Competing orders and cascade of degeneracy lifting in doped Bernal bilayer graphene

TL;DR

This paper proposes a mechanism for lifting electronic band degeneracies in doped Bernal bilayer graphene through electric fields and electronic orders, explaining various doping regimes and emergent superconductivity.

Contribution

It introduces a comprehensive model linking electric displacement, layer polarization, and electronic orders to degeneracy lifting and superconductivity in bilayer graphene.

Findings

Degeneracy lifting explains quarter metal and half-metal phases.

Disappearance of QAH order restores valley degeneracy.

Triplet f-wave pairing emerges under in-plane magnetic field.

Abstract

Motivated by recent experiments [H. Zhou, \emph{et al.}, Science {\bf 375}, 774 (2022) and S. C. de la Barrera, \emph{et al.}, arXiv:2110.13907], here we propose a general mechanism for valley and/or spin degeneracy lifting of the electronic bands in doped Bernal bilayer graphene, subject to electric displacement ($D$) fields. A $D$-field induced layer polarization (LP), when accompanied by Hubbard repulsion driven layer antiferromagnet (LAF) and next-nearest-neighbor repulsion driven quantum anomalous Hall (QAH) orders, lifts the four-fold degeneracy of electronic bands, yielding a quarter metal for small doping, as also observed in ABC trilayer graphene. With the disappearance of the QAH order, electronic bands recover two-fold valley degeneracy, thereby forming a conventional or compensated (with majority and minority carriers) half-metal at moderate doping, depending on the relative strength of LP and LAF. At even higher doping and for weak $D$-field only LAF survives and the Fermi surface recovers four-fold degeneracy. We also show that a pure repulsive electronic interaction mediated triplet $f$-wave pairing emerges from a parent correlated nematic liquid or compensated half-metal when an in-plane magnetic field is applied to the system.