We investigate theoretically the properties of a Bose-Einstein condensate located in a spatially periodic potential. The excitations of the condensate are obtained from linear equations involving the periodic potential and the periodic condensate density. The Bloch ansatz therefore applies and the excitation spectrum exhibits band structure. If the periodic potential is accelerated, the condensate may undergo Bloch oscillations in the accelerated frame, corresponding to an acceleration without spreading of the entire condensate.

• Received 23 February 1998

DOI:https://doi.org/10.1103/PhysRevA.58.1480