12,911 research outputs found
Influence of Cooper pairing on the inelastic processes in a gas of Fermi atoms
Correlation properties in ultracold Fermi gas with negative scattering length
and its impact on the three-body recombination is analyzed. We find that Cooper
pairing enhances the recombination rate in contrast to the decrease of this
rate accompanying Bose-Einstein condensation in a Bose gas. This trend is
characteristic for all interval of temperatures T<Tc
Motion of a condensate in a shaken and vibrating harmonic trap
The dynamics of a Bose-Einstein condensate (BEC) in a time-dependent harmonic
trapping potential is determined for arbitrary variations of the position of
the center of the trap and its frequencies. The dynamics of the BEC wavepacket
is soliton-like. The motion of the center of the wavepacket, and the spatially
and temporally dependent phase (which affects the coherence properties of the
BEC) multiplying the soliton-like part of the wavepacket, are analytically
determined.Comment: Accepted for publication in J. Phys. B: At Mol Opt Phy
Expansion of an interacting Fermi gas
We study the expansion of a dilute ultracold sample of fermions initially
trapped in a anisotropic harmonic trap. The expansion of the cloud provides
valuable information about the state of the system and the role of
interactions. In particular the time evolution of the deformation of the
expanding cloud behaves quite differently depending on whether the system is in
the normal or in the superfluid phase. For the superfluid phase, we predict an
inversion of the deformation of the sample, similarly to what happens with
Bose-Einstein condensates. Viceversa, in the normal phase, the inversion of the
aspect ratio is never achieved, if the mean field interaction is attractive and
collisions are negligible.Comment: 4 pages, 3 figures, final versio
Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal Fixed Point
Critical dynamics of an ultracold Bose gas far from equilibrium is studied in
two spatial dimensions. Superfluid turbulence is created by quenching the
equilibrium state close to zero temperature. Instead of immediately
re-thermalizing, the system approaches a meta-stable transient state,
characterized as a non-thermal fixed point. A focus is set on the vortex
density and vortex-antivortex correlations which characterize the evolution
towards the non-thermal fixed point and the departure to final
(quasi-)condensation. Two distinct power-law regimes in the vortex-density
decay are found and discussed in terms of a vortex binding-unbinding transition
and a kinetic description of vortex scattering. A possible relation to decaying
turbulence in classical fluids is pointed out. By comparing the results to
equilibrium studies of a two-dimensional Bose gas, an intuitive understanding
of the location of the non-thermal fixed point in a reduced phase space is
developed.Comment: 11 pages, 13 figures; PRA versio
Community, work and family and the metamorphosis of social change
On the ten year anniversary of this biannual conference we revisit our hopes/aims and expectations when Community, Work and Family was first developed, as a journal and later as a conferences. We reflect on where we are now; and to look forwards to the challenges posed by current and future contexts. We note a number of gathering storms – global social trends which can be described as crises and form the contemporary context for communities, workplaces and families and their interface. These include: economic crisis; demographic crisis including population mobility; gender crisis; crisis of care and crisis of violence
Measurement of positive and negative scattering lengths in a Fermi gas of atoms
An exotic superfluid phase has been predicted for an ultracold gas of
fermionic atoms. This phase requires strong attractive interactions in the gas,
or correspondingly atoms with a large, negative s-wave scattering length. Here
we report on progress toward realizing this predicted superfluid phase. We
present measurements of both large positive and large negative scattering
lengths in a quantum degenerate Fermi gas of atoms. Starting with a
two-component gas that has been evaporatively cooled to quantum degeneracy, we
create controllable, strong interactions between the atoms using a
magnetic-field Feshbach resonance. We then employ a novel rf spectroscopy
technique to directly measure the mean-field interaction energy, which is
proportional to the s-wave scattering length. Near the peak of the resonance we
observe a saturation of the interaction energy; it is in this strongly
interacting regime that superfluidity is predicted to occur. We have also
observed anisotropic expansion of the gas, which has recently been suggested as
a signature of superfluidity. However, we find that this can be attributed to a
purely collisional effect
Non-exponential one-body loss in a Bose-Einstein condensate
We have studied the decay of a Bose-Einstein condensate of metastable helium
atoms in an optical dipole trap. In the regime where two- and three-body losses
can be neglected we show that the Bose-Einstein condensate and the thermal
cloud show fundamentally different decay characteristics. The total number of
atoms decays exponentially with time constant tau; however, the thermal cloud
decays exponentially with time constant (4/3)tau and the condensate decays much
faster, and non-exponentially. We show that this behaviour, which should be
present for all BECs in thermal equilibrium with a considerable thermal
fraction, is due to a transfer of atoms from the condensate to the thermal
cloud during its decay.Comment: The intuitive explanation of the atomic transfer effect has been
correcte
Superconductivity from repulsive interactions in the two dimensional electron gas
We present a well-controlled perturbative renormalization group (RG)
treatment of superconductivity from short-ranged repulsive interactions in a
variety of model two dimensional electronic systems. Our analysis applies in
the limit where the repulsive interactions between the electrons are small
compared to their kinetic energy.Comment: 10 pages 3 figure
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