We report on the measurement of the spin-dipole (SD) polarizability and of the frequency of the SD oscillation of a two-component Bose-Einstein condensate of sodium atoms occupying the |3 2 S 1/2 , F = 1, mF = ±1 hyperfine states. This binary spin-mixture presents the important properties of being, at the same time, fully miscible and rid of the limit set by buoyancy. It is also characterized by a huge enhancement of the SD polarizability and by the consequent softening of the frequency of the SD oscillation, due to the vicinity to the transition to the immiscible phase. The experimental data are successfully compared with the predictions of theory.

The spin dynamics of a harmonically trapped Bose-Einstein condensed binary mixture of sodium atoms is experimentally investigated at finite temperature. In the collisional regime the motion of the thermal component is shown to be damped because of spin drag, while the two condensates exhibit a counterflow oscillation without friction, thereby providing direct evidence for spin superfluidity. Results are also reported in the collisionless regime where the spin components of both the condensate and thermal part oscillate without damping, their relative motion being driven by a mean-field effect. We also measure the static polarizability of the condensed and thermal parts and we find a large increase of the condensate polarizability with respect to the T=0 value, in agreement with the predictions of theory.

We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D1 optical transition (3s 2 S 1/2 → 3p 2 P 1/2 ) at 589.8 nm. The technique is applied to samples containing 3 × 10 9 atoms, previously cooled to 350 µK in a magneto-optical trap, and it leads to temperatures as low as 9 µK and phase-space densities in the range of 10 −4 . The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to 10 11 cm −3 .