Landau-Zener tunneling of Bose-Einstein condensates in an optical lattice

In this paper, we develop a theory of the LZ tunneling between the lowest bands of a periodic potential. Our approach reveals the role of modulational instability (MI) on the atom transfer between zones as well as the effect of (partial) suppression of the MI by the lattice acceleration. Our consideration is based on the Gross-Pitaevskii equation, which in the two-zone approximation is reduced to the linearly coupled nonlinear Schrodinger equations. In the presence of nonlinearity, the Bloch states can become unstable and localized states can appear in the gaps between bands (gap-solitons) in which can dramatically affect the atomic transfer rate. The stability properties constitute the main reason of asymmetry of the LZ tunneling