554 research outputs found
Quantum-Monte-Carlo Calculations for Bosons in a Two-Dimensional Harmonic Trap
Path-Integral-Monte-Carlo simulation has been used to calculate the
properties of a two-dimensional (2D) interacting Bose system. The bosons
interact with hard-core potentials and are confined to a harmonic trap. Results
for the density profiles, the condensate fraction, and the superfluid density
are presented. By comparing with the ideal gas we easily observe the effects of
finite size and the depletion of the condensate because of interactions. The
system is known to have no phase transition to a Bose-Einstein condensation in
2D, but the finite system shows that a significant fraction of the particles
are in the lowest state at low temperatures.Comment: six pages, two figures; Contribution to QFS98; To be published in
Journ. Low. Temp. Phy
The Two-Dimensional Bose-Einstein Condensate
We study the Hartree-Fock-Bogoliubov mean-field theory as applied to a
two-dimensional finite trapped Bose gas at low temperatures and find that, in
the Hartree-Fock approximation, the system can be described either with or
without the presence of a condensate; this is true in the thermodynamic limit
as well. Of the two solutions, the one that includes a condensate has a lower
free energy at all temperatures. However, the Hartree-Fock scheme neglects the
presence of phonons within the system, and when we allow for the possibility of
phonons we are unable to find condensed solutions; the uncondensed solutions,
on the other hand, are valid also in the latter, more general scheme. Our
results confirm that low-energy phonons destabilize the two-dimensional
condensate.Comment: 8 pages, 3 figures, REVTeX 4. To appear in J. Low Temp. Phys.
Corrected a mistake in a calculation and changed the conclusions accordingl
Path-integral Monte Carlo and the squeezed trapped Bose-Einstein gas
Bose-Einstein condensation has been experimentally found to take place in
finite trapped systems when one of the confining frequencies is increased until
the gas becomes effectively two-dimensional (2D). We confirm the plausibility
of this result by performing path-integral Monte Carlo (PIMC) simulations of
trapped Bose gases of increasing anisotropy and comparing them to the
predictions of finite-temperature many-body theory. PIMC simulations provide an
essentially exact description of these systems; they yield the density profile
directly and provide two different estimates for the condensate fraction. For
the ideal gas, we find that the PIMC column density of the squeezed gas
corresponds quite accurately to that of the exact analytic solution and,
moreover, is well mimicked by the density of a 2D gas at the same temperature;
the two estimates for the condensate fraction bracket the exact result. For the
interacting case, we find 2D Hartree-Fock solutions whose density profiles
coincide quite well with the PIMC column densities and whose predictions for
the condensate fraction are again bracketed by the PIMC estimates.Comment: 2 pages, 3 figure
Theory of cooling by flow through narrow pores
We consider the possibility of adding a stage to a dilution refrigerator to
provide additional cooling by ``filtering out'' hot atoms. Three methods are
considered: 1) Effusion, where holes having diameters larger than a mean-free
path allow atoms to pass through easily; 2) Particle waveguide-like motion
using very narrow channels that greatly restrict the quantum states of the
atoms in a channel. 3) Wall-limited diffusion through channels, in which the
wall scattering is disordered so that local density equilibrium is established
in a channel. We assume that channel dimension are smaller than the mean-free
path for atom-atom interactions. The particle waveguide and the wall-limited
diffusion methods using channels on order of the de Broglie wavelength give
cooling. Recent advances in nano-filters give this method some hope of being
practical.Comment: 10 pages, 3 figures. Corrected typos and made some minor wording
change
Absence of Fragmentation in Two-Dimensional Bose-Einstein Condensation
We investigate the possibility that the BEC-like phenomena recently detected
on two-dimensional finite trapped systems consist of fragmented condensates. We
derive and diagonalize the one-body density matrix of a two-dimensional
isotropically trapped Bose gas at finite temperature. For the ideal gas, the
procedure reproduces the exact harmonic-oscillator eigenfunctions and the Bose
distribution. We use a new collocation-minimization method to study the
interacting gas in the Hartree-Fock approximation and obtain a ground-state
wavefunction and condensate fraction consistent with those obtained by other
methods. The populations of the next few eigenstates increase at the expense of
the ground state but continue to be negligible; this supports the conclusion
that two-dimensional BEC is into a single state.Comment: 6 pages, 1 figur
Home Ranges of Rat Snakes (Colubridae: Elaphe) in Different Habitats
Based on our findings, we suggest that rat snakes represent not only a major predator of kites, but also of other canopy and mid-story nesting species in the southeastern United States. For example, rat snakes are the most dominant snake nest predator of bird nests throughout the Southeast (DeGregorio et al. 2014) and are skilled tree climbers that often occupy arboreal habitats (Jackson 1976, Keller and Heske 2000, Sperry et al. 2009), particularly in bottomland forests (Mullin et al. 2000, Carfagno and Weatherhead 2009). Thus, the role of rat snakes as predators of nests above the understory is likely underappreciated because of the paucity of information on causes of failure among mid-story and canopy nest
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