22,097 research outputs found
Semiclassical Theory of Chaotic Quantum Transport
We present a refined semiclassical approach to the Landauer conductance and
Kubo conductivity of clean chaotic mesoscopic systems. We demonstrate for
systems with uniformly hyperbolic dynamics that including off-diagonal
contributions to double sums over classical paths gives a weak-localization
correction in quantitative agreement with results from random matrix theory. We
further discuss the magnetic field dependence. This semiclassical treatment
accounts for current conservation.Comment: 4 pages, 1 figur
Mg I emission lines at 12 and 18 micrometer in K giants
The solar Mg I emission lines at 12 micrometer have already been observed and
analyzed well. Previous modeling attempts for other stars have, however, been
made only for Procyon and two cool evolved stars, with unsatisfactory results
for the latter. We present high-resolution observational spectra for the K
giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines
at 12 micrometer as compared to the Sun. We also present the first observed
stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and
presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ
standard non-LTE modeling for trace elements in cool stellar photospheres. We
compute model atmospheres with the MARCS code, apply a comprehensive magnesium
model atom, and use the radiative transfer code MULTI to solve for the
magnesium occupation numbers in statistical equilibrium. We successfully
reproduce the observed Mg I emission lines simultaneously in the giants and in
the Sun, but show how the computed line profiles depend critically on atomic
input data and how the inclusion of energy levels with n > 9 and collisions
with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Prevalence of marginally unstable periodic orbits in chaotic billiards
The dynamics of chaotic billiards is significantly influenced by coexisting
regions of regular motion. Here we investigate the prevalence of a different
fundamental structure, which is formed by marginally unstable periodic orbits
and stands apart from the regular regions. We show that these structures both
{\it exist} and {\it strongly influence} the dynamics of locally perturbed
billiards, which include a large class of widely studied systems. We
demonstrate the impact of these structures in the quantum regime using
microwave experiments in annular billiards.Comment: 6 pages, 5 figure
Sistemas silvipastoriles agroecológicos con espécies nativas como estratégia de conservassem de la biodiversidad.
IUFROLAT 2013. Disponibilizado online. Resumen
Sampling functions for multimode homodyne tomography with a single local oscillator
We derive various sampling functions for multimode homodyne tomography with a
single local oscillator. These functions allow us to sample multimode
s-parametrized quasidistributions, density matrix elements in Fock basis, and
s-ordered moments of arbitrary order directly from the measured quadrature
statistics. The inevitable experimental losses can be compensated by proper
modification of the sampling functions. Results of Monte Carlo simulations for
squeezed three-mode state are reported and the feasibility of reconstruction of
the three-mode Q-function and s-ordered moments from 10^7 sampled data is
demonstrated.Comment: 12 pages, 8 figures, REVTeX, submitted Phys. Rev.
Molecular observation of contour-length fluctuations limiting topological confinement in polymer melts
In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time
The 5-hydroxytryptamine(4a) receptor is palmitoylated at two different sites and acylation is critically involved in regulation of receptor constitutive activity
Interfaces Within Graphene Nanoribbons
We study the conductance through two types of graphene nanostructures:
nanoribbon junctions in which the width changes from wide to narrow, and curved
nanoribbons. In the wide-narrow structures, substantial reflection occurs from
the wide-narrow interface, in contrast to the behavior of the much studied
electron gas waveguides. In the curved nanoribbons, the conductance is very
sensitive to details such as whether regions of a semiconducting armchair
nanoribbon are included in the curved structure -- such regions strongly
suppress the conductance. Surprisingly, this suppression is not due to the band
gap of the semiconducting nanoribbon, but is linked to the valley degree of
freedom. Though we study these effects in the simplest contexts, they can be
expected to occur for more complicated structures, and we show results for
rings as well. We conclude that experience from electron gas waveguides does
not carry over to graphene nanostructures. The interior interfaces causing
extra scattering result from the extra effective degrees of freedom of the
graphene structure, namely the valley and sublattice pseudospins.Comment: 19 pages, published version, several references added, small changes
to conclusion
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