Revisiting flat band superconductivity: dependence on minimal quantum metric and band touchings

TL;DR

This paper clarifies the role of quantum geometry, especially the minimal quantum metric, in flat band superconductivity, and explores how band touchings can enhance superconductivity, challenging previous incomplete models.

Contribution

The authors derive complete equations for superfluid weight, relate the minimal quantum metric to Cooper pair mass, and analyze the effects of band touchings on flat band superconductivity.

Findings

Minimal quantum metric is related to superfluid weight in flat bands.

Band touchings can enhance superconductivity even in non-isolated flat bands.

Complete derivation corrects previous incomplete models of superfluid weight.

Abstract

A critical result in superconductivity is that flat bands, though dispersionless, can still host nonzero superfluid weight due to quantum geometry. We show that the derivation of the mean field superfluid weight in previous literature is incomplete, which can lead to severe quantitative and even qualitative errors. We derive the complete equations and demonstrate that the minimal quantum metric -- the metric with minimum trace -- is related to the superfluid weight in isolated flat bands. We complement this result with an exact calculation of the Cooper pair mass in attractive Hubbard models with the uniform pairing condition. When the orbitals are located at high symmetry positions, the Cooper pair mass is exactly given by the quantum metric, which is guaranteed to be minimal. Moreover, we study the effect of closing the band gap between the flat and dispersive bands, and develop a mean-field theory of pairing for different band-touching points via the $S$-matrix construction. In mean field, we show that a non-isolated flat band can actually be beneficial for superconductivity. This is a promising result in the search for high temperature superconductivity as the material does not need to have flat bands that are isolated from other bands by the thermal energy. Our work resolves a fundamental caveat in understanding the relation of multiband superconductivity to quantum geometry, and the results on band touchings widen the class of systems advantageous for the search of high temperature flat band superconductivity.