2,063 research outputs found
An empirical analysis of the distribution of overshoots in a stationary Gaussian stochastic process
The frequency distribution of overshoots in a stationary Gaussian stochastic process is analyzed. The primary processes involved in this analysis are computer simulation and statistical estimation. Computer simulation is used to simulate stationary Gaussian stochastic processes that have selected autocorrelation functions. An analysis of the simulation results reveals a frequency distribution for overshoots with a functional dependence on the mean and variance of the process. Statistical estimation is then used to estimate the mean and variance of a process. It is shown that for an autocorrelation function, the mean and the variance for the number of overshoots, a frequency distribution for overshoots can be estimated
Dissipative dynamics of vortex arrays in trapped Bose-condensed gases: neutron stars physics on K scale
We develop a theory of dissipative dynamics of large vortex arrays in trapped
Bose-condensed gases. We show that in a static trap the interaction of the
vortex array with thermal excitations leads to a non-exponential decay of the
vortex structure, and the characteristic lifetime depends on the initial
density of vortices. Drawing an analogy with physics of pulsar glitches, we
propose an experiment which employs the heating of the thermal cloud in the
course of the decay of the vortex array as a tool for a non-destructive study
of the vortex dynamics.Comment: 4 pages, revtex; revised versio
Manifestation of superfluidity in an evolving Bose-condensed gas
We study the generation of excitations due to an ''impurity''(static
perturbation) placed into an oscillating Bose-condensed gas in the
time-dependent trapping field. It is shown that there are two regions for the
position of the local perturbation. In the first region the condensate flows
around the ''impurity'' without generation of excitations demonstrating
superfluid properties. In the second region the creation of excitations occurs,
at least within a limited time interval, revealing destruction of
superfluidity. The phenomenon can be studied by measuring the damping of
condensate oscillations at different positions of the ''impurity''
Periodically-dressed Bose-Einstein condensates: a superfluid with an anisotropic and variable critical velocity
Two intersecting laser beams can produce a spatially-periodic coupling
between two components of an atomic gas and thereby modify the dispersion
relation of the gas according to a dressed-state formalism. Properties of a
Bose-Einstein condensate of such a gas are strongly affected by this
modification. A Bogoliubov transformation is presented which accounts for
interparticle interactions to obtain the quasiparticle excitation spectrum in
such a condensate. The Landau critical velocity is found to be anisotropic and
can be widely tuned by varying properties of the dressing laser beams.Comment: 5 pages, 4 figure
Creation of a monopole in a spinor condensate
We propose a method to create a monopole structure in a spin-1 spinor
condensate by applying the basic methods used to create vortices and solitons
experimentally in single-component condensates. We show, however, that by using
a two-component structure for a monopole, we can simplify our proposed
experimental approach and apply it also to ferromagnetic spinor condensates. We
also discuss the observation and dynamics of such a monopole structure, and
note that the dynamics of the two-component monopole differs from the dynamics
of the three-component monopole.Comment: The focus of the paper is shifted towards creation and observation of
monopole
Vortex lattices in a stirred Bose-Einstein condensate
We stir with a focused laser beam a Bose-Einstein condensate of Rb
atoms confined in a magnetic trap. We observe the formation of a single vortex
for a stirring frequency exceeding a critical value. At larger rotation
frequencies we produce states of the condensate for which up to eleven vortices
are simultaneously present. We present measurements of the decay of a vortex
array once the stirring laser beam is removed
Impact of Human Presence and Visual Access on Barking Behavior in Shelter Dogs
Shelters can be stressful for dogs due to lack of predictability and control, social isolation, and busy environments. Providing dogs with more social opportunities and environmental predictability may improve their welfare. Barking may indicate stress and contribute to noise levels that are harmful to dogs and people. We investigated the impact of human presence and line of sight on barking. We manipulated line of sight by partially removing a crate barrier to allow the dogs visual access to other dogs and a better view of the room. We collected data on barking on 17 focal dogs as well as overall barking in the room during pre-treatment (no visual access), treatment (visual access), and post-treatment (no visual access) and noted if a person other than the observer was in the room. We found that in-room barking was significantly higher when a person was in the room (Wilcoxon Signed Ranks Test, Z= -4.048, p \u3c .001). Based on these results, shelters should consider limiting the human activity in the room to reduce noise levels. Since barking did not significantly increase with the addition of visual access, shelters may also consider providing the dogs visual access as a way to allow beneficial social interaction
Electron Production in Proton Collisions: Total Cross Sections
Existing data on the ionization of neutral atoms and molecules by proton impact are reviewed, and electron production cross-section data are collected. The three major experimental methods are discussed and possible sources of error identified. Some theoretical cross sections are discussed, and well-established methods of relating them to measured cross sections are reviewed. A mathematical equation is fitted to the weighted experimental data for each target, and these fits are adjusted to be consistent with appropriate theoretical calculations and with electron impact and photoionization data. Recommended values of total cross sections for proton-impact ionization are given
Absorption and wavepackets in optically excited semiconductor superlattices driven by dc-ac fields
Within the one-dimensional tight-binding minibands and on-site
Coloumbic interaction approximation, the absorption spectrum and coherent
wavepacket time evolution in an optically excited semiconductor superlattice
driven by dc-ac electric fields are investigated using the semiconductor Bloch
equations.
The dominating roles of the ratios of dc-Stark to external ac frequency, as
well as ac-Stark to external ac frequency, is emphasized. If the former is an
integer , then also harmonics are present within one Stark
frequency, while the fractional case leads to the formation of excitonic
fractional ladders. The later ratio determines the size and profile of the
wavepacket. In the absence of excitonic interaction it controls the maximum
size wavepackets reach within one cycle, while the interaction produces a
strong anisotropy and tends to palliate the dynamic wavepacket localization.Comment: 14 pages, 7 postscript figure
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