Nodal band-off-diagonal superconductivity in twisted graphene superlattices

TL;DR

This paper proposes a new type of superconductivity in twisted graphene systems, characterized by band-off-diagonal pairing that can be nodal or gapped, explaining recent experimental phenomena.

Contribution

It introduces the concept of nodal band-off-diagonal superconductivity, revealing its symmetry properties, conditions for occurrence, and relevance to experimental observations in twisted graphene.

Findings

Band-off-diagonal pairing can be nodal or gapped depending on parameters.

Nodal pairing can be favored by phonons and intervalley-coherent fluctuations.

The theory explains transition phenomena observed in experiments.

Abstract

The superconducting state and mechanism are among the least understood phenomena in twisted graphene systems. For instance, recent tunneling experiments indicate a transition between nodal and gapped pairing with electron filling, which is not naturally understood within current theory. We demonstrate that the coexistence of superconductivity and flavor polarization leads to pairing channels that are guaranteed by symmetry to be entirely band-off-diagonal, with a variety of unusual consequences: most notably, the pairing invariant under all symmetries can have protected nodal lines or be fully gapped, depending on parameters, and the band-off-diagonal chiral d-wave state exhibits transitions between gapped and nodal regions upon varying the chemical potential. We demonstrate that nodal band-off-diagonal pairing can be the leading state when only phonons are considered, and is also uniquely favored by fluctuations of a time-reversal-symmetric intervalley-coherent order motivated by recent experiments. Consequently, band-off-diagonal superconductivity allows for the reconciliation of several key experimental observations in graphene moir\'e systems.