Switching between superconductivity and current density waves in Bernal bilayer graphene

TL;DR

This paper explores how superconductivity in Bernal bilayer graphene can be switched on or off by magnetic fields or spin-orbit coupling, revealing a competition with a current density wave driven by electronic interactions.

Contribution

It introduces a theoretical framework showing how spin degeneracy breaking shifts the balance from a current density wave to superconductivity in bilayer graphene.

Findings

Spin degeneracy breaking favors superconductivity over current density waves.

Superconductivity exhibits p/d-wave pairing symmetry driven by repulsive interactions.

CrDW explains non-linear I-V behavior and potential Hall effects in the normal state.

Abstract

An out-of-plane magnetic field can always suppress superconductivity. In Bernal-stacked bilayer graphene (BBG), recently observed activation of superconductivity (SC) through either in-plane magnetic fields or proximate spin-orbit coupling (SOC) offers a rare instance of switching superconductivity on. To understand this, we must first examine the non-superconducting state. We propose an incommensurate current density wave (CrDW) driven by van Hove singularities away from the zone corners as a competing order. We note that the two switches, the in-plane field and the SOC, both break spin degeneracy. Our parquet renormalization group analysis reveals that breaking spin degeneracy shifts the balance from CrDW, favored under spin degeneracy, to SC when degeneracy is lifted. Driven by purely repulsive interactions, the pairing symmetry of the resulting SC is $p/d$-wave. The presence of CrDW accounts for the non-linear $I-V$ behavior in the normal state and suggests potential anomalous Hall effects due to time-reversal symmetry breaking. We further predict that reducing screening could enhance SC.