We report the observation of the scissors mode of a Bose-Einstein condensed gas of ${}^{87}\mathrm{Rb}$ 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.

• Received 18 October 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.2056