Oscillations of rotating trapped Bose-Einstein condensates

The tensor-virial method is applied for a study of oscillation modes of uniformly rotating Bose-Einstein condensed gases, whose rigid body rotation is supported by an vortex array. The second order virial equations are derived in the hydrodynamic regime for an arbitrary external harmonic trapping potential assuming that the condensate is a superfluid at zero temperature. The axisymmetric equilibrium shape of the condensate is determined as a function of the deformation of the trap; its domain of stability is bounded by the constraint $\Omega<1$ on the rotation rate (measured in units of the trap frequency $\omega_0$.) The oscillations of the axisymmetric condensate are stable with respect to the transverse-shear, toroidal and quasi-radial modes of oscillations, corresponding to the $l= 2$, $| m| = 0,1,2$ surface deformations. In non-axisymmetric traps, the equilibrium constrains the (dimensionless) deformation in the plane orthogonal to the rotation to the domain $A_2 > \Omega^2$ with $\Omega< 1$. The second harmonic oscillation modes in non-axisymmetric traps separate into two classes which have even or odd parity with respect to the direction of the rotation axis. Numerical solutions show that these modes are stable in the parameter domain where equilibrium figures exist.