Coupled superfluidity of binary Bose mixtures in two dimensions
Exotic superfluid phases in 2D binary Bose mixtures.
We study a two-component Bose gas in two dimensions at low temperatures with short-range repulsive interactions.
When both components are superfluid, they are coupled through the Andreev–Bashkin effect, which induces a nondissipative drag between the two flows:
Here, \({\tilde{n}}_i\) are the renormalized superfluid densities and \(n_{\mathrm{dr}}\) is the drag density.
At finite temperature, topological vortex excitations destroy superfluidity through a Berezinskii–Kosterlitz–Thouless (BKT) transition. We extend the BKT renormalization group flow to coupled mixtures and find that vortices in one species can induce the breakdown of superfluidity in the other. This leads to a locking of critical temperatures:
\[T_c^{(1)} \approx T_c^{(2)} ,\]a striking departure from the uncoupled case where each component has its own transition. These results highlight how quantum fluctuations and vortex interactions create coupled superfluid phases, relevant for cold atoms, bilayer systems, multiband superconductors, and even neutron star matter.
