Scissors mode in a superfluid Fermi gas

We evaluate the frequencies of scissors modes for density and concentration fluctuations in a vapor of fermionic atoms placed in two hyperfine levels inside a spherical harmonic trap. Both the superfluid and normal states are considered, with inclusion of the interactions at the random-phase level. Two main results are obtained: (i) the transition to the superfluid state is signaled by the disappearance of soft transverse modes of the normal fluid in the collisionless regime; and (ii) the eigenfrequency of the density fluctuations in the superfluid coincides with that of the normal fluid in the collisional regime. The latter property is related to the opening of the gap in the single-pair spectrum.