Momentum transfer to and elementary excitations of a Bose-Einstein condensate by a time-dependent optical potential

We present results of calculations on Bose-Einstein-condensed [Formula Presented] atoms subjected to a moving standing-wave light potential of the form [Formula Presented] We calculate the mean-field dynamics (the order parameter) of the condensate and determine the resulting condensate momentum in the z direction, [Formula Presented] where [Formula Presented] is the peak optical potential strength and [Formula Presented] is the pulse duration. Although the local-density approximation for the Bogoliubov excitation spectral distribution is a good approximation for very low optical intensities, long pulse duration and sufficiently large values of the wave vector q of the light potential, for small q, short duration pulses, or for not-so-low intensities, the local-density perturbative description of the excitation spectrum breaks down badly, as shown by our results. 5555 2002 The American Physical Society.