An algebraic approach to the study of weakly excited states for a condensate in a ring geometry

We determine the low-energy spectrum and the eigenstates for a two-bosonic mode nonlinear model by applying the Inonu-Wigner contraction method to the Hamiltonian algebra. This model is known to well represent a Bose-Einstein condensate rotating in a thin torus endowed with two angular-momentum modes as well as a condensate in a double-well potential characterized by two space modes. We consider such a model in the presence of both an attractive and a repulsive boson interaction and investigate regimes corresponding to different values of the inter-mode tunnelling parameter. We show that the results ensuing from our approach are in many cases extremely satisfactory. To this end, we compare our results with the ground state obtained both numerically and within a standard semiclassical approximation based on su(2) coherent states.