zone, we investigate a toy
model which exhibits thermalization only in the transverse directions but
undergoes collisionless free-streaming expansion in the longitudinal direction.
For collisions at RHIC energies, elliptic flow results from the model are
compared with those from hydrodynamics. With the final particle yield and
\kt-distribution fixed, the transversally thermalized model is shown not to
be able to produce the measured amount of elliptic flow. This investigation
provides further support for very rapid local kinetic equilibration at RHIC. It
also yields interesting novel results for the elliptic flow of massless
particles such as direct photons.Comment: revtex4, 15 pages + 10 embedded EPS figure
Non-equilibrium chemistry and dust formation in AGB stars as probed by SiO line emission
We have performed high spatial resolution observations of SiO line emission
for a sample of 11 AGB stars using the ATCA, VLA and SMA interferometers.
Detailed radiative transfer modelling suggests that there are steep chemical
gradients of SiO in their circumstellar envelopes. The emerging picture is one
where the radial SiO abundance distribution starts at an initial high
abundance, in the case of M-stars consistent with LTE chemistry, that
drastically decreases at a radius of ~1E15 cm. This is consistent with a
scenario where SiO freezes out onto dust grains. The region of the wind with
low abundance is much more extended, typically ~1E16 cm, and limited by
photodissociation. The surpisingly high SiO abundances found in carbon stars
requires non-equilibrium chemical processes.Comment: 2 pages, 1 figure. To be published in the proceedings of the
conference "Why Galaxies Care about AGB Stars", held in Vienna, August 7-11,
2006; F. Kerschbaum, C. Charbonnel, B. Wing eds, ASP Conf.Ser. in pres
Topo-Geometric Filtration Scheme for Geometric Active Contours and Level Sets: Application to Cerebrovascular Segmentation
One of the main problems of the existing methods for the
segmentation of cerebral vasculature is the appearance in the segmentation
result of wrong topological artefacts such as the kissing vessels.
In this paper, a new approach for the detection and correction of such
errors is presented. The proposed technique combines robust topological
information given by Persistent Homology with complementary geometrical
information of the vascular tree. The method was evaluated on 20
images depicting cerebral arteries. Detection and correction success rates
were 81.80% and 68.77%, respectively
Cryogenic spectroscopy of ultra-low density colloidal lead chalcogenide quantum dots on chip-scale optical cavities towards single quantum dot near-infrared cavity QED
We present evidence of cavity quantum electrodynamics from a sparse density
of strongly quantum-confined Pb-chalcogenide nanocrystals (between 1 and 10)
approaching single-dot levels on moderately high-Q mesoscopic silicon optical
cavities. Operating at important near-infrared (1500-nm) wavelengths, large
enhancements are observed from devices and strong modifications of the QD
emission are achieved. Saturation spectroscopy of coupled QDs is observed at
77K, highlighting the modified nanocrystal dynamics for quantum information
processing.Comment: * new additional figures and text * 10 pages, 5 figure
Current-Induced Spin Polarization in Gallium Nitride
Electrically generated spin polarization is probed directly in bulk GaN using
Kerr rotation spectroscopy. A series of n-type GaN epilayers are grown in the
wurtzite phase both by molecular beam epitaxy (MBE) and metalorganic chemical
vapor deposition (MOCVD) with a variety of doping densities chosen to broadly
modulate the transverse spin lifetime, T2*. The spin polarization is
characterized as a function of electrical excitation energy over a range of
temperatures. Despite weak spin-orbit interactions in GaN, a current-induced
spin polarization (CISP) is observed in the material at temperatures of up to
200 K.Comment: 16 pages, 3 figure
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