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arXiv:cond-mat/0303079 · Physical Review Letters

Measurement of the Interaction Energy near a Feshbach Resonance in a Li6 Fermi Gas

Thomas Bourdel, J. Cubizolles, Lev Khaykovich, K. M. F. Magalhães, S. J. J. M. F. Kokkelmans, Georgy V. Shlyapnikov, Ch Salomon

Published Jul 11, 2003• 339 citations
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Abstract

We investigate the strongly interacting regime in an optically trapped 6Li Fermi mixture near a Feshbach resonance. The resonance is found at 800(40) G in good agreement with theory. Anisotropic expansion of the gas is interpreted by collisional hydrodynamics. We observe an unexpected and large shift (80 G) between the resonance peak and both the maximum of atom loss and the change of sign of the interaction energy.

DOI
10.1103/physrevlett.91.020402

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We investigate the strongly interacting regime in an optically trapped 6Li Fermi mixture near a Feshbach resonance. The resonance is found at 800(40) G in good agreement with theory. Anisotropic expansion of the gas is interpreted by collisional hydrodynamics. We observe an unexpected and large shift (80 G) between the resonance peak and both the maximum of atom loss and the change of sign of the interaction energy.
We report on experiments in 6Li Fermi gases near Feshbach resonances. A broad s‐wave resonance is used to form a Bose‐Einstein condensate of weakly bound 6Li2 molecules in a crossed optical trap. The measured molecule‐molecule scattering length of 170−60+100 nm at 770 G is found in good agreement with theory. The expansion energy of the cloud in the BEC‐BCS crossover region is measured. Finally we discuss the properties of p‐wave Feshbach resonances observed near 200 Gauss and new s‐wave resonances in the heteronuclear 6Li‐ 7Li mixture.
We report on the observation of six large Feshbach resonances in a Fermi-Bose mixture of 6Li^{6}\mathrm{Li}6Li and 85Rb^{85}\mathrm{Rb}85Rb atoms. Near the resonances, both the elastic and inelastic collision rates are enhanced, and this results primarily in the loss of 6Li^{6}\mathrm{Li}6Li from an optical dipole trap since it is less massive and less confined than its 85Rb^{85}\mathrm{Rb}85Rb collision partner. We interpret our experimental data using a coupled-channels calculation which fully characterizes the ground-state scattering properties in any combination of spin states.
We observe collective oscillations of a trapped, degenerate Fermi gas of 6Li atoms at a magnetic field just above a Feshbach resonance, where the two-body physics does not support a bound state. The gas exhibits a radial breathing mode at a frequency of 2837(05) Hz, in excellent agreement with the frequency of nu(H) identical with sqrt[10nu(x)nu(y)/3]=2830(20) Hz predicted for a hydrodynamic Fermi gas with unitarity-limited interactions. The measured damping times and frequencies are inconsistent with predictions for both the collisionless mean field regime and for collisional hydrodynamics. These observations provide the first evidence for superfluid hydrodynamics in a resonantly interacting Fermi gas.
In summary, we use all-optical methods with evaporative cooling near a Feshbach resonance to produce a strongly interacting degenerate Fermi gas. We observe hydrodynamic behavior in the expansion dynamics. At low temperatures, collisions may not explain the expansion dynamics. We observe hydrodynamics in the trapped gas. Our observations include collisionally-damped excitation spectra at high temperature which were not discussed above. In addition, we observe weakly damped breathing modes at low temperature. The observed temperature dependence of the damping time and hydrodynamic frequency are not consistent with collisional dynamics nor with collisionless mean field interactions. These observations constitute the first evidence for superfluid hydrodynamics in a Fermi gas.
We report on the observation of a highly degenerate, strongly interacting Fermi gas of atoms. Fermionic lithium-6 atoms in an optical trap are evaporatively cooled to degeneracy using a magnetic field to induce strong, resonant interactions. Upon abruptly releasing the cloud from the trap, the gas is observed to expand rapidly in the transverse direction while remaining nearly stationary in the axial direction. We interpret the expansion dynamics in terms of collisionless superfluid and collisional hydrodynamics. For the data taken at the longest evaporation times, we find that collisional hydrodynamics does not provide a satisfactory explanation, whereas superfluidity is plausible.
We measure the magnetic-field dependence of the frequency and damping time for the radial breathing mode of an optically trapped Fermi gas of 6Li^{6}\mathrm{Li}6Li atoms near a Feshbach resonance. The measurements address the apparent discrepancy between the results of Kinast et al. [Phys. Rev. Lett. 92, 150402 (2004)] and those of Bartenstein et al. [Phys. Rev. Lett. 92, 203201 (2004)]. Over the range of magnetic field from 770to910G770\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}910\phantom{\rule{0.3em}{0ex}}\mathrm{G}770to910G, the measurements confirm the results of Kinast et al. Close to resonance, the measured frequencies are in excellent agreement with predictions for a unitary hydrodynamic gas. At a field of 925G925\phantom{\rule{0.3em}{0ex}}\mathrm{G}925G, the measured frequency begins to decrease below predictions. For fields near 1080G1080\phantom{\rule{0.3em}{0ex}}\mathrm{G}1080G, we observe a breakdown of hydrodynamic behavior, which is manifested by a sharp increase in frequency and damping rate. The observed breakdown is in qualitative agreement with the sharp transition observed by Bartenstein et al. at 910G910\phantom{\rule{0.3em}{0ex}}\mathrm{G}910G.