6,315 research outputs found
The influence of temporal coherence on the dynamical Casimir effect
We study the dynamical Casimir effect in the presence of a finite coherence
time, which is associated with a finite quality factor of the optical cavity.
We use the time refraction model, where a fixed cavity with a modulated optical
medium, replaces the empty cavity with a vibrating mirror. Temporal coherence
is described with the help of cavity quasi-mode operators. Asymptotic
expressions for the number of photon pairs generated from vacuum are derived.Comment: 8 pages, 1 figur
Exploring the Venus global super-rotation using a comprehensive General Circulation Model
The atmospheric circulation in Venus is well known to exhibit strong
super-rotation. However, the atmospheric mechanisms responsible for the
formation of this super-rotation are still not fully understood. In this work,
we developed a new Venus general circulation model to study the most likely
mechanisms driving the atmosphere to the current observed circulation. Our
model includes a new radiative transfer, convection and suitably adapted
boundary layer schemes and a dynamical core that takes into account the
dependence of the heat capacity at constant pressure with temperature.
The new Venus model is able to simulate a super-rotation phenomenon in the
cloud region quantitatively similar to the one observed. The mechanisms
maintaining the strong winds in the cloud region were found in the model
results to be a combination of zonal mean circulation, thermal tides and
transient waves. In this process, the semi-diurnal tide excited in the upper
clouds has a key contribution in transporting axial angular momentum mainly
from the upper atmosphere towards the cloud region. The magnitude of the
super-rotation in the cloud region is sensitive to various radiative parameters
such as the amount of solar radiative energy absorbed by the surface, which
controls the static stability near the surface. In this work, we also discuss
the main difficulties in representing the flow below the cloud base in Venus
atmospheric models.
Our new radiative scheme is more suitable for 3D Venus climate models than
those used in previous work due to its easy adaptability to different
atmospheric conditions. This flexibility of the model was crucial to explore
the uncertainties in the lower atmospheric conditions and may also be used in
the future to explore, for example, dynamical-radiative-microphysical
feedbacks.Comment: Accepted for publication in Planet. Space Sc
Vacuum effects in a vibrating cavity: time refraction, dynamical Casimir effect, and effective Unruh acceleration
Two different quantum processes are considered in a perturbed vacuum cavity:
time refraction and dynamical Casimir effect. They are shown to be physically
equivalent, and are predicted to be unstable, leading to an exponential growth
in the number of photons created in the cavity. The concept of an effective
Unruh acceleration for these processes is also introduced, in order to make a
comparison in terms of radiation efficiency, with the Unruh radiation
associated with an accelerated frame in unbounded vacuum.Comment: 5 pages, version to appear in Physics Letters
Analytical Models of Exoplanetary Atmospheres. II. Radiative Transfer via the Two-stream Approximation
We present a comprehensive analytical study of radiative transfer using the
method of moments and include the effects of non-isotropic scattering in the
coherent limit. Within this unified formalism, we derive the governing
equations and solutions describing two-stream radiative transfer (which
approximates the passage of radiation as a pair of outgoing and incoming
fluxes), flux-limited diffusion (which describes radiative transfer in the deep
interior) and solutions for the temperature-pressure profiles. Generally, the
problem is mathematically under-determined unless a set of closures (Eddington
coefficients) is specified. We demonstrate that the hemispheric (or
hemi-isotropic) closure naturally derives from the radiative transfer equation
if energy conservation is obeyed, while the Eddington closure produces spurious
enhancements of both reflected light and thermal emission. We concoct recipes
for implementing two-stream radiative transfer in stand-alone numerical
calculations and general circulation models. We use our two-stream solutions to
construct toy models of the runaway greenhouse effect. We present a new
solution for temperature-pressure profiles with a non-constant optical opacity
and elucidate the effects of non-isotropic scattering in the optical and
infrared. We derive generalized expressions for the spherical and Bond albedos
and the photon deposition depth. We demonstrate that the value of the optical
depth corresponding to the photosphere is not always 2/3 (Milne's solution) and
depends on a combination of stellar irradiation, internal heat and the
properties of scattering both in optical and infrared. Finally, we derive
generalized expressions for the total, net, outgoing and incoming fluxes in the
convective regime.Comment: Accepted by ApJS. 23 pages, 11 figures, 3 tables, 158 equations. No
change from previous version except for title (to match ApJS convention
Eosinophilic fasciitis (Shulman's disease): review and comparative evaluation of seven patients.
OBJECTIVES:
Eosinophilic fasciitis (EF) was described in 1974 by Shulman as a rare fibrosing connective tissue disease of unknown etiology. An undetermined trigger is thought to lead to the degranulation of eosinophils that interact with fibroblasts and express fibrogenic cytokines including the transforming factor of tumor growth a and b and interleukins 1 and 6. The purpose of this study was to summarize seven cases of EF in a central hospital.
MATERIAL AND METHODS:
This was a retrospective and descriptive study of a population with EF of a central hospital. All patients diagnosed with EF in a hospital unit were admitted to the study between January 1, 2005, and April 30, 2018.
RESULTS:
A total of seven patients diagnosed with EF were analyzed. The median age of the population at the time of diagnosis was 56 years, and 57% of the patients were women. All patients had elevated peripheral eosinophilia and sedimentation rate, and only one patient had hypergammaglobulinemia. All patients had edema and cutaneous thickening of the limbs, 57% had constitutional symptoms, and 57% had inflammatory arthritis with joint contracture. Prednisolone (PDN) therapy was initiated in all patients, and only in two was the association of PDN with methotrexate (MTX) initially performed. In one patient triple therapy of PDN, MTX, and cyclosporine was required. At the time of this publication, only one patient maintains active disease, and tocilizumab has been initiated.
CONCLUSIONS:
Recent studies show a more favorable response from the combination of PDN and MTX than from PDN alone. Considering the rarity of the disease, more long-term studies are needed regarding the etiopathogenetics, progression, recurrence of EF, and new effective therapies.info:eu-repo/semantics/publishedVersio
Two-stream instability in quasi-one-dimensional Bose-Einstein condensates
We apply a kinetic model to predict the existence of an instability mechanism in elongated Bose-Einstein condensates. Our kinetic description, based on the Wigner formalism, is employed to highlight the existence of unstable Bogoliubov waves that may be excited in the counterpropagation configuration. We identify a dimensionless parameter, the Mach number at T=0, that tunes different regimes of stability. We also estimate the magnitude of the main parameters at which two-stream instability is expected to be observed under typical experimental conditions
Modulational instability of spatially broadband nonlinear optical pulses in four-state atomic systems
The modulational instability of broadband optical pulses in a four-state
atomic system is investigated. In particular, starting from a recently derived
generalized nonlinear Schr\"odinger equation, a wave-kinetic equation is
derived. A comparison between coherent and random phase wave states is made. It
is found that the spatial spectral broadening can contribute to the nonlinear
stability of ultra-short optical pulses. In practical terms, this could be
achieved by using random phase plate techniques.Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Cascaded Nondegenerate Four-Wave Mixing Technique for High-Power Single-Cycle Pulse Synthesis in the Visible and Ultraviolet Ranges
We present a new technique to synthesize high-power single-cycle pulses in
the visible and ultraviolet ranges by coherent superposition of a multiband
octave-spanning spectrum obtained by highly-nondegenerate cascaded four-wave
mixing of femtosecond pulses in bulk isotropic nonresonant media. The
generation of coherent spectra spanning over two octaves in bandwidth is
experimentally demonstrated using a thin fused silica slide. Full
characterization of the intervening multicolored fields using
frequency-resolved optical gating, where multiple cascaded orders have been
measured simultaneously for the first time, supports the possibility of direct
synthesis of near-single-cycle 2.2 fs visible-UV pulses without recurring to
complex amplitude or phase control, which should enable many applications in
science and technology.Comment: 13 pages, 4 figures. Submitted to Physical Review
Effective action for a quantum scalar field in warped spaces
We investigate the one-loop corrections at zero, as well as finite
temperature, of a scalar field taking place in a braneworld motived warped
background. After to reach a well defined problem, we calculate the effective
action with the corresponding quantum corrections to each case.Comment: 10 pages, to appear in The European Physical Journal
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