11,495 research outputs found
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
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
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
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
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
- …