Activating superconductivity in a repulsive system by high-energy degrees of freedom

TL;DR

This paper proposes a novel superconductivity mechanism driven by Coulomb repulsion in a narrow band system, where high-energy degrees of freedom facilitate pairing, with potential relevance to twisted bilayer graphene.

Contribution

It introduces a new pairing mechanism based on repulsion-assisted scattering involving high-energy bands, emphasizing the role of unscreened Coulomb interactions in two-dimensional systems.

Findings

Repulsion-assisted pairing can occur in narrow band systems with specific bandstructure design.

Unscreened Coulomb interactions enhance pairing strength in low-density, two-dimensional systems.

Predicted signatures could be experimentally tested in twisted bilayer graphene.

Abstract

We discuss superconducting pairing in a narrow conduction band sandwiched between unoccupied and occupied bands, an arrangement that enables an unconventional pairing mechanism governed by Coulomb repulsion. Pairing interaction originates from repulsion-assisted scattering between far-out pair states in the higher-energy bands and those at the Fermi level. Optimizing the bandstructure design and carrier density in order to bring plasma frequency below the bandgap renders the repulsion unscreened for the processes with a large frequency transfer. This allows the pairing to fully benefit from the pristine Coulomb repulsion strength. The repulsion-induced attraction is particularly strong in two dimensions and is assisted by a low density of carriers and the resulting low plasma frequency values. We assess the possible connection of this mechanism to superconductivity in magic-angle twisted bilayer graphene where the bandstructure features wide dispersive upper and lower minibands. We use a simple model to illustrate the importance of the far-out pairs in these bands and predict testable signatures of this superconductivity mechanism.