Drag effect and topological complexes in strongly interacting two-component lattice superfluids

The mutual drag in strongly interacting two-component superfluids in optical lattices is discussed. Two competing drag mechanisms are the vacancy-assisted motion and proximity to the quasi-molecular state, in which an integer number $q$ of atoms (or holes) of one component might be bound to one atom (or hole) of the other component. Then the lowest energy topological excitation (vortex or persistent current) becomes a composite object consisting of $q$ circulation quanta of one component and one circulation of the other. In the SQUID-type geometry, the value of $q$ can become fractional. These topological complexes can be detected by absorptive imaging. We present both the mean field and Monte Carlo results. The drag effects in optical lattices are drastically different from the Galilean invariant Andreev-Bashkin effect in liquid helium.