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

Abstract

Motivated by recent experiments [1,2], 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.

Acoustic-phonon-mediated superconductivity in moiréless graphene multilayers

“…Since the band structures of RTG and BBG are simpler and better established compared with twisted moiré graphene systems, it is easier to make direct (semi-)quantitative comparisons between theory and experiment here. Besides the acoustic-phonon mechanism, which we consider, a number of alternative theoretical ideas focusing on inter-electron interactions have also been proposed for RTG [25][26][27][28][29][30][31][32] and BBG [33]. We mention here that the case for robust acoustic phonon mediated superconductivity in twisted graphene systems has already been made in the literature, based on the enhancement of the effective electron-phonon coupling in moiré systems by virtue of the suppression of the graphene Fermi velocity [20,22], but the current work, by contrast, is specifically on moiréless graphene multilayers.…”