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

Feshbach resonances in the 6Li-40K Fermi-Fermi mixture: Elastic versus inelastic interactions

We present a detailed theoretical and experimental study of Feshbach resonances in the 6Li-40K mixture. Particular attention is given to the inelastic scattering properties, which have not been considered before. As an important example, we thoroughly investigate both elastic and inelastic scattering properties of a resonance that occurs near 155 G. Our theoretical predictions based on a coupled channels calculation are found in excellent agreement with the experimental results. We also present theoretical results on the molecular state that underlies the 155G resonance, in particular concerning its lifetime against spontaneous dissociation. We then present a survey of resonances in the system, fully characterizing the corresponding elastic and inelastic scattering properties. This provides the essential information to identify optimum resonances for applications relying on interaction control in this Fermi-Fermi mixture.Comment: Submitted to EPJD, EuroQUAM special issues "Cold Quantum Matter - Achievements and Prospects", v2 with updated calibration of magnetic field (+4mG correction) and updated figures 4 and

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

Two-phonon 1- state in 112Sn observed in resonant photon scattering

Results of a photon scattering experiment on 112Sn using bremsstrahlung with an endpoint energy of E_0 = 3.8 MeV are reported. A J = 1 state at E_x = 3434(1) keV has been excited. Its decay width into the ground state amounts to Gamma_0 = 151(17) meV, making it a candidate for a [2+ x 3-]1- two-phonon state. The results for 112Sn are compared with quasiparticle-phonon model calculations as well as the systematics of the lowest-lying 1- states established in other even-mass tin isotopes. Contrary to findings in the heavier stable even-mass Sn isotopes, no 2+ states between 2 and 3.5 MeV excitation energy have been detected in the present experiment.Comment: 10 pages, including 2 figures, Phys. Rev. C, in pres

Pairing-gap, pseudo-gap, and no-gap phases in the radio-frequency spectra of a trapped unitary 6Li gas

Radio frequency spectra of a trapped unitary 6Li gas are reported and analyzed in terms of a theoretical approach that includes both final-state and trap effects. Final-state effects play a crucial role in evidencing two main peaks both above and below the critical temperature Tc as being associated with two distinct phases that reside in different trap regions. These are the pairing-gap and pseudo-gap phases below Tc, which evolve into the pseudo-gap and no-gap phases above Tc. In this way, a long standing puzzle about the interpretation of rf spectra for 6Li in a trap is solved.Comment: 5 pages, 6 figures (final version