15,297 research outputs found
Adequacy of the Dicke model in cavity QED: a counter-"no-go" statement
The long-standing debate whether the phase transition in the Dicke model can
be realized with dipoles in electromagnetic fields is yet an unsettled one. The
well-known statement often referred to as the "no-go theorem", asserts that the
so-called A-square term, just in the vicinity of the critical point, becomes
relevant enough to prevent the system from undergoing a phase transition. At
variance with this common belief, in this paper we prove that the Dicke model
does give a consistent description of the interaction of light field with the
internal excitation of atoms, but in the dipole gauge of quantum
electrodynamics. The phase transition cannot be excluded by principle and a
spontaneous transverse-electric mean field may appear. We point out that the
single-mode approximation is crucial: the proper treatment has to be based on
cavity QED, wherefore we present a systematic derivation of the dipole gauge
inside a perfect Fabry-P\'erot cavity from first principles. Besides the impact
on the debate around the Dicke phase transition, such a cleanup of the
theoretical ground of cavity QED is important because currently there are many
emerging experimental approaches to reach strong or even ultrastrong coupling
between dipoles and photons, which demand a correct treatment of the Dicke
model parameters
Dissipative flow and vortex shedding in the Painlev\'e boundary layer of a Bose Einstein condensate
Raman et al. have found experimental evidence for a critical velocity under
which there is no dissipation when a detuned laser beam is moved in a
Bose-Einstein condensate. We analyze the origin of this critical velocity in
the low density region close to the boundary layer of the cloud. In the frame
of the laser beam, we do a blow up on this low density region which can be
described by a Painlev\'e equation and write the approximate equation satisfied
by the wave function in this region. We find that there is always a drag around
the laser beam. Though the beam passes through the surface of the cloud and the
sound velocity is small in the Painlev\'e boundary layer, the shedding of
vortices starts only when a threshold velocity is reached. This critical
velocity is lower than the critical velocity computed for the corresponding 2D
problem at the center of the cloud. At low velocity, there is a stationary
solution without vortex and the drag is small. At the onset of vortex shedding,
that is above the critical velocity, there is a drastic increase in drag.Comment: 4 pages, 4 figures (with 9 ps files
Force-extension relation of cross-linked anisotropic polymer networks
Cross-linked polymer networks with orientational order constitute a wide
class of soft materials and are relevant to biological systems (e.g., F-actin
bundles). We analytically study the nonlinear force-extension relation of an
array of parallel-aligned, strongly stretched semiflexible polymers with random
cross-links. In the strong stretching limit, the effect of the cross-links is
purely entropic, independent of the bending rigidity of the chains. Cross-links
enhance the differential stretching stiffness of the bundle. For hard
cross-links, the cross-link contribution to the force-extension relation scales
inversely proportional to the force. Its dependence on the cross-link density,
close to the gelation transition, is the same as that of the shear modulus. The
qualitative behavior is captured by a toy model of two chains with a single
cross-link in the middle.Comment: 7 pages, 4 figure
The structure of trailing vortices generated by model rotor blades
Hot-wire anemometry to analyze the structure and geometry of rotary wing trailing vortices is studied. Tests cover a range of aspect ratios and blade twist. For all configurations, measured vortex strength correlates well with maximum blade-bound circulation. Measurements of wake geometry are in agreement with classical data for high-aspect ratios. The detailed vortex structure is similar to that found for fixed wings and consists of four well defined regions--a viscous core, a turbulent mixing region, a merging region, and an inviscid outer region. A single set of empirical formulas for the entire set of test data is described
Ion Trap Mass Spectrometers for Identity, Abundance and Behavior of Volatiles on the Moon
NASA GSFC and The Open University (UK) are collaborating to deploy an Ion Trap Mass Spectrometer on the Moon to investigate the lunar water cycle. The ITMS is flight-proven throughthe Rosetta Philae comet lander mission. It is also being developed under ESA funding to analyse samples drilled from beneath the lunar surface on the Roscosmos Luna-27 lander (2025).Now, GSFC and OU will now develop a compact ITMS instrument to study the near-surface lunar exosphere on board a CLPS Astrobotic lander at Lacus Mortis in 2021
Renormalized One-loop Theory of Correlations in Disordered Diblock Copolymers
A renormalized one-loop theory (ROL) is used to calculate corrections to the
random phase approximation (RPA) for the structure factor \Sc(q) in
disordered diblock copolymer melts. Predictions are given for the peak
intensity , peak position , and single-chain
statistics for symmetric and asymmetric copolymers as functions of ,
where is the Flory-Huggins interaction parameter and is the degree
of polymerization. The ROL and Fredrickson-Helfand (FH) theories are found to
yield asymptotically equivalent results for the dependence of the peak
intensity upon for symmetric diblock copolymers in the
limit of strong scattering, or large , but yield qualitatively
different predictions for symmetric copolymers far from the ODT and for
asymmetric copolymers. The ROL theory predicts a suppression of
and a decrease of for large values of , relative to the RPA
predictions, but an enhancement of and an increase in
for small (). By separating intra- and
inter-molecular contributions to , we show that the decrease in
near the ODT is caused by the dependence of the intermolecular
direct correlation function, and is unrelated to any change in single-chain
statistics, but that the increase in at small values of is
a result of non-Gaussian single-chain statistics.Comment: 16 pages, 13 figures, submitted to J. Chem. Phy
Evaluation of exercises taken from the Druker thesis of first grade reading materials of high interest level.
Thesis (Ed.M.)--Boston Universit
Caddisflies (Trichoptera) of Wildcat Creek, Pickens County, South Carolina
Sixty-two species of caddisflies (Trichoptera) were identified from collections make from Wildcat Creek over a period of 33 years. A new distributional record for South Carolina was obtained for Diplectrona metaqui. Eight species, Polycentropus carlsoni, Wormaldia thyria, Neotrichia collata, Stactobiella delira, Neophylax atlanta, Goera fuscula, Pseudogoera singularis, and Agarodes griseus, are considered to be threatened in South Carolina
Strong scaling of general-purpose molecular dynamics simulations on GPUs
We describe a highly optimized implementation of MPI domain decomposition in
a GPU-enabled, general-purpose molecular dynamics code, HOOMD-blue (Anderson
and Glotzer, arXiv:1308.5587). Our approach is inspired by a traditional
CPU-based code, LAMMPS (Plimpton, J. Comp. Phys. 117, 1995), but is implemented
within a code that was designed for execution on GPUs from the start (Anderson
et al., J. Comp. Phys. 227, 2008). The software supports short-ranged pair
force and bond force fields and achieves optimal GPU performance using an
autotuning algorithm. We are able to demonstrate equivalent or superior scaling
on up to 3,375 GPUs in Lennard-Jones and dissipative particle dynamics (DPD)
simulations of up to 108 million particles. GPUDirect RDMA capabilities in
recent GPU generations provide better performance in full double precision
calculations. For a representative polymer physics application, HOOMD-blue 1.0
provides an effective GPU vs. CPU node speed-up of 12.5x.Comment: 30 pages, 14 figure
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