Collective oscillations in two-dimensional Bose-Einstein condensate

We study the effect of lower-dimensional geometry on the frequencies of the collective oscillations of a Bose-Einstein condensate confined in a trap. To study the effect of two-dimensional geometry we consider a pancake-shaped condensate confined in a harmonic trap and employ various models for the coupling constant depending on the thickness of the condensate relative to the value of the scattering length. These models correspond to different scattering regimes ranging from quasi-three-dimensional to strictly two-dimensional regimes. Using these models for the coupling parameter and sum rule approach of the many-body response theory we derive analytical expressions for the frequencies of the monopole and the quadrupole modes. We show that the frequencies of monopole mode of the collective oscillations are significantly altered by the reduced dimensionality and also study the evolution of the frequencies as the system makes transition from one regime to another. (C) 2004 Elsevier B.V. All rights reserved.