A practical finite temperature theory is developed for the superfluid regime of a weakly interacting Bose gas in an optical lattice with additional harmonic confinement. We derive an extended Bose-Hubbard model that is valid for shallow lattices and when excited bands are occupied. Using the Hartree-Fock-Bogoliubov-Popov mean-field approach, and applying local density and coarse-grained envelope approximations, we arrive at a theory that can be numerically implemented accurately and efficiently. We present results for a threedimensional system, characterizing the importance of the features of the extended Bose-Hubbard model and compare against other theoretical results and show an improved agreement with experimental data. PACS numbers: 67.85.Hj, 03.75.Hh, 05.30.Jp
III. MEAN-FIELD APPROXIMATION
A. Mean-field approach: condensate and non-condensateWe assume that the local number of condensate atoms is either macroscopic or zero [57,58], so that the field operator,Ψ(r), can be separated into a c-number condensate component (the order parameter), Φ(r), and a non-condensate field operator,ψ(r), defined by the usual broken symmetry approach, Φ(r) ≡ Ψ (r) ,ψ(r) ≡Ψ(r) − Φ(r) so that ψ (r) = 0.The assumption that Φ(r) is a c-number is inaccurate near the edges of the condensate, where the local condensate density, |Φ(r)| 2 , is small and just below the critical temperature, since fluctuations are important in such regions.