1,204 research outputs found
A Dynamical Self-Consistent Finite Temperature Kinetic Theory: The ZNG Scheme
We review a self-consistent scheme for modelling trapped weakly-interacting
quantum gases at temperatures where the condensate coexists with a significant
thermal cloud. This method has been applied to atomic gases by Zaremba, Nikuni,
and Griffin, and is often referred to as ZNG. It describes both
mean-field-dominated and hydrodynamic regimes, except at very low temperatures
or in the regime of large fluctuations. Condensate dynamics are described by a
dissipative Gross-Pitaevskii equation (or the corresponding quantum
hydrodynamic equation with a source term), while the non-condensate evolution
is represented by a quantum Boltzmann equation, which additionally includes
collisional processes which transfer atoms between these two subsystems. In the
mean-field-dominated regime collisions are treated perturbatively and the full
distribution function is needed to describe the thermal cloud, while in the
hydrodynamic regime the system is parametrised in terms of a set of local
variables. Applications to finite temperature induced damping of collective
modes and vortices in the mean-field-dominated regime are presented.Comment: Unedited version of chapter to appear in Quantum Gases: Finite
Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms Series). N.P.
Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds. Imperial
College Press, London (in press). See
http://www.icpress.co.uk/physics/p817.htm
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.
The level of chemokine CXCL5 in the cerebrospinal fluid is increased during the first 24 hours of ischaemic stroke and correlates with the size of early brain damage
Inflammation is an important feature of the pathophysiological response to
ischaemic stroke. The ischaemic brain-invading leukocytes, neutrophils in particular,
contribute to the exacerbation of tissue injury in stroke. Chemokines are
a growing family of proteins performing chemotactic activity on selective leukocyte
subpopulations. Chemokines are broadly divided into two major subfamilies on
the basis of the arrangement of the two N-terminal cysteine residues, CXC and CC,
depending on whether the first two cysteine residues have an amino acid between
them (CXC) or are adjacent (CC). CXC chemokines possessing, close to the
N terminus, the amino acid sequence glutamic acid-leucine-arginine (ELR motif)
specifically act on neutrophils. CXCL5 is one of the ELR-expressing CXC chemokines
and is a potent neutrophil attractant and activator. The objective of the study
was to detect CXCL5 levels in the cerebrospinal fluid (CSF) and sera of stroke
patients and to investigate the relation between these levels and the volume of
brain computed tomography (CT) hypodense areas representing early ischaemic
lesions. A total of 23 ischaemic stroke patients were studied. CSF and blood sampling
and brain CT were performed within the first 24 hours of stroke. The control
group consisted of 15 patients with tension headache. CXCL5 levels were determined
by the ELISA method. CSF CXCL5 levels in stroke patients were significantly
higher in comparison with the control group (38.2 ± 18.4 pg/ml vs. 18.7 ± 8.2 pg/ml; p < 0.001). No significant differences in serum CXCL5 levels were
found between the stroke patients and the control group. CSF CXCL5 levels correlated
positively with the volume of early brain CT hypodense areas (p < 0.0001).
The results suggest that CXCL5 may play a role in the inflammatory reaction during
the early phase of ischaemic stroke
Transverse Magnetoresistance of GaAs/AlGaAs Heterojunctions in the Presence of Parallel Magnetic Fields
We have calculated the resistivity of a GaAs\slash AlGaAs heterojunction in
the presence of both an in--plane magnetic field and a weak perpendicular
component using a semiclassical Boltzmann transport theory. These calculations
take into account fully the distortion of the Fermi contour which is induced by
the parallel magnetic field. The scattering of electrons is assumed to be due
to remote ionized impurities. A positive magnetoresistance is found as a
function of the perpendicular component, in good qualitative agreement with
experimental observations. The main source of this effect is the strong
variation of the electronic scattering rate around the Fermi contour which is
associated with the variation in the mean distance of the electronic states
from the remote impurities. The magnitude of the positive magnetoresistance is
strongly correlated with the residual acceptor impurity density in the GaAs
layer. The carrier lifetime anisotropy also leads to an observable anisotropy
in the resistivity with respect to the angle between the current and the
direction of the in--plane magnetic field.Comment: uuencoded file containing a 26 page RevTex file and 14 postscript
figures. Submitted to Phys. Rev.
Evaluation of a semi-active gravity gradient system. Volume I - Technical summary
Semi-active gravity gradient system for attitude control of earth oriented spacecraf
Evaluation of a semi-active gravity gradient system. Volume II - Appendices
Evaluation of semi-active gravity gradient system - appendixe
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