Unconventional superconductivity with preformed pairs in twisted bilayer graphene

TL;DR

This paper develops a theoretical model for unconventional superconductivity in twisted bilayer graphene, emphasizing preformed pairs and phase transitions influenced by magnetic fields, with implications for high critical magnetic fields.

Contribution

It introduces a strong coupling model with preformed Cooper pairs and predicts a new phase transition influenced by magnetic fields in twisted bilayer graphene.

Findings

Preformed Cooper pairs exist inside grains in the strong coupling regime.

A phase transition between preformed pairs and FFLO state can be induced by magnetic fields.

Upper critical magnetic field is enhanced in the strong coupling scenario.

Abstract

We present a theory of superconductivity in magic-angle twisted bilayer graphene and analyze the superconducting phase diagram in presence of the magnetic field. Namely, we consider a model of a granular array hosting localized states, which are hybridized via the delocalized fermions in the inter-grain regions. We study a strong coupling situation when the interactions lead to an incoherent state with preformed Cooper pairs inside the grains. The Andreev scattering among different grains manifests itself through the global phase-coherent superconducting state at lower temperatures. We demonstrate that a new phase transition between the preformed Cooper pairing state and the Larkin-Ovchinnikov-Fulde-Ferrell state might be induced by the spin pair-breaking effect of in-plane magnetic field. The upper critical magnetic field is shown to be enhanced in the strong coupling case.