Floquet-Bloch operator for the Bose-Hubbard model with static field.

This paper deals with the spectral properties of the one-dimensional Bose-Hubbard Hamiltonian amended by an external static field-a model for cold spinless atoms loaded in a quasi-one-dimensional optical lattice and subject to an additional static (for example, gravitational) force. The analysis is performed in terms of the Floquet-Bloch operator, defined as the evolution operator of the system over one Bloch period. Depending on the particular choice of parameters, the spectrum is found to be either regular or chaotic. Moreover, in the chaotic case, the matrix of the Floquet-Bloch operator is well characterized as a random matrix of the circular orthogonal ensemble.