Exploring the BEC-BCS Crossover with an Ultracold Gas of 6^6Li 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 $^6$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

Observation of the Pairing Gap in a Strongly Interacting Fermi Gas

We study fermionic pairing in an ultracold two-component gas of $^6$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

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

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

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 $^6$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