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 Rb-87 atoms subjected to a moving standing-wave light potential of the form V-L(z,t) = V-0(t)cos(qz-omegat) We calculate the mean-field dynamics (the order parameter) of the condensate and determine the resulting condensate momentum in the z direction, P-z(q,omega, V-0, t(p)), where V-0 is the peak optical potential strength and t(p) 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.