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 a quasimolecular state. In a case of strong drag, the lowest energy topological excitation (vortex or persistent current) can consist of several circulation quanta. In the SQUID-type geometry, the circulation can become fractional. 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.

Figure 1

The drag coefficient $k$ for the $J$-current analog of the Hamiltonian (6) as a function of the relative interaction, with the value $V_{\mathrm{int}}/V_c=1$ corresponding to the 2SF-SCF phase transition. The horizontal dashed lines indicate a domain where $1+q$ vortex complex with $q=1$ has lower energy than any single-circulation vortex. Error bars are much smaller then symbol sizes. Solid line is the eye guide.

Figure 2

The drag coefficient $k$ for the $J$-model analog of the Hamiltonian (6) in the limit $V_{\mathrm{int}}\to \infty$ as a function of concentration of vacancies $x_v$ for symmetric case: $N_A=N_B$, $t_a=t_b$. Error bars are shown for all points. Solid line is the eye guide.