Counterflow Superfluidity of Two-Species Ultracold Atoms in a Commensurate Optical Lattice

If two species of ultracold atoms are loaded in a sufficiently tight optical lattice at a commensurate total filling factor, the net number-of-atoms transport is suppressed by the Mott-Hubbard localization. Nonetheless, the counterflow low-frequency dynamics of the two components may survive. We consider corresponding effective Hamiltonians for the three classes of the two-species insulators-- fermion-fermion, boson-boson, and boson-fermion type-- and reveal the conditions when the resulting groundstate supports super-counter-fluidity (SCF) of the two components. Alternative groundstates are found to be phase-segregated states. We emphasize a crucial role of breaking the isotopic symmetry between the species for realizing the SCF phase.