984 research outputs found
Kinetic theory and dynamic structure factor of a condensate in the random phase approximation
We present the microscopic kinetic theory of a homogeneous dilute Bose
condensed gas in the generalized random phase approximation (GRPA), which
satisfies the following requirements: 1) the mass, momentum and energy
conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the
recovery of the Bogoliubov theory at zero temperature \cite{condenson}. In this
approach, the condensate influences the binary collisional process between the
two normal atoms, in the sense that their interaction force results from the
mediation of a Bogoliubov collective excitation traveling throughout the
condensate. Furthermore, as long as the Bose gas is stable, no collision
happens between condensed and normal atoms. In this paper, we show how the
kinetic theory in the GRPA allows to calculate the dynamic structure factor at
finite temperature and when the normal and superfluid are in a relative motion.
The obtained spectrum for this factor provides a prediction which, compared to
the experimental results, allows to validate the GRPA.
PACS numbers:03.75.Hh, 03.75.Kk, 05.30.-dComment: 6 pages, 1 figures, QFS2004 conferenc
Nickel and gtp modulate helicobacter pylori ureg structural flexibility
UreG is a P-loop GTP hydrolase involved in the maturation of nickel-containing urease, an essential enzyme found in plants, fungi, bacteria, and archaea. This protein couples the hydrolysis of GTP to the delivery of Ni(II) into the active site of apo-urease, interacting with other urease chaperones in a multi-protein complex necessary for enzyme activation. Whereas the conformation of Helicobacter pylori (Hp) UreG was solved by crystallography when it is in complex with two other chaperones, in solution the protein was found in a disordered and flexible form, defining it as an intrinsically disordered enzyme and indicating that the well-folded structure found in the crystal state does not fully reflect the behavior of the protein in solution. Here, isothermal titration calorimetry and site-directed spin labeling coupled to electron paramagnetic spectroscopy were successfully combined to investigate HpUreG structural dynamics in solution and the effect of Ni(II) and GTP on protein mobility. The results demonstrate that, although the protein maintains a flexible behavior in the metal and nucleotide bound forms, concomitant addition of Ni(II) and GTP exerts a structural change through the crosstalk of different protein regions
Finite temperature theory of the scissors mode in a Bose gas using the moment method
We use a generalized Gross-Pitaevskii equation for the condensate and a
semi-classical kinetic equation for the noncondensate atoms to discuss the
scissors mode in a trapped Bose-condensed gas at finite temperatures. Both
equations include the effect of collisions between the condensate and
noncondensate atoms. We solve the coupled moment equations describing
oscillations of the quadrupole moments of the condensate and noncondensate
components to find the collective mode frequencies and collisional damping
rates as a function of temperature. Our calculations extend those of
Gu\'ery-Odelin and Stringari at T=0 and in the normal phase. They complement
the numerical results of Jackson and Zaremba, although Landau damping is left
out of our approach. Our results are also used to calculate the quadrupole
response function, which is related to the moment of inertia. It is shown
explicitly that the moment of inertia of a trapped Bose gas at finite
temperatures involves a sum of an irrotational component from the condensate
and a rotational component from the thermal cloud atoms.Comment: 18 pages, 8 figure
Overcritical Rotation of a Trapped Bose-Einstein Condensate
The rotational motion of an interacting Bose-Einstein condensate confined by
a harmonic trap is investigated by solving the hydrodynamic equations of
superfluids, with the irrotationality constraint for the velocity field. We
point out the occurrence of an overcritical branch where the system can rotate
with angular velocity larger than the oscillator frequencies. We show that in
the case of isotropic trapping the system exhibits a bifurcation from an
axisymmetric to a triaxial configuration, as a consequence of the interatomic
forces. The dynamical stability of the rotational motion with respect to the
dipole and quadrupole oscillations is explicitly discussed.Comment: 6 pages, 3 postscript figure
Finite-temperature simulations of the scissors mode in Bose-Einstein condensed gases
The dynamics of a trapped Bose-condensed gas at finite temperatures is
described by a generalized Gross-Pitaevskii equation for the condensate order
parameter and a semi-classical kinetic equation for the thermal cloud, solved
using -body simulations. The two components are coupled by mean fields as
well as collisional processes that transfer atoms between the two. We use this
scheme to investigate scissors modes in anisotropic traps as a function of
temperature. Frequency shifts and damping rates of the condensate mode are
extracted, and are found to be in good agreement with recent experiments.Comment: 4 pages, 3 figure
Reframing Gender Justice in an Unequal, Volatile World: IDS’ Directions for Future Research on Gender and Sexuality in Development
At a time of major changes in global politics and trends, and major shifts in international development following the establishment of the global Sustainable Development Goals in Agenda 2030, the Gender and Sexuality cluster at the Institute of Development Studies engaged in collaborative discussions and
consultations over nine months with partner organisations, networks, external experts and policymakers. Following this process through most of 2016, this brief outlines our priority directions for future research on gender and sexuality in development in an increasingly unequal, polarised and volatile world.UK Department for International Developmen
Quantized vortices and collective oscillations of a trapped Bose condensed gas
Using a sum rule approach we calculate the frequency shifts of the quadrupole
oscillations of a harmonically trapped Bose gas due to the presence of a
quantized vortex. Analytic results are obtained for positive scattering lengths
and large N where the shift relative to excitations of opposite angular
momentum is found to be proportional to the quantum circulation of the vortex
and to decrease as N^{-2/5}. Results are also given for smaller values of N
covering the transition between the ideal gas and the Thomas-Fermi limit. For
negative scattering lengths we predict a macroscopic instability of the vortex.
The splitting of the collective frequencies in toroidal configurations is also
discussed.Comment: Rextex, 4 pages, 1 postscript figur
Phase diagram of quantized vortices in a trapped Bose-Einstein condensed gas
We investigate the thermodynamic stability of quantized vortices in a dilute
Bose gas confined by a rotating harmonic trap at finite temperature.
Interatomic forces play a crucial role in characterizing the resulting phase
diagram, especially in the large Thomas-Fermi regime. We show that the
critical temperature for the creation of stable vortices exhibits a maximum as
a function of the frequency of the rotating trap and that the corresponding
transition is associated with a discontinuity in the number of atoms in the
condensate. Possible strategies for approaching the vortical region are
discussed.Comment: Revtex, 4 pages, 2 figure
Detection of vorticity in Bose-Einstein condensed gases by matter-wave interference
A phase-slip in the fringes of an interference pattern is an unmistakable
characteristic of vorticity. We show dramatic two-dimensional simulations of
interference between expanding condensate clouds with and without vorticity. In
this way, vortices may be detected even when the core itself cannot be
resolved.Comment: 3 pages, RevTeX, plus 6 PostScript figure
Kelvin Modes of a fast rotating Bose-Einstein Condensate
Using the concept of diffused vorticity and the formalism of rotational
hydrodynamics we calculate the eigenmodes of a harmonically trapped
Bose-Einstein condensate containing an array of quantized vortices. We predict
the occurrence of a new branch of anomalous excitations, analogous to the
Kelvin modes of the single vortex dynamics. Special attention is devoted to the
excitation of the anomalous scissors mode.Comment: 7 pages, 3 figures, submitted to Phys. Rev.
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