Phase transition in quasi-flat band superconductors

Abstract

We investigate superconductivity in a two-dimensional material described by a two-band heavy-fermion model, where hybridization between a dispersive band and a flat band introduces a quasi-flat dispersion to the otherwise localized flat-band electrons. The enhanced density of states in the quasi-flat band raises the crossover temperature for an inhomogeneous preformed Cooper pair state. The superconducting phase stiffness and the Berezinskii-Kosterlitz-Thouless (BKT) temperature are governed by the Fermi surface contribution induced by hybridization. We compute the crossover and BKT temperatures, revealing a dome-like dependence on doping. When the pairing amplitude exceeds the energy width of the quasi-flat band, superconductivity is suppressed, and the inhomogeneous pairing regime expands linearly with increasing interaction strength. However, in the opposite case, the BKT temperature reaches a maximum value that is only numerically less than the energy width of the quasi-flat band. We also discuss our results in the context of superconductivity in graphene-based systems.