15,258 research outputs found
Partially suppressed long-range order in the Bose-Einstein condensation of polaritons
We adopt a kinetic theory of polariton non-equilibrium Bose-Einstein
condensation, to describe the formation of off-diagonal long-range order. The
theory accounts properly for the dominant role of quantum fluctuations in the
condensate. In realistic situations with optical excitation at high energy, it
predicts a significant depletion of the condensate caused by long-wavelength
fluctuations. As a consequence, the one-body density matrix in space displays a
partially suppressed long-range order and a pronounced dependence on the finite
size of the system
Advanced composite aileron for L-1011 transport aircraft: Design and analysis
Detail design of the composite aileron has been completed. The aileron design is a multi-rib configuration with single piece upper and lower covers mechanically fastened to the substructure. Covers, front, spar and ribs are fabricated with graphite/epoxy tape or fabric composite material. The design has a weight savings of 23 percent compared to the aluminum aileron. The composite aileron has 50 percent fewer fasteners and parts than the metal aileron and is predicted to be cost competitive. Structural integrity of the composite aileron was verified by structural analysis and an extensive test program. Static, failsafe, and vibration analyses have been conducted on the composite aileron using finite element models and specialized computer programs for composite material laminates. The fundamental behavior of the composite materials used in the aileron was determined by coupon tests for a variety of environmental conditions. Critical details of the design were interrogated by static and fatigue tests on full-scale subcomponents and subassemblies of the aileron
Advanced composite aileron for L-1011 transport aircraft, task 1
Structural design and maintainability criteria were established and used as a guideline for evaluating a variety of configurations and materials for each of the major subcomponents. From this array of subcomponent designs, several aileron assemblies were formulated and analyzed. The selected design is a multirib configuration with sheet skin covers mechanically fastened to channel section ribs and spars. Qualitative analysis of currently available composite material systems led to the selection of three candidate materials on which comparative structural tests were conducted to measure the effects of environment and impact damage on mechanical property retention. In addition, each system was evaluated for producibility characteristics. From these tests, Thornel 300/5208 unidirectional tape was selected for the front spar and covers, and Thornel 300 fabric/5208 was chosen for the ribs
Propagation of Second sound in a superfluid Fermi gas in the unitary limit
We study sound propagation in a uniform superfluid gas of Fermi atoms in the
unitary limit. The existence of normal and superfluid components leads to
appearance of two sound modes in the collisional regime, referred to as first
and second sound. The second sound is of particular interest as it is a clear
signal of a superfluid component. Using Landau's two-fluid hydrodynamic theory,
we calculate hydrodynamic sound velocities and these weights in the density
response function. The latter is used to calculate the response to a sudden
modification of the external potential generating pulse propagation. The
amplitude of a pulse which is proportional to the weight in the response
function, is calculated the basis of the approach of Nozieres and Schmitt-Rink
(NSR) for the BCS-BEC crossover. We show that, in a superfluid Fermi gas at
unitarity, the second sound pulse is excited with an appreciate amplitude by
density perturbations.Comment: 12 pages, 9 figures. This version includes an erratum concerning the
temperature dependence of hydrodynamic sound weights in Phys. Rev. A 80,
043613 (2009
Long-term material compatibility testing system
System includes procedure for hermetically sealing solid materials and fluids in glass ampoule and use of temperature-controlled facility containing sample holder, which permits sample containers to be retrieved safely and conveniently. Solid material and fluid are sealed within chemically-clean glass ampoule according to highly detailed procedure
Scalable Peer-to-Peer Streaming for Live Entertainment Content
We present a system for streaming live entertainment content over the Internet originating from a single source to a scalable number of consumers without resorting to centralized or provider-provisioned resources. The system creates a peer-to-peer overlay network, which attempts to optimize use of existing capacity to ensure quality of service, delivering low startup delay and lag in playout of the live content. There are three main aspects of our solution: first, a swarming mechanism that constructs an overlay topology for minimizing propagation delays from the source to end consumers; second, a distributed overlay anycast system that uses a location-based search algorithm for peers to quickly find the closest peers in a given stream; and finally, a novel incentive mechanism that encourages peers to donate capacity even when the user is not actively consuming content
Dynamic spin response of a strongly interacting Fermi gas
We present an experimental investigation of the dynamic spin response of a
strongly interacting Fermi gas using Bragg spectroscopy. By varying the
detuning of the Bragg lasers, we show that it is possible to measure the
response in the spin and density channels separately. At low Bragg energies,
the spin response is suppressed due to pairing, whereas the density response is
enhanced. These experiments provide the first independent measurements of the
spin-parallel and spin-antiparallel dynamic and static structure factors and
open the way to a complete study of the structure factors at any momentum. At
high momentum the spin-antiparallel dynamic structure factor displays a
universal high frequency tail, proportional to , where is the probe energy.Comment: Replaced with final versio
Viscosity of strongly interacting quantum fluids: spectral functions and sum rules
The viscosity of strongly interacting systems is a topic of great interest in
diverse fields.
We focus here on the bulk and shear viscosities of \emph{non-relativistic}
quantum fluids, with particular emphasis on strongly interacting ultracold
Fermi gases. We use Kubo formulas for the bulk and shear viscosity spectral
functions, and respectively, to derive exact,
non-perturbative results. Our results include: a microscopic connection between
the shear viscosity and the normal fluid density ; sum rules for
and and their evolution through the BCS-BEC
crossover; universal high-frequency tails for and the dynamic
structure factor . We use our sum rules to show that, at
unitarity, is identically zero and thus relate
to density-density correlations. We predict that frequency-dependent shear
viscosity of the unitary Fermi gas can be experimentally
measured using Bragg spectroscopy.Comment: Published versio
Microscopic calculation of the phonon-roton branch in superfluid He
Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk
liquid He at zero temperature are presented. The sign problem associated to
the excited wave function has been dealt both with the fixed-node approximation
and the released-node technique. The upper bounds provided by the fixed-node
approximation are shown to become exact when using the released-node method. An
excellent agreement with experimental data is achieved both at the equilibrium
and near the freezing densities.Comment: 12 pages, RevTex, 3 ps figures include
Collective and single-particle excitations in 2D dipolar Bose gases
The Berezinskii-Kosterlitz-Thouless transition in 2D dipolar systems has been
studied recently by path integral Monte Carlo (PIMC) simulations [A. Filinov et
al., PRL 105, 070401 (2010)]. Here, we complement this analysis and study
temperature-coupling strength dependence of the density (particle-hole) and
single-particle (SP) excitation spectra both in superfluid and normal phases.
The dynamic structure factor, S(q,omega), of the longitudinal excitations is
rigorously reconstructed with full information on damping. The SP spectral
function, A(q,omega), is worked out from the one-particle Matsubara Green's
function. A stochastic optimization method is applied for reconstruction from
imaginary times. In the superfluid regime sharp energy resonances are observed
both in the density and SP excitations. The involved hybridization of both
spectra is discussed. In contrast, in the normal phase, when there is no
coupling, the density modes, beyond acoustic phonons, are significantly damped.
Our results generalize previous zero temperature analyses based on variational
many-body wavefunctions [F. Mazzanti et al., PRL 102, 110405 (2009), D. Hufnagl
et al., PRL 107, 065303 (2011)], where the underlying physics of the excitation
spectrum and the role of the condensate has not been addressed.Comment: 27 pages, 15 figures, 7 table
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