5,398 research outputs found
A Formal Model For Real-Time Parallel Computation
The imposition of real-time constraints on a parallel computing environment-
specifically high-performance, cluster-computing systems- introduces a variety
of challenges with respect to the formal verification of the system's timing
properties. In this paper, we briefly motivate the need for such a system, and
we introduce an automaton-based method for performing such formal verification.
We define the concept of a consistent parallel timing system: a hybrid system
consisting of a set of timed automata (specifically, timed Buchi automata as
well as a timed variant of standard finite automata), intended to model the
timing properties of a well-behaved real-time parallel system. Finally, we give
a brief case study to demonstrate the concepts in the paper: a parallel matrix
multiplication kernel which operates within provable upper time bounds. We give
the algorithm used, a corresponding consistent parallel timing system, and
empirical results showing that the system operates under the specified timing
constraints.Comment: In Proceedings FTSCS 2012, arXiv:1212.657
ECONOMIC EFFECTS OF U.S. DAIRY PROGRAMS
Based on econometric analysis, this article estimates effects of terminating the milk order system and milk price support, singly and together, over the period 1966-90. Since 1980, milk orders have raised the national blend price by 1-2%; price support has raised the blend price to well above the market clearing price, by over 21% in 1983. Short- and long-run benefits and costs are estimated for various policy options under 1990 conditions.Agricultural and Food Policy,
Finite temperature phase diagram of a spin-polarized ultracold Fermi gas in a highly elongated harmonic trap
We investigate the finite temperature properties of an ultracold atomic Fermi
gas with spin population imbalance in a highly elongated harmonic trap.
Previous studies at zero temperature showed that the gas stays in an exotic
spatially inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid
state at the trap center; while moving to the edge, the system changes into
either a non-polarized Bardeen-Cooper-Schriffer superfluid () or a fully
polarized normal gas (), depending on the smallness of the spin
polarization , relative to a critical value . In this work, we show how
these two phase-separation phases evolve with increasing temperature, and
thereby construct a finite temperature phase diagram. For typical interactions,
we find that the exotic FFLO phase survives below one-tenth of Fermi degeneracy
temperature, which seems to be accessible in the current experiment. The
density profile, equation of state, and specific heat of the polarized system
have been calculated and discussed in detail. Our results are useful for the
on-going experiment at Rice University on the search for FFLO states in
quasi-one-dimensional polarized Fermi gases.Comment: 9 pages and 8 figures; Published version in Phys. Rev.
Exact few-body results for strongly correlated quantum gases in two dimensions
The study of strongly correlated quantum gases in two dimensions has
important ramifications for understanding many intriguing pheomena in solid
materials, such as high- superconductivity and the fractional quantum
Hall effect. However, theoretical methods are plagued by the existence of
significant quantum fluctuations. Here, we present two- and three-body exact
solutions for both fermions and bosons trapped in a two-dimensional harmonic
potential, with an arbitrary -wave scattering length. These few-particle
solutions link in a natural way to the high-temperature properties of
many-particle systems via a quantum virial expansion. As a concrete example,
using the energy spectrum of few fermions, we calculate the second and third
virial coefficients of a strongly interacting Fermi gas in two dimensions, and
consequently investigate its high-temperature thermodynamics. Our thermodynamic
results may be useful for ongoing experiments on two-dimensional Fermi gases.
These exact results also provide an unbiased benchmark for quantum Monte Carlo
simulations of two-dimensional Fermi gases at high temperatures.Comment: 11 pages, 6 figure
Exploring extra dimensions through inflationary tensor modes
Predictions of inflationary schemes can be influenced by the presence of
extra dimensions. This could be of particular relevance for the spectrum of
gravitational waves in models where the extra dimensions provide a brane-world
solution to the hierarchy problem. Apart from models of large as well as
exponentially warped extra dimensions, we analyze the size of tensor modes in
the Linear Dilaton scheme recently revived in the discussion of the "clockwork
mechanism". The results are model dependent, significantly enhanced tensor
modes on one side and a suppression on the other. In some cases we are led to a
scheme of "remote inflation", where the expansion is driven by energies at a
hidden brane. In all cases where tensor modes are enhanced, the requirement of
perturbativity of gravity leads to a stringent upper limit on the allowed
Hubble rate during inflation.Comment: 29 pages, 7 figures; v2: added discussion on the emergence of
curvature singularities and removed discussion on the NKKK case with horizon
in the bulk, conclusions unaltered, matches the published versio
Static structure factor of a strongly correlated Fermi gas at large momenta
We theoretically investigate the static structure factor of an interacting
Fermi gas near the BEC-BCS crossover at large momenta. Due to short-range
two-body interactions, we predict that the structure factor of unlike spin
correlations falls off as in a universal
scaling region with large momentum and large scattering length. The
scaling coefficient is determined by the celebrated Tan's contact parameter,
which links the short-range behavior of many-body systems to their universal
thermodynamic properties. By implementing this new Tan relation together with
the random-phase approximation and the virial expansion theory in various
limiting cases, we show how to calculate at zero
and finite temperatures for arbitrary interaction strengths, at momentum
transfer higher than the Fermi momentum. Our results provide a way to
experimentally confirm a new Tan relation and to accurately measure the value
of contact parameter.Comment: 8 pages, 3 figures; revised according to the Referee's suggestions;
publised versio
Investigating the Transition Region Explosive Events and Their Relationship to Network Jets
Recent imaging observations with the Interface Region Imaging Spectrograp
(IRIS) have revealed prevalent intermittent jets with apparent speeds of
80--250 km~s from the network lanes in the solar transition region (TR).
On the other hand, spectroscopic observations of the TR lines have revealed the
frequent presence of highly non-Gaussian line profiles with enhanced emission
at the line wings, often referred as explosive events (EEs). Using simultaneous
imaging and spectroscopic observations from IRIS, we investigate the
relationship between EEs and network jets. We first identify EEs from the
Si~{\sc{iv}}~1393.755 {\AA} line profiles in our observations, then examine
related features in the 1330 {\AA} slit-jaw images. Our analysis suggests that
EEs with double peaks or enhancements in both wings appear to be located at
either the footpoints of network jets, or transient compact brightenings. These
EEs are most likely produced by magnetic reconnection. We also find that EEs
with enhancements only at the blue wing are mainly located on network jets,
away from the footpoints. These EEs clearly result from the superposition of
the high-speed network jets on the TR background. In addition, EEs showing
enhancement only at the red wing of the line are often located around the jet
footpoints, possibly caused by the superposition of reconnection downflows on
the background emission. Moreover, we find some network jets that are not
associated with any detectable EEs. Our analysis suggests that some EEs are
related to the birth or propagation of network jets, and that others are not
connected to network jets.Comment: 9 figures; to appear in Ap
Mean-field study of itinerant ferromagnetism in trapped ultracold Fermi gases: Beyond the local density approximation
We theoretically investigate the itinerant ferromagnetic transition of a
spherically trapped ultracold Fermi gas with spin imbalance under strongly
repulsive interatomic interactions. Our study is based on a self-consistent
solution of the Hartree-Fock mean-field equations beyond the widely used local
density approximation. We demonstrate that, while the local density
approximation holds in the paramagnetic phase, after the ferromagnetic
transition it leads to a quantitative discrepancy in various thermodynamic
quantities even with large atom numbers. We determine the position of the phase
transition by monitoring the shape change of the free energy curve with
increasing the polarization at various interaction strengths.Comment: 7 pages, 5 figures; published version in Phys. Rev.
- β¦