8,168 research outputs found
Critical Temperature of a Trapped Bose Gas: Mean-Field Theory and Fluctuations
We investigate the possibilities of distinguishing the mean-field and
fluctuation effects on the critical temperature of a trapped Bose gas with
repulsive interatomic interactions. Since in a direct measurement of the
critical temperature as a function of the number of trapped atoms these effects
are small compared to the ideal gas results, we propose to observe
Bose-Einstein condensation by adiabatically ramping down the trapping
frequency. Moreover, analyzing this adiabatic cooling scheme, we show that
fluctuation effects can lead to the formation of a Bose condensate at
frequencies which are much larger than those predicted by the mean-field
theory.Comment: 4 pages of ReVTeX and 3 figures. Submitted to Physical Review
Scaling laws for the movement of people between locations in a large city
Large scale simulations of the movements of people in a ``virtual'' city and
their analyses are used to generate new insights into understanding the dynamic
processes that depend on the interactions between people. Models, based on
these interactions, can be used in optimizing traffic flow, slowing the spread
of infectious diseases or predicting the change in cell phone usage in a
disaster. We analyzed cumulative and aggregated data generated from the
simulated movements of 1.6 million individuals in a computer (pseudo
agent-based) model during a typical day in Portland, Oregon. This city is
mapped into a graph with nodes representing physical locations such
as buildings. Connecting edges model individual's flow between nodes. Edge
weights are constructed from the daily traffic of individuals moving between
locations. The number of edges leaving a node (out-degree), the edge weights
(out-traffic), and the edge-weights per location (total out-traffic) are fitted
well by power law distributions. The power law distributions also fit subgraphs
based on work, school, and social/recreational activities. The resulting
weighted graph is a ``small world'' and has scaling laws consistent with an
underlying hierarchical structure. We also explore the time evolution of the
largest connected component and the distribution of the component sizes. We
observe a strong linear correlation between the out-degree and total
out-traffic distributions and significant levels of clustering. We discuss how
these network features can be used to characterize social networks and their
relationship to dynamic processes.Comment: 18 pages, 10 figure
Negative Ion Drift Velocities In Mixtures Of Methane And Sulphur Hexafluoride
The velocities of negative ions drifting in mixtures of methane and Sulphur hexafluoride have been measured. Measurements were made as a function of gas composition, reduced electric field strength between 3 and 185 Td, and total gas pressure in the range 100-600 Torr at 300K. The data indicate that SF 6- (SF6) ions are the dominant ion in all of the gas mixtures studied. Extrapolation of the data using Blanc\u27s Law was used to determine the velocity of Sulphur hexafluoride ions drifting in pure methane
Production Efficiency of Ultracold Feshbach Molecules in Bosonic and Fermionic Systems
We investigate the production efficiency of ultracold molecules in bosonic
Rb and fermionic K when the magnetic field is swept across a
Feshbach resonance. For adiabatic sweeps of the magnetic field, the conversion
efficiency of each species is solely determined by the phase space density of
the atomic cloud, in contrast to a number of theoretical predictions. Our novel
model for the adiabatic pairing process, developed from general physical
principles, accurately predicts the conversion efficiency for {\it both}
ultracold gases of bosons and of fermions. In the non-adiabatic regime our
measurements of the Rb molecule conversion efficiency follow a Landau
Zener model, with a conversion efficiency that is characterized by the density
divided by the time derivative of the magnetic field.Comment: 5 pages, 3 figure
Normal-superfluid interaction dynamics in a spinor Bose gas
Coherent behavior of spinor Bose-Einstein condensates is studied in the
presence of a significant uncondensed (normal) component. Normal-superfluid
exchange scattering leads to a near-perfect local alignment between the spin
fields of the two components. Through this spin locking, spin-domain formation
in the condensate is vastly accelerated as the spin populations in the
condensate are entrained by large-amplitude spin waves in the normal component.
We present data evincing the normal-superfluid spin dynamics in this regime of
complicated interdependent behavior.Comment: 5 pages, 4 fig
Electron-ion Recombination In Gas Mixtures Of Helium, Nitrogen, And Carbon Dioxide
A study has been conducted to determine the electron-ion recombination rates in gas mixtures of helium, nitrogen, and carbon dioxide. Measurements were made as functions of electric field strength, gas pressure, and gas composition. In gas mixtures containing only nitrogen and carbon dioxide, the rates were dependent on pressure. This dependence is interpreted as being due to the separate effects of two- and three-body recombination processes. No pressure dependence was observed for mixtures containing helium. In such mixtures, two-body recombination appears to be dominant. Moreover, the addition of relatively low concentrations of helium to the discharge leads to a dramatic change in the bulk recombination rate. Two possible explanations for this observation are suggested. © 1984 American Institute of Physics
Output coupling of a Bose-Einstein condensate formed in a TOP trap
Two distinct mechanisms are investigated for transferring a pure 87Rb
Bose-Einstein condensate in the F = 2, mF = 2 state into a mixture of
condensates in all the mF states within the F = 2 manifold. Some of these
condensates remain trapped whilst others are output coupled in the form of an
elementary pulsed atom laser. Here we present details of the condensate
preparation and results of the two condensate output coupling schemes. The
first scheme is a radio frequency technique which allows controllable transfer
into available mF states, and the second makes use of Majorana spin flips to
equally populate all the manifold sub-states.Comment: 12 Pages, 5 Figures, submitted to J. Phys.
Recent Experiments with Bose-Condensed Gases at JILA
We consider a binary mixture of two overlapping Bose-Einstein condensates in
two different hyperfine states of \Rb87 with nearly identical magnetic moments.
Such a system has been simply realized through application of radiofrequency
and microwave radiation which drives a two-photon transition between the two
states. The nearly identical magnetic moments afford a high degree of spatial
overlap, permitting a variety of new experiments. We discuss some of the
conditions under which the magnetic moments are identical, with particular
emphasis placed on the requirements for a time-averaged orbiting potential
(TOP) magnetic trap.Comment: 9 pages, 5 figures; corrected post-publication editio
Modeling the iron oxides and oxyhydroxides for the prediction of environmentally sensitive phase transformations
Iron oxides and oxyhydroxides are challenging to model computationally as
competing phases may differ in formation energies by only several kJ/mol, they
undergo magnetization transitions with temperature, their structures may
contain partially occupied sites or long-range ordering of vacancies, and some
loose structures require proper description of weak interactions such as
hydrogen bonding and dispersive forces. If structures and transformations are
to be reliably predicted under different chemical conditions, each of these
challenges must be overcome simultaneously, while preserving a high level of
numerical accuracy and physical sophistication. Here we present comparative
studies of structure, magnetization, and elasticity properties of iron oxides
and oxyhydroxides using density functional theory calculations with plane-wave
and locally-confined-atomic-orbital basis sets, which are implemented in VASP
and SIESTA packages, respectively. We have selected hematite, maghemite,
goethite, lepidocrocite, and magnetite as model systems from a total of 13
known iron oxides and oxyhydroxides; and use same convergence criteria and
almost equivalent settings in order to make consistent comparisons. Our results
show both basis sets can reproduce the energetic stability and magnetic
ordering, and are in agreement with experimental observations. There are
advantages to choosing one basis set over the other, depending on the intended
focus. In our case, we find the method using PW basis set most appropriate, and
combine our results to construct the first phase diagram of iron oxides and
oxyhydroxides in the space of competing chemical potentials, generated entirely
from first principlesComment: 46 pages - Accepted for publication in PRB (19 journal pages),
January 201
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