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We present a theoretical treatment of the surprisingly large damping observedrecently in one-dimensional Bose-Einstein atomic condensates in opticallattices. We show that time-dependent Hartree-Fock-Bogoliubov (HFB)calculations can describe qualitatively the main features of the dampingobserved over a range of lattice depths. We also derive a formula of thefluctuation-dissipation type for the damping, based on a picture in which thecoherent motion of the condensate atoms is disrupted as they try to flowthrough the random local potential created by the irregular motion ofnoncondensate atoms. We expect this irregular motion to result from thewell-known dynamical instability exhibited by the mean-field theory for thesesystems. When parameters for the characteristic strength and correlation timesof the fluctuations, obtained from the HFB calculations, are substituted in thedamping formula, we find very good agreement with the experimentally-observeddamping, as long as the lattice is shallow enough for the fraction of atoms inthe Mott insulator phase to be negligible. We also include, for completeness,the results of other calculations based on the Gutzwiller ansatz, which appearto work better for the deeper lattices.Comment: Extended and revised version, August 200

Mean-field treatment of the damping of the oscillations of a one-dimensional Bose gas in an optical lattice