Superconductivity and geometric superfluid weight of a tunable flat band system

Abstract

We study superconductivity and superfluid weight of the two-dimensional $\alpha$-$\mathcal{T}_3$ lattice with on-site asymmetries, hosting an isolated quasi-flat band with tunable bandwidth via a parameter $\alpha$. Within a mean-field approximation of the attractive Hubbard model, we obtain the superconducting order parameters on the three inequivalent sublattices and show their strong dependence on $\alpha$, interaction strength, and electron filling. At quasi-flat band filling, a superconducting gap opens and grows power-law fast with interaction strength, instead of the usual slow exponential growth, due to diverging density of states. We calculate the superfluid weight from linear response theory and study its band dispersion and geometric contributions. While the conventional part proportional to band derivatives is suppressed in the quasi-flat band regime, the contribution dominated by the quantum metric grows linearly for small interaction strength. We further demonstrate how tuning $\alpha$ enhances the quantum metric and thus the geometric superfluid weight especially near half-filling, while increasing on-site asymmetries increases the conventional contribution by broadening the quasi-flat band. We obtain the Berezinskii-Kosterlitz-Thouless transition temperature and demonstrate its strong dependence and enhancement with the parameter $\alpha$. Our results establish a tunable flat band system, the $\alpha$-$\mathcal{T}_3$ lattice model, as a candidate for tunable quantum geometry and superfluid weight and as a prototype of related behavior in tunable quantum materials.