Shape deformations and angular momentum transfer in trapped   Bose-Einstein condensates
  Angular momentum can be transferred to a trapped Bose-Einstein condensate by distorting its shape with an external rotating field, provided the rotational frequency is larger than a critical frequency fixed by the energy and angular momentum of the excited states of the system. By using the Gross-Pitaevskii equation and sum rules, we explore the dependence of such a critical frequency on the multipolarity of the excitations and the asymmetry of the confining potential. We also discuss its possible relevance for vortex nucleation in rotating traps. 
F. Dalfovo (Univ. Catt. Brescia) and S. Stringari (Univ. Trento)