Phase transition in quasi-flat band superconductors

TL;DR

This paper explores how hybridization-induced quasi-flat bands in a two-dimensional heavy-fermion model influence superconductivity, revealing a dome-shaped doping dependence of transition temperatures and the conditions for optimal superconducting phases.

Contribution

It introduces a detailed analysis of superconductivity in quasi-flat band systems, highlighting the role of hybridization and inhomogeneous pairing regimes, with implications for graphene-based materials.

Findings

Crossover and BKT temperatures show dome-like doping dependence.

Superconductivity is suppressed when pairing exceeds the quasi-flat band width.

Maximum BKT temperature is close to the quasi-flat band energy width.

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.