932 research outputs found
Warm turbulence in the Boltzmann equation
We study the single-particle distributions of three-dimensional hard sphere
gas described by the Boltzmann equation. We focus on the steady homogeneous
isotropic solutions in thermodynamically open conditions, i.e. in the presence
of forcing and dissipation. We observe nonequilibrium steady state solution
characterized by a warm turbulence, that is an energy and particle cascade
superimposed on the Maxwell-Boltzmann distribution. We use a dimensional
analysis approach to relate the thermodynamic quantities of the steady state
with the characteristics of the forcing and dissipation terms. In particular,
we present an analytical prediction for the temperature of the system which we
show to be dependent only on the forcing and dissipative scales. Numerical
simulations of the Boltzmann equation support our analytical predictions.Comment: 4 pages, 5 figure
Efficiency and spectrum of internal gamma-ray burst shocks
We present an analysis of the Internal Shock Model of GRBs, where gamma-rays
are produced by internal shocks within a relativistic wind. We show that
observed GRB characteristics impose stringent constraints on wind and source
parameters. We find that a significant fraction, of order 20 %, of the wind
kinetic energy can be converted to radiation, provided the distribution of
Lorentz factors within the wind has a large variance and provided the minimum
Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s
is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy
breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time
R/c < 1 ms, suggesting a possible explanation for the observed clustering of
spectral break energies in this range. The lower limit to wind Lorenz factor
and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies
are set by Thomson optical depth due to electron positron pairs produced by
synchrotron photons. Natural consequences of the model are absence of bursts
with peak emission energy significantly exceeding 1 MeV, and existence of low
luminosity bursts with low, 1 keV to 10 keV, break energies.Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and
electron energy fraction. Accepted for publication in Astrophysical Journa
Vibrational Enhancement of the Effective Donor - Acceptor Coupling
The paper deals with a simple three sites model for charge transfer phenomena
in an one-dimensional donor (D) - bridge (B) - acceptor (A) system coupled with
vibrational dynamics of the B site. It is found that in a certain range of
parameters the vibrational coupling leads to an enhancement of the effective
donor - acceptor electronic coupling as a result of the formation of the
polaron on the B site. This enhancement of the charge transfer efficiency is
maximum at the resonance, where the effective energy of the fluctuating B site
coincides with the donor (acceptor) energy.Comment: 5 pages, 3 figure
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
Bilayer Membrane in Confined Geometry: Interlayer Slide and Steric Repulsion
We derived free energy functional of a bilayer lipid membrane from the first
principles of elasticity theory. The model explicitly includes
position-dependent mutual slide of monolayers and bending deformation. Our free
energy functional of liquid-crystalline membrane allows for incompressibility
of the membrane and vanishing of the in-plane shear modulus and obeys
reflectional and rotational symmetries of the flat bilayer. Interlayer slide at
the mid-plane of the membrane results in local difference of surface densities
of the monolayers. The slide amplitude directly enters free energy via the
strain tensor. For small bending deformations the ratio between bending modulus
and area compression coefficient, Kb/KA, is proportional to the square of
monolayer thickness, h. Using the functional we performed self-consistent
calculation of steric potential acting on bilayer between parallel confining
walls separated by distance 2d. We found that temperature-dependent curvature
at the minimum of confining potential is enhanced four times for a bilayer with
slide as compared with a unit bilayer. We also calculate viscous modes of
bilayer membrane between confining walls. Pure bending of the membrane is
investigated, which is decoupled from area dilation at small amplitudes. Three
sources of viscous dissipation are considered: water and membrane viscosities
and interlayer drag. Dispersion has two branches. Confinement between the walls
modifies the bending mode with respect to membrane in bulk solution.
Simultaneously, inter-layer slipping mode, damped by viscous drag, remains
unchanged by confinement.Comment: 23 pages,3 figures, pd
Surface Screening in the Casimir Force
We calculate the corrections to the Casimir force between two metals due to
the spatial dispersion of their response functions. We employ model-independent
expressions for the force in terms of the optical coefficients. We express the
non-local corrections to the Fresnel coefficients employing the surface
parameter, which accounts for the distribution of the surface
screening charge. Within a self-consistent jellium calculation, spatial
dispersion increases the Casimir force significatively for small separations.
The nonlocal correction has the opposite sign than previously predicted
employing hydrodynamic models and assuming abruptly terminated surfaces.Comment: 5 pages, 2 figure
Finite temperature Casimir effect for graphene
We adopt the Dirac model for quasiparticles in graphene and calculate the
finite temperature Casimir interaction between a suspended graphene layer and a
parallel conducting surface. We find that at high temperature the Casimir
interaction in such system is just one half of that for two ideal conductors
separated by the same distance. In this limit single graphene layer behaves
exactly as a Drude metal. In particular, the contribution of the TE mode is
suppressed, while one of the TM mode saturates the ideal metal value. Behaviour
of the Casimir interaction for intermediate temperatures and separations
accessible for an experiment is studied in some detail. We also find an
interesting interplay between two fundamental constants of graphene physics:
the fine structure constant and the Fermi velocity.Comment: 13 pages, 2 figures, to appear in Physical Review
Excitation of surface plasmon-polaritons in metal films with double periodic modulation: anomalous optical effects
We perform a thorough theoretical analysis of resonance effects when an
arbitrarily polarized plane monochromatic wave is incident onto a double
periodically modulated metal film sandwiched by two different transparent
media. The proposed theory offers a generalization of the theory that had been
build in our recent papers for the simplest case of one-dimensional structures
to two-dimensional ones. A special emphasis is placed on the films with the
modulation caused by cylindrical inclusions, hence, the results obtained are
applicable to the films used in the experiments. We discuss a spectral
composition of modulated films and highlight the principal role of
``resonance'' and ``coupling'' modulation harmonics. All the originating
multiple resonances are examined in detail. The transformation coefficients
corresponding to different diffraction orders are investigated in the vicinity
of each resonance. We make a comparison between our theory and recent
experiments concerning enhanced light transmittance and show the ways of
increasing the efficiency of these phenomena. In the appendix we demonstrate a
close analogy between ELT effect and peculiarities of a forced motion of two
coupled classical oscillators.Comment: 24 pages, 17 figure
Polyakov Loops in Strongly-Coupled Plasmas with Gravity Duals
We study the properties of the Polyakov loop in strongly-coupled gauge
plasmas that are conjectured to be dual to five dimensional theories of gravity
coupled to a nontrivial single scalar field. We find a gravity dual that can
describe the thermodynamic properties and also the expectation value of the
Polyakov loop in the deconfined phase of quenched SU(3) QCD up to .Comment: 7 pages, 2 figures, talk presented at the International Conference on
Strangeness in Quark Matter, Buzios, Rio de Janeiro, Brazil, Sept. 27 - oct.
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Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces
The concept of optical phase discontinuities is applied to the design and
demonstration of aberration-free planar lenses and axicons, comprising a phased
array of ultrathin subwavelength spaced optical antennas. The lenses and
axicons consist of radial distributions of V-shaped nanoantennas that generate
respectively spherical wavefronts and non-diffracting Bessel beams at telecom
wavelengths. Simulations are also presented to show that our aberration-free
designs are applicable to high numerical aperture lenses such as flat
microscope objectives
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