Phase transition in quasi-flat band superconductor

In this talk, we explore superconductivity in a two-dimensional material using a minimal two-band model with two types of electron bands: one that is dispersive and another that is flat. The hybridization between the bands introduces a finite dispersion to the otherwise localized flat-band electrons. The enhanced density of states in such a quasi-flat band raises the crossover temperature for an inhomogeneous preformed Cooper pair state (or dimers in the Helium-3 context). The superconducting phase stiffness and the Berezinskii-Kosterlitz-Thouless (BKT) temperature are governed by the Fermi surface contribution of the quasi-flat band, revealing a dome-like dependence on doping. The BKT temperature reaches a maximum value that is only numerically smaller than the energy width of the quasi-flat band.