Superfluid stiffness for the attractive Hubbard model on a honeycomb optical lattice

In addition to the conventional contribution that is directly controlled by the single-particle energy spectrum, the superfluid phase stiffness of a two-component Fermi gas has a geometric contribution that is governed by the quantum metric of the honeycomb's band structure. Here we take both contributions into account and construct phase diagrams for the critical superfluid transition temperature as a function of the chemical potential, particle filling, onsite interaction, and next-nearest-neighbor hopping. Our theoretical approach is based on a self-consistent solution of the BCS mean-field theory for the stationary Cooper pairs and the universal Berezinskii-Kosterlitz-Thouless (BKT) relation for the phase fluctuations.