The scissors mode and superfluidity of a Bose-Einstein condensed gas

This thesis deals with the fundamental relation between Bose-Einstein condensation and superfluidity. In particular an extensive study of the scissors mode of excitation in a Bose-Einstein condensed gas of 87Rb atoms is presented. The complex apparatus constructed for the condensate production and the pathway that leads to the quantum degeneracy through laser cooling, magnetic trapping and evaporative cooling of the atomic sample are described. Temperatures as low as 300 nK are achieved. At these temperatures about 2x104 atoms condense in the ground state of the magnetic trap, realizing the phenomenon predicted by A. Einstein in 1925. The scissors mode is excited in our experiment by a sudden rotation of the trap through a small angle. Its observation shows that a condensate is irrotational and as such a superfluid. Above the transition temperature the scissors oscillation occurs at two frequencies. The high-lying frequency corresponds to an irrotational velocity flow that is the classical counterpart of the superfluid oscillation. The low-lying one is instead related to rotational flow. Its absence and the occurrence of the single frequency oscillation for the condensate are the key features that indicate superfluidity. A systematic experimental study of the temperature dependent damping and frequency shifts of the mode is presented and compared with available theoretical predictions, finding a good agreement. This enabled the investigation of the relative motion of the superfluid through the normal fluid. The frequency shift of the mode as a function of temperature is consistent with a quenching of the moment of inertia as the temperature is reduced because of superfluidity. The fundamental relation between the scissors mode and the moment of inertia is also pointed out for partially condensed clouds in the context of linear response theory. In a semiclassical approximation the moment of inertia is directly derived from our scissors mode data. (author)