8,713 research outputs found
Surface losses and self-pumping effects in a long Josephson junction - a semi-analytical approach
The flux-flow dynamics in a long Josephson junction is studied both
analytically and numerically. A realistic model of the junction is considered
by taking into account a nonuniform current distribution, surface losses and
self-pumping effects. An approximate analytical solution of the modified
sine-Gordon equation is derived in the form of a unidirectional dense fluxon
train accompanied by two oppositely directed plasma waves. Next, some
macroscopic time-averaged quantities are calculated making possible to evaluate
the current-voltage characteristic of the junction. The results obtained by the
present method are compared with direct numerical simulations both for the
current-voltage characteristics and for the loss factor modulated spatially due
to the self-pumping. The comparison shows very good agreement for typical
junction parameters but indicates also some limitations of the method.Comment: 7 pages, 5 figure
PUPHS2D 2.0 User\u27s Manual
The Purdue University Program for Heterostructure Simulation in Two Dimensions (PUPHS2D) solves Poisson\u27s equation and the electron and hole continuity equations within a two-dimensional heterostructure device. The program will compute the electrostatic potential, electron and hole densities, recombination rate, and other quantities of interest as a function of applied bias. Like its predecessor, version 2.0 allows extensive analysis of solar cells, including computation of the current-voltage characteristics of two-terminal devices, solar cell parameters, quantum efficiency, and current versus solar intensity. Extensions to version 2.Q include transient analysis and bipolar transistor capability. The heterojunction bipolar transistor routines allow computation of dc currents as a function of applied bias, as well as quasi-static evaluation of the high-frequency behavior. A simplified energy balance equation has been added in the interest of more accurately computing high-field characteristics, and should be viewed as a preliminary step toward this goal. PUPHS2D stands as an accurate model for computing low-field device characteristics and recombinative losses. While PUPHS2D was written for the ternary AlxGai1-xAs, all material specific parameters are contained within a single subroutine (BANDX), except for absorption coefficient and carrier mobilities which are computed in subroutines ALGABS and SETMOB, respectively. Material-specific parameters used for the energy balance equation are found in subroutines INITMU and INITAU. The program may be readily modified to analyze other semiconductors. For a more thorough discussion of the theoretical basis and numerical implementation of PUPHS2D, the user is directed to the references. Materials parameters are described in reference [I]. Various phases of the development of PUPHS2D have been supported by the Semiconductor Research Corporation, Sandia National Laboratories/ the Eastman Kodak Company, and by Research Triangle Institute
FISH1D 2.1 User’s Manual
FISH1D is a computer program that solves the one-dimensional Poisson equation for electrostatic Fields In Semiconductor Heterostructures. The program will print or plot the electrostatic potential, electric field, electron and hole densities, dopant density, ionized dopant density, and other quantities of interest versus position at an applied bias voltage (assuming zero current). A capacitance or sheet carrier concentration versus voltage analysis may also be performed. While FISH1D was originally written for the ternary AlxGa1_xAs, it has been modified to simulate CdxHg1_xTe, ZnSe, GexSi1_x, and Si as well, and the program can be readily modified to analyze other semiconductors through the addition of new material subroutines or using the most recent option, the MATDEF card. This card enables the user to enter new material definitions by layers in the input deck without having to recompile, an advantage of FISH1D 2.1 over FISH1D 2.0. The primary purpose of this document is explain how to use FISH1D; for a more thorough discussion of the numerical implementation of FISH1D, the user is directed to the references. A theoretical basis for FISH1D is provided in Appendix I of this manual. The development of FISH1D was supported by the Semiconductor Research Corporation, the National Science Foundation Materials Research Laboratory, and by the Eastman Kodak Company
Gas Purity effect on GEM Performance in He and Ne at Low Temperatures
The performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne, He+H2
and Ne+H2 was studied at temperatures in the range of 3-293 K. This paper
reports on previously published measurements and additional studies on the
effects of the purity of the gases in which the GEM performance is evaluated.
In He, at temperatures between 77 and 293 K, triple-GEM structures operate at
rather high gains, exceeding 1000. There is an indication that this high gain
is achieved through the Penning effect as a result of impurities in the gas. At
lower temperatures the gain-voltage characteristics are significantly modified
probably due to the freeze-out of these impurities. Double-GEM and single-GEM
structures can operate down to 3 K at gains reaching only several tens at a gas
density of about 0.5 g/l; at higher densities the maximum gain drops further.
In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in
Ne at low temperatures can be re-established in Penning mixtures of Ne+H2: very
high gains, exceeding 104, have been obtained in these mixtures at 30-77 K, at
a density of 9.2 g/l which corresponds to saturated Ne vapor density at 27 K.
The addition of small amounts of H2 in He also re-establishes large GEM gains
above 30 K but no gain was observed in He+H2 at 4 K and a density of 1.7 g/l
(corresponding to roughly one-tenth of the saturated vapor density). These
studies are, in part, being pursued in the development of two-phase He and Ne
detectors for solar neutrino detection.Comment: 4 pages, 7 figure
Slow Quenches Produce Fuzzy, Transient Vortices
We examine the Zurek scenario for the production of vortices in quenches of
liquid in the light of recent experiments. Extending our previous
results to later times, we argue that short wavelength thermal fluctuations
make vortices poorly defined until after the transition has occurred. Further,
if and when vortices appear, it is plausible that that they will decay faster
than anticipated from turbulence experiments, irrespective of quench rates.Comment: 4 pages, Revtex file, no figures Apart from a more appropriate title,
this paper differs from its predecessor by including temperature, as well as
pressure, quenche
Gapless finite- theory of collective modes of a trapped gas
We present predictions for the frequencies of collective modes of trapped
Bose-condensed Rb atoms at finite temperature. Our treatment includes a
self-consistent treatment of the mean-field from finite- excitations and the
anomolous average. This is the first gapless calculation of this type for a
trapped Bose-Einstein condensed gas. The corrections quantitatively account for
the downward shift in the excitation frequencies observed in recent
experiments as the critical temperature is approached.Comment: 4 pages Latex and 2 postscript figure
Comparison of mean-field theories for vortices in trapped Bose-Einstein condensates
We compute structures of vortex configurations in a harmonically trapped
Bose-Einstein condensed atom gas within three different gapless self-consistent
mean-field theories. Outside the vortex core region, the density profiles for
the condensate and the thermal gas are found to differ only by a few percent
between the Hartree-Fock-Bogoliubov-Popov theory and two of its recently
proposed gapless extensions. In the core region, however, the differences in
the density profiles are substantial. The structural differences are reflected
in the energies of the quasiparticle states localized near the vortex core.
Especially, the predictions for the energy of the lowest quasiparticle
excitation differ considerably between the theoretical models investigated.Comment: 4 pages, 2 figure
Internal Vortex Structure of a Trapped Spinor Bose-Einstein Condensate
The internal vortex structure of a trapped spin-1 Bose-Einstein condensate is
investigated. It is shown that it has a variety of configurations depending on,
in particular, the ratio of the relevant scattering lengths and the total
magnetization.Comment: replacement; minor grammatical corrections but with additional
figure
Salmonella contamination of pork carcasses : UK baseline culture-based data determined by sponge sampling during 2006
During 2006-7, microbiological baseline data on the frequency and distribution of Salmonella contamination of pig carcasses in UK slaughterhouses were collected. Data were generated from four separate abattoirs which were determined as having practices representative of the UK slaughter industry. Studies were designed to provide estimates of the prevalence and levels of Salmonella contamination of the UK pork industry
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