We present an approximate analytical theory and direct numerical computation
of defect modes of a Bose-Einstein condensate loaded in an optical lattice and
subject to an additional localized (defect) potential. Some of the modes are
found to be remarkably stable and can be driven along the lattice by means of a
defect moving following a step-like function defined by the period of Josephson
oscillations and the macroscopic stability of the atoms.Comment: 4 pages, 5 figure
