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 present the Bogoliubov theory for the Bose-Einstein condensation of Feshbach molecules in a balanced Fermi mixture. Because the Bogoliubov theory includes (Gaussian) fluctuations, we can in this manner accurately incorporate both the two-body and many-body aspects of the BEC-BCS crossover that occurs near a Feshbach resonance. We apply the theory in particular to the very broad Feshbach resonance in atomic
6Li at a magnetic field of
B0=834G and find good agreement with experiments in that case. The BEC-BCS crossover for more narrow Feshbach resonances is also discussed.
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 6Li atoms, where a Feshbach 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.
We observe characteristic atomic behaviors in the Bose–Einstein-condensation-Bardeen–Cooper–Schrieffer (BECBCS) crossover, by accurately tuning the magnetic field across the Feshbach resonance of lithium atoms. The magnetic field is calibrated by measuring the Zeeman shift of the optical transition. A non-monotonic anisotropic expansion is observed across the Feshbach resonance. The density distribution is explored in different interacting regimes, where a condensate of diatomic molecules forms in the BEC limit with the indication of a bimodal distribution. We also measure the three-body recombination atom loss in the BEC-BCS crossover, and find that the magnetic field of the maximum atom loss is in the BEC limit and gets closer to the Feshbach resonance when decreasing the atom temperature, which agrees with previous experiments and theoretical prediction. This work builds up a controllable platform for the study on the strongly interacting Fermi gas.
We systematically study the BCS-BEC crossover and the quantum phase transition in ultracold
6Li and
40K atoms across a wide Feshbach resonance. The background scattering lengths for
6Li and
40K have opposite signs, which lead to very different behaviors for these two types of atoms. For
40K, both the two-body and the many-body calculations show that the system always has two branches of solutions: one corresponds to a deeply bound molecule state; and the other, the one accessed by the current experiments, corresponds to a weakly bound state with population always dominantly in the open channel. For
6Li, there is only a unique solution with the standard crossover from the weakly bound Cooper pairs to the deeply bound molecules as one sweeps the magnetic field through the crossover region. Because of this difference, for the experimentally accessible state of
40K, there is a quantum phase transition at zero temperature from the superfluid to the normal Fermi gas at the positive detuning of the magnetic field where the
s-wave scattering length passes its zero point. For
6Li, however, the system changes continuously across the zero point of the scattering length. For both types of atoms, we also give a detailed comparison between the results from the two-channel and the single-channel model over the whole region of the magnetic field detuning.
Experiments on ultra-cold fermionic /sup 6/Li confined in a crossed dipole trap. /sup 6/Li possesses Feshbach resonances both in the s-wave and p-wave channels. This features enables the exploration of both s-wave and p-wave pairing mechanisms. We first discuss our observation of Bose-Einstein condensation of /sup 6/Li/sub 2/ molecules in the low-magnetic field wing the s-wave Feshbach located at /spl sim/ 834 G. The molecular BEC offers a good starting point for the exploration of the BEC-BCS (Bardeen-Cooper-Schrieffer) crossover. We have also located p-wave Feshbach resonances in the two lowest hyperfine states of /sup 6/Li. The experimental positions of these resonances are in good agreement with those predicted by coupled channel calculations. We have studied the atom losses close to these resonances and have developed a semi-analytical model capturing our experimental findings.