10,229 research outputs found
Dynamical Hartree-Fock-Bogoliubov Theory of Vortices in Bose-Einstein Condensates at Finite Temperature
We present a method utilizing the continuity equation for the condensate
density to make predictions of the precessional frequency of single off-axis
vortices and of vortex arrays in Bose-Einstein condensates at finite
temperature. We also present an orthogonalized Hartree-Fock-Bogoliubov (HFB)
formalism. We solve the continuity equation for the condensate density
self-consistently with the orthogonalized HFB equations, and find stationary
solutions in the frame rotating at this frequency. As an example of the utility
of this formalism we obtain time-independent solutions for
quasi-two-dimensional rotating systems in the co-rotating frame. We compare
these results with time-dependent predictions where we simulate stirring of the
condensate.Comment: 13 pages, 11 figures, 1 tabl
Modeling the buckling and delamination of thin films
I study numerically the problem of delamination of a thin film elastically
attached to a rigid substrate. A nominally flat elastic thin film is modeled
using a two-dimensional triangular mesh. Both compression and bending
rigidities are included to simulate compression and bending of the film. The
film can buckle (i.e., abandon its flat configuration) when enough compressive
strain is applied. The possible buckled configurations of a piece of film with
stripe geometry are investigated as a function of the compressive strain. It is
found that the stable configuration depends strongly on the applied strain and
the Poisson ratio of the film. Next, the film is considered to be attached to a
rigid substrate by springs that can break when the detaching force exceeds a
threshold value, producing the partial delamination of the film. Delamination
is induced by a mismatch of the relaxed configurations of film and substrate.
The morphology of the delaminated film can be followed and compared with
available experimental results as a function of model parameters.
`Telephone-cord', polygonal, and `brain-like' patterns qualitatively similar to
experimentally observed configurations are obtained in different parameter
regions. The main control parameters that select the different patterns are the
mismatch between film and substrate and the degree of in-plane relaxation
within the unbuckled regions.Comment: 8 pages, 10 figure
Quantifying Finite Temperature Effects in Atom Chip Interferometry of Bose-Einstein Condensates
We quantify the effect of phase fluctuations on atom chip interferometry of
Bose-Einstein condensates. At very low temperatures, we observe small phase
fluctuations, created by mean-field depletion, and a resonant production of
vortices when the two clouds are initially in anti-phase. At higher
temperatures, we show that the thermal occupation of Bogoliubov modes makes
vortex production vary smoothly with the initial relative phase difference
between the two atom clouds. We also propose a technique to observe vortex
formation directly by creating a weak link between the two clouds. The position
and direction of circulation of the vortices is subsequently revealed by kinks
in the interference fringes produced when the two clouds expand into one
another. This procedure may be exploited for precise force measurement or
motion detection.Comment: 7 pages, 5 figure
A Gender Comparison of Economists' Publications
An ordered probit model is used to examine the impact of gender and the quality of the PhD g ranting in stitution o n the publication r ecord of m ale an d f emale economists who received t heir doctorate i n 1985. This analysis indicates th at men an d women have different publ ication pa tterns regardless of where t hey r eceived t heir P hD and t hat the quality of t he P hD granting institution has n o m easurable effect o n an individual's publication record.gender, ordered probit model, journal publications
Effects of temperature upon the collapse of a Bose-Einstein condensate in a gas with attractive interactions
We present a study of the effects of temperature upon the excitation
frequencies of a Bose-Einstein condensate formed within a dilute gas with a
weak attractive effective interaction between the atoms. We use the
self-consistent Hartree-Fock Bogoliubov treatment within the Popov
approximation and compare our results to previous zero temperature and
Hartree-Fock calculations The metastability of the condensate is monitored by
means of the excitation frequency. As the number of atoms in the
condensate is increased, with held constant, this frequency goes to zero,
signalling a phase transition to a dense collapsed state. The critical number
for collapse is found to decrease as a function of temperature, the rate of
decrease being greater than that obtained in previous Hartree-Fock
calculations.Comment: 4 pages LaTeX, 3 eps figures. To appear as a letter in J. Phys.
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The impact of low energy proton damage on the operational characteristics of EPIC-MOS CCDs
The University of TĂŒbingen 3.5 MeV Van de Graaf accelerator facility was used to investigate the effect of low energy protons on the performance of the European Photon Imaging Camera (EPIC), metalâoxide semiconductor (MOS), charge coupled devices (CCDs). Two CCDs were irradiated in different parts of their detecting areas using different proton spectra and dose rates. Iron-55 was the calibration source in all cases and was used to measure any increases in charge transfer inefficiency (CTI) and spectral resolution of the CCDs. Additional changes in the CCD bright pixel table and changes in the low X-ray energy response of the device were examined.
The Monte Carlo code Stopping Range of Ions in Matter (SRIM) was used to model the effect of a 10 MeV equivalent fluence of protons interacting with the CCD. Since the non-ionising energy loss (NIEL) function could not be applied effectively at such low proton energies. From the 10 MeV values, the expected CTI degradation could be calculated and then compared to the measured CTI changes
Dissipation in nanocrystalline-diamond nanomechanical resonators
We have measured the dissipation and frequency of nanocrystalline-diamond nanomechanical resonators with resonant frequencies between 13.7 MHz and 157.3 MHz, over a temperature range of 1.4â274 K. Using both magnetomotive network analysis and a time-domain ring-down technique, we have found the dissipation in this material to have a temperature dependence roughly following T^(0.2), with Q^(â1) â 10^(â4) at low temperatures. The frequency dependence of a large dissipation feature at ~35â55 K is consistent with thermal activation over a 0.02 eV barrier with an attempt frequency of 10 GHz
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