3,862 research outputs found
Exploring the BEC-BCS Crossover with an Ultracold Gas of Li Atoms
We present an overview of our recent measurements on the crossover from a
Bose-Einstein condensate of molecules to a Bardeen-Cooper-Schrieffer
superfluid. The experiments are performed on a two-component spin-mixture of
Li atoms, where a Fesh\-bach resonance serves as the experimental key to
tune the s-wave scattering length and thus to explore the various interaction
regimes. In the BEC-BCS crossover, we have characterized the interaction energy
by measuring the size of the trapped gas, we have studied collective excitation
modes, and we have observed the pairing gap. Our observations provide strong
evidence for superfluidity in the strongly interacting Fermi gas.Comment: Proceedings of ICAP-2004 (Rio de Janeiro). Review on Innsbruck
BEC-BCS crossover experiments with updated Feshbach resonance positio
Finite-Temperature Collective Dynamics of a Fermi Gas in the BEC-BCS Crossover
We report on experimental studies on the collective behavior of a strongly
interacting Fermi gas with tunable interactions and variable temperature. A
scissors mode excitation in an elliptical trap is used to characterize the
dynamics of the quantum gas in terms of hydrodynamic or near-collisionless
behavior. We obtain a crossover phase diagram for collisional properties,
showing a large region where a non-superfluid strongly interacting gas shows
hydrodynamic behavior. In a narrow interaction regime on the BCS side of the
crossover, we find a novel temperature-dependent damping peak, suggesting a
relation to the superfluid phase transition
Observation of the Pairing Gap in a Strongly Interacting Fermi Gas
We study fermionic pairing in an ultracold two-component gas of Li atoms
by observing an energy gap in the radio-frequency excitation spectra. With
control of the two-body interactions via a Feshbach resonance we demonstrate
the dependence of the pairing gap on coupling strength, temperature, and Fermi
energy. The appearance of an energy gap with moderate evaporative cooling
suggests that our full evaporation brings the strongly interacting system deep
into a superfluid state.Comment: 18 pages, 3 figure
Dynamics of a strongly interacting Fermi gas: the radial quadrupole mode
We report on measurements of an elementary surface mode in an ultracold,
strongly interacting Fermi gas of 6Li atoms. The radial quadrupole mode allows
us to probe hydrodynamic behavior in the BEC-BCS crossover without being
influenced by changes in the equation of state. We examine frequency and
damping of this mode, along with its expansion dynamics. In the unitarity limit
and on the BEC side of the resonance, the observed frequencies agree with
standard hydrodynamic theory. However, on the BCS side of the crossover, a
striking down shift of the oscillation frequency is observed in the
hydrodynamic regime as a precursor to an abrupt transition to collisionless
behavior; this indicates coupling of the oscillation to fermionic pairs.Comment: 11 pages, 11 figures v2: minor change
Optimization of the cooling circuit and thermo-mechanical analysis for the extraction grid of ELISE
Precision Measurements of Collective Oscillations in the BEC-BCS Crossover
We report on precision measurements of the frequency of the radial
compression mode in a strongly interacting, optically trapped Fermi gas of Li-6
atoms. Our results allow for a test of theoretical predictions for the equation
of state in the BEC-BCS crossover. We confirm recent quantum Monte-Carlo
results and rule out simple mean-field BCS theory. Our results show the
long-sought beyond-mean-field effects in the strongly interacting BEC regime.Comment: improved discussion of small ellipticity and anharmonicity
correction
Collective oscillations of a Fermi gas in the unitarity limit: Temperature effects and the role of pair correlations
We present detailed measurements of the frequency and damping of three
different collective modes in an ultracold trapped Fermi gas of Li atoms
with resonantly tuned interactions. The measurements are carried out over a
wide range of temperatures. We focus on the unitarity limit, where the
scattering length is much greater than all other relevant length scales. The
results are compared to theoretical calculations that take into account Pauli
blocking and pair correlations in the normal state above the critical
temperature for superfluidity. We show that these two effects nearly compensate
each other and the behavior of the gas is close to the one of a classical gas.Comment: 8 pages, 5 figure
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