19,764 research outputs found
The plasma-insulator transition of spin-polarized Hydrogen
A mixed classical-quantum density functional theory is used to calculate pair
correlations and the free energy of a spin-polarized Hydrogen plasma. A
transition to an atomic insulator phase is estimated to occur around r_s=2.5 at
T=10^4K, and a pressure . Spin polarization is imposed to
prevent the formation of H_2 molecules.Comment: 10 pages, 4 figure
Examining Spillover Effects from Teach For America Corps Members in Miami-Dade County Public Schools
Despite a large body of evidence documenting the effectiveness of Teach For America (TFA) corps members at raising the math test scores of their students, little is known about the program's impact at the school level. TFA's recent placement strategy in the Miami-Dade County Public Schools (M-DCPS), where large numbers of TFA corps members are placed as clusters into a targeted set of disadvantaged schools, provides an opportunity to evaluate the impact of the TFA program on broader school performance. This study examines whether the influx of TFA corps members led to a spillover effect on other teachers' performance. We find that many of the schools chosen to participate in the cluster strategy experienced large subsequent gains in math achievement. These gains were driven in part by the composition effect of having larger numbers of effective TFA corps members. However, we do not find any evidence that the clustering strategy led to any spillover effect on school-wide performance. In other words, our estimates suggest that extra student gains for TFA corps members under the clustering strategy would be equivalent to the gains that would result from an alternate placement strategy where corps members were evenly distributed across schools
Near-Optimal Detection in MIMO Systems using Gibbs Sampling
In this paper we study a Markov Chain Monte Carlo (MCMC) Gibbs sampler for
solving the integer least-squares problem. In digital communication the problem
is equivalent to performing Maximum Likelihood (ML) detection in Multiple-Input
Multiple-Output (MIMO) systems. While the use of MCMC methods for such problems
has already been proposed, our method is novel in that we optimize the
"temperature" parameter so that in steady state, i.e. after the Markov chain
has mixed, there is only polynomially (rather than exponentially) small
probability of encountering the optimal solution. More precisely, we obtain the
largest value of the temperature parameter for this to occur, since the higher
the temperature, the faster the mixing. This is in contrast to simulated
annealing techniques where, rather than being held fixed, the temperature
parameter is tended to zero. Simulations suggest that the resulting Gibbs
sampler provides a computationally efficient way of achieving approximative ML
detection in MIMO systems having a huge number of transmit and receive
dimensions. In fact, they further suggest that the Markov chain is rapidly
mixing. Thus, it has been observed that even in cases were ML detection using,
e.g. sphere decoding becomes infeasible, the Gibbs sampler can still offer a
near-optimal solution using much less computations.Comment: To appear in Globecom 200
Stabilization of nonlinear velocity profiles in athermal systems undergoing planar shear flow
We perform molecular dynamics simulations of model granular systems
undergoing boundary-driven planar shear flow in two spatial dimensions with the
goal of developing a more complete understanding of how dense particulate
systems respond to applied shear. In particular, we are interested in
determining when these systems will possess linear velocity profiles and when
they will develop highly localized velocity profiles in response to shear. In
previous work on similar systems we showed that nonlinear velocity profiles
form when the speed of the shearing boundary exceeds the speed of shear waves
in the material. However, we find that nonlinear velocity profiles in these
systems are unstable at very long times. The degree of nonlinearity slowly
decreases in time; the velocity profiles become linear when the granular
temperature and density profiles are uniform across the system at long times.
We measure the time required for the velocity profiles to become linear
and find that increases as a power-law with the speed of the shearing
boundary and increases rapidly as the packing fraction approaches random close
packing. We also performed simulations in which differences in the granular
temperature across the system were maintained by vertically vibrating one of
the boundaries during shear flow. We find that nonlinear velocity profiles form
and are stable at long times if the difference in the granular temperature
across the system exceeds a threshold value that is comparable to the glass
transition temperature in an equilibrium system at the same average density.
Finally, the sheared and vibrated systems form stable shear bands, or highly
localized velocity profiles, when the applied shear stress is lowered below the
yield stress of the static part of the system.Comment: 11 pages, 14 figure
Microlensing pulsars
We investigate the possibilities that pulsars act as the lens in
gravitational microlensing events towards the galactic bulge or a spiral arm.
Our estimation is based on expectant survey and observations of FAST (Five
hundred meter Aperture Spherical Telescope) and SKA (Square Kilometer Array),
and two different models of pulsar distribution are used. We find that the
lensing rate is > 1 event/decade, being high enough to search the real events.
Therefore, the microlensing observations focusing on pulsars identified by FAST
or SKA in the future are meaningful. As an independent determination of pulsar
mass, a future detection of microlensing pulsars should be significant in the
history of studying pulsars, especially in constraining the state of matter
(either hadronic or quark matter) at supra-nuclear densities. The observations
of such events by using advanced optical facilities (e.g., the James Webb Space
Telescope and the Thirty Meter Telescope) in future are highly suggested.Comment: 5pages, 2figure
A Spectral Method for Elliptic Equations: The Neumann Problem
Let be an open, simply connected, and bounded region in
, , and assume its boundary is smooth.
Consider solving an elliptic partial differential equation over with a Neumann boundary condition. The problem is converted
to an equivalent elliptic problem over the unit ball , and then a spectral
Galerkin method is used to create a convergent sequence of multivariate
polynomials of degree that is convergent to . The
transformation from to requires a special analytical calculation
for its implementation. With sufficiently smooth problem parameters, the method
is shown to be rapidly convergent. For
and assuming is a boundary, the convergence of
to zero is faster than any power of .
Numerical examples in and show experimentally
an exponential rate of convergence.Comment: 23 pages, 11 figure
Climate Change, Water Supply, and Agriculture in the Arid Western United States: Eighty Years of Agricultural Census Observations from Idaho
As the population and agricultural development in the U.S. expanded west throughout the 20th century, major water infrastructure projects were initiated in order to meet irrigation demand and to reduce the risk and uncertainty associated with highly variable water supplies.2 Agriculture in the arid and semi-arid western U.S. is particularly vulnerable to variability in water supply, and has evolved to rely heavily on major water infrastructure projects.3 In 2007, of the 57 million acres of irrigated cropland and pastureland in the U.S., nearly three-quarters was in the 17 western-most contiguous states; in 2008, irrigated agriculture applied 91.2 million acre-feet of water, with over four-fifths being used in the arid west
The asymptotic tails of limit distributions of continuous time Markov chains
This paper investigates tail asymptotics of stationary distributions and
quasi-stationary distributions of continuous-time Markov chains on a subset of
the non-negative integers. A new identity for stationary measures is
established. In particular, for continuous-time Markov chains with asymptotic
power-law transition rates, tail asymptotics for stationary distributions are
classified into three types by three easily computable parameters: (i)
Conley-Maxwell-Poisson distributions (light-tailed), (ii) exponential-tailed
distributions, and (iii) heavy-tailed distributions. Similar results are
derived for quasi-stationary distributions. The approach to establish tail
asymptotics is different from the classical semimartingale approach. We apply
our results to biochemical reaction networks (modeled as continuous-time Markov
chains), a general single-cell stochastic gene expression model, an extended
class of branching processes, and stochastic population processes with bursty
reproduction, none of which are birth-death processes
- …