Observation of the scissors mode and evidence for superfluidity of a trapped Bose-Einstein condensed gas

We report the observation of the scissors mode of a Bose-Einstein condensed gas of Rb-87 atoms in a magnetic trap, which gives direct evidence of superfluidity in this system. The scissors mode of oscillation is excited by a sudden rotation of the anisotropic trapping potential. For a gas above T-c (normal fluid) we detect the occurrence of oscillations at two frequencies, with the lower frequency corresponding to the rigid body value of the moment of inertia. Well below T-c the condensate oscillates at a single frequency, without damping, as expected for a superfluid.