Bosonic mixtures in optical lattices are of interest for the study of the two-component Bose-Hubbard model with its connection to quantum magnetism, impurities and disorder, and of phonon-mediated effects. My talk will concentrate on experiments with variable hyperfine-state mixtures of rubidium-87 atoms in an optical lattice, whose depth is state-dependent along one axis and can be tuned.
We have studied how superfluid coherence near the Mott regime is affected by a variable bosonic background medium. We found a significant reduction when the latter either experiences a strongly localizing lattice potential or none at all, qualitatively consistent with recent discussions of impurity scattering, and of atom-phonon coupling for atoms immersed in a superfluid. Moreover, we have observed four-wave mixing between two superfluid components after release from the lattice. This occurs in a collinear geometry, in which the seed and output modes share the same momentum state, but are in internally distinct states. This novel effect not only can play an important role in studies of bosonic mixtures, but it should also be of interest in the context of quantum atom optics.