1,917 research outputs found
Chaotic scattering with direct processes: A generalization of Poisson's kernel for non-unitary scattering matrices
The problem of chaotic scattering in presence of direct processes or prompt
responses is mapped via a transformation to the case of scattering in absence
of such processes for non-unitary scattering matrices, \tilde S. In the absence
of prompt responses, \tilde S is uniformly distributed according to its
invariant measure in the space of \tilde S matrices with zero average, < \tilde
S > =0. In the presence of direct processes, the distribution of \tilde S is
non-uniform and it is characterized by the average (\neq 0). In
contrast to the case of unitary matrices S, where the invariant measures of S
for chaotic scattering with and without direct processes are related through
the well known Poisson kernel, here we show that for non-unitary scattering
matrices the invariant measures are related by the Poisson kernel squared. Our
results are relevant to situations where flux conservation is not satisfied.
For example, transport experiments in chaotic systems, where gains or losses
are present, like microwave chaotic cavities or graphs, and acoustic or elastic
resonators.Comment: Added two appendices and references. Corrected typo
Statistical fluctuations of the parametric derivative of the transmission and reflection coefficients in absorbing chaotic cavities
Motivated by recent theoretical and experimental works, we study the
statistical fluctuations of the parametric derivative of the transmission T and
reflection R coefficients in ballistic chaotic cavities in the presence of
absorption. Analytical results for the variance of the parametric derivative of
T and R, with and without time-reversal symmetry, are obtained for both
asymmetric and left-right symmetric cavities. These results are valid for
arbitrary number of channels, in completely agreement with the one channel case
in the absence of absorption studied in the literature.Comment: Modified version as accepted in PR
Observation of electronic and atomic shell effects in gold nanowires
The formation of gold nanowires in vacuum at room temperature reveals a
periodic spectrum of exceptionally stable diameters. This is identified as
shell structure similar to that which was recently discovered for alkali metals
at low temperatures. The gold nanowires present two competing `magic' series of
stable diameters, one governed by electronic structure and the other by the
atomic packing.Comment: 4 pages, 4 figure
Statistical wave scattering through classically chaotic cavities in the presence of surface absorption
We propose a model to describe the statistical properties of wave scattering
through a classically chaotic cavity in the presence of surface absorption.
Experimentally, surface absorption could be realized by attaching an "absorbing
patch" to the inner wall of the cavity. In our model, the cavity is connected
to the outside by a waveguide with N open modes (or channels), while an
experimental patch is simulated by an "absorbing mirror" attached to the inside
wall of the cavity; the mirror, consisting of a waveguide that supports Na
channels, with absorption inside and a perfectly reflecting wall at its end, is
described by a subunitary scattering matrix Sa. The number of channels Na, as a
measure of the geometric cross section of the mirror, and the lack of unitarity
of Sa as a measure of absorption, are under our control: these parameters have
an important physical significance for real experiments. The absorption
strength in the cavity is quantified by the trace of the lack of unitarity. The
statistical distribution of the resulting S matrix for N=1 open channel and
only one absorbing channel, Na =1, is solved analytically for the orthogonal
and unitary universality classes, and the results are compared with those
arising from numerical simulations. The relation with other models existing in
the literature, in some of which absorption has a volumetric character, is also
studied.Comment: 6 pages, 3 figures, submitted to Phys. Rev.
Second order equation of motion for electromagnetic radiation back-reaction
We take the viewpoint that the physically acceptable solutions of the
Lorentz--Dirac equation for radiation back-reaction are actually determined by
a second order equation of motion, the self-force being given as a function of
spacetime location and velocity. We propose three different methods to obtain
this self-force function. For two example systems, we determine the second
order equation of motion exactly in the nonrelativistic regime via each of
these three methods, the three methods leading to the same result. We reveal
that, for both systems considered, back-reaction induces a damping proportional
to velocity and, in addition, it decreases the effect of the external force.Comment: 13 page
K^-/K^+ ratio at GSI in hot and dense matter
The ratio in heavy-ion collisions at GSI energies is studied
including the properties of the participating hadrons in hot and dense matter.
The determination of the temperature and chemical potential at freeze-out
conditions compatible with the ratio is very delicate, and depends on
the approach adopted for the antikaon self-energy. Three approaches for the
self-energy are considered: non-interacting , on-shell self-energy
and single-particle spectral density. With respect to the on-shell approach,
the use of an energy dependent spectral density, including both s-
and p-wave components of the interaction, lowers considerably the
freeze-out temperature and gives rise to the "broad-band equilibration"
advocated by Brown, Rho and Song.Comment: 8 pages, 5 figures, talk given at the Strange Quark Matter
Conference, Atlantic Beach, North Carolina, March 12-17, 200
Functional and emotional outcomes after transient ischaemic attack: A 12-month prospective controlled cohort study
Background: Symptoms of transient ischemic attack are believed to fully resolve within 24 h of onset. Emerging evidence suggests that there may be prolonged functional and psychological impact, although studies have not been able to robustly identify whether these are the effect of transient ischemic attack or changes usually associated with ageing. We describe trajectories of disability and risk of anxiety and depression among patients seen at transient ischemic attack clinics over 12 months, compared to healthy controls. Methods: Thirty transient ischemic attack clinics across England participated. A total of 1320 participants were included: 373 diagnosed with transient ischemic attack, 186 with minor stroke, 310 with “possible transient ischemic attack,” 213 with another condition mimicking a transient ischemic attack and 238 controls recruited from primary care providers. Participants completed questionnaires after diagnosis then after 3, 6 and 12 months. Outcomes were the Nottingham Extended Activities of Daily Living Scale and the Hospital Anxiety and Depression Scale. Mixed effects regression was used to estimate group differences and trajectories. Results: At baseline, confirmed transient ischemic attack patients scored 1.31 HADS-Anxiety points (s.e. = 0.28; p < 0.001), 0.51 HADS-Depression points (s.e. = 0.26; p = 0.056), and 2.6 NEADL points (s.e. = 1.1; p = 0.020) worse than controls. At 12 months, the deficits were 0.78 (s.e. = 0.30; p = 0.008), 0.97 (s.e. = 0.23; p < 0.001), and 0.96 (s.e. = 0.92; p = 0.294) respectively. Differences among patients diagnosed with minor stroke were like or worse than transient ischemic attack patients. Conclusions: Transient ischemic attack clinic patients may have functional and emotional impairments compared to the general population irrespective of final diagnosis. The presence of emotional symptoms or risk of developing anxiety or depression did not always fully recover and may increase
Reduced Leukocyte Infiltration in Absence of Eosinophils Correlates With Decreased Tissue Damage and Disease Susceptibility in ΔdblGATA Mice During Murine Neurocysticercosis
Neurocysticercosis (NCC) is one of the most common helminth parasitic diseases of the central nervous system (CNS) and the leading cause of acquired epilepsy worldwide. NCC is caused by the presence of the metacestode larvae of the tapeworm Taenia solium within brain tissues. NCC patients exhibit a long asymptomatic phase followed by a phase of symptoms including increased intra-cranial pressure and seizures. While the asymptomatic phase is attributed to the immunosuppressive capabilities of viable T. solium parasites, release of antigens by dying organisms induce strong immune responses and associated symptoms. Previous studies in T. solium-infected pigs have shown that the inflammatory response consists of various leukocyte populations including eosinophils, macrophages, and T cells among others. Because the role of eosinophils within the brain has not been investigated during NCC, we examined parasite burden, disease susceptibility and the composition of the inflammatory reaction in the brains of infected wild type (WT) and eosinophil-deficient mice (ΔdblGATA) using a murine model of NCC in which mice were infected intracranially with Mesocestoides corti, a cestode parasite related to T. solium. In WT mice, we observed a time-dependent induction of eosinophil recruitment in infected mice, contrasting with an overall reduced leukocyte infiltration in ΔdblGATA brains. Although, ΔdblGATA mice exhibited an increased parasite burden, reduced tissue damage and less disease susceptibility was observed when compared to infected WT mice. Cellular infiltrates in infected ΔdblGATA mice were comprised of more mast cells, and αβ T cells, which correlated with an abundant CD8+ T cell response and reduced CD4+ Th1 and Th2 responses. Thus, our data suggest that enhanced inflammatory response in WT mice appears detrimental and associates with increased disease susceptibility, despite the reduced parasite burden in the CNS. Overall reduced leukocyte infiltration due to absence of eosinophils correlates with attenuated tissue damage and longer survival of ΔdblGATA mice. Therefore, our study suggests that approaches to clear NCC will require strategies to tightly control the host immune response while eradicating the parasite with minimal damage to brain tissue
Adiabatic quantum pump in the presence of external ac voltages
We investigate a quantum pump which in addition to its dynamic pump
parameters is subject to oscillating external potentials applied to the
contacts of the sample. Of interest is the rectification of the ac currents
flowing through the mesoscopic scatterer and their interplay with the quantum
pump effect. We calculate the adiabatic dc current arising under the
simultaneous action of both the quantum pump effect and classical
rectification. In addition to two known terms we find a third novel
contribution which arises from the interference of the ac currents generated by
the external potentials and the ac currents generated by the pump. The
interference contribution renormalizes both the quantum pump effect and the ac
rectification effect. Analysis of this interference effect requires a
calculation of the Floquet scattering matrix beyond the adiabatic approximation
based on the frozen scattering matrix alone. The results permit us to find the
instantaneous current. In addition to the current generated by the oscillating
potentials, and the ac current due to the variation of the charge of the frozen
scatterer, there is a third contribution which represents the ac currents
generated by an oscillating scatterer. We argue that the resulting pump effect
can be viewed as a quantum rectification of the instantaneous ac currents
generated by the oscillating scatterer. These instantaneous currents are an
intrinsic property of a nonstationary scattering process.Comment: 11 pages, 1 figur
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