  The superfluidity of Bose-Einstein condensates in optical lattices are
investigated. Apart from the usual Landau instability which occurs when a BEC
flows faster than the speed of sound, the BEC can also suffer a dynamical
instability, resulting in period doubling and other sorts of symmetry breaking
of the system. Such an instability may play a crucial role in the dissipative
motion of a trapped BEC in an optical lattice recently observed [1].
