We report Bose-Einstein condensation of weakly bound 6 Li2 molecules in a crossed optical trap near a Feshbach resonance. We measure a molecule-molecule scattering length of 170 +100−60 nm at 770 G, in good agreement with theory. We study the 2D expansion of the cloud and show deviation from hydrodynamic behavior in the BEC-BCS crossover region.PACS numbers: 03.75. Ss, 05.30.Fk, 32.80.Pj, By applying a magnetic field to a gas of ultra-cold atoms, it is possible to tune the strength and the sign of the effective interaction between particles. This phenomenon, known as Feshbach resonance, offers in the case of fermions the unique possibility to study the crossover between situations governed by Bose-Einstein and FermiDirac statistics. Indeed, when the scattering length a characterizing the 2-body interaction at low temperature is positive, the atoms are known to pair in a bound molecular state. When the temperature is low enough, these bosonic dimers can form a Bose-Einstein condensate (BEC) as observed very recently in 40 K [1] and 6 Li [2,3]. On the side of the resonance where a is negative, one expects the well known Bardeen-Cooper-Schrieffer (BCS) model for superconductivity to be valid. However, this simple picture of a BEC phase on one side of the resonance and a BCS phase on the other is valid only for small atom density n. When n|a| 3 > ∼ 1 the system enters a strongly interacting regime that represents a challenge for many-body theories [4,5,6] and that now begins to be accessible to experiments [7,8,9].In this letter, we report on Bose-Einstein condensation of 6 Li dimers in a crossed optical dipole trap and a study of the BEC-BCS crossover region. Unlike all previous observations of molecular BEC made in single beam dipole traps with very elongated geometries, our condensates are formed in nearly isotropic traps. Analyzing free expansions of pure condensates with up to 4×10 4 molecules, we measure the molecule-molecule scattering length a m = 170 +100 −60 nm at a magnetic field of 770 gauss. This measurement is in good agreement with the value deduced from the resonance position [9] and the relation a m = 0.6 a of ref. [10]. Combined with tight confinement, these large scattering lengths lead to a regime of strong interactions where the chemical potential µ is on the order of k B T C where T C ≃ 1.5 µK is the condensation temperature. As a consequence, we find an important modification of the thermal cloud time of flight expansion induced by the large condensate mean field. Moreover, the gas parameter n m a 3 m is no longer small but on the order of 0.3. In this regime, the validity of mean field theory becomes questionable [11,12,13]. We show, in particular, that the anisotropy and gas energy released during expansion varies monotonically across the Feshbach resonance.Our experimental setup has been described previously [14,15]. A gas of 6 Li atoms is prepared in the absolute ground state |1/2, 1/2 in a Nd-YAG crossed beam optical dipole trap. The horizontal beam (resp. vertical) propagates along x (y)...

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.