Excitations of the ν = 0 state of bilayer graphene

The filling factor n = 0 state of bilayer graphene (BLG) exhibits a rich phase diagram driven by a displacement field D, a perpendicular magnetic field B_perp and an in-plane magnetic field B_ip. In a small B_perp and in the vicinity of D = 0, experiments show that an increasing B_ip drives the BLG from an insulating, presumably canted anti-ferromagnet (CAF) phase to a quantum spin Hall (QSH) phase, which is ferromagnetic in the bulk and carries metallic edge states. Systematic experimental studies of these interesting phases, however, are lacking. In high-quality, dual h-BN encapsulated and gated BLG devices, we have performed temperature-dependent conductance measurements G (T) of the CAF and the QSH states in tilted B-field up to 31T. G (T) of the CAF state continuously decreases as a function of T without an apparent activation gap. G (T) of the QSH state is non-monotonic in the range of 0.3 K < T < 50 K, where dG/dT changes sign a few times. Even at the lowest temperatures, dG/dT does not exhibit a universal sign among the several devices we measured although all exhibit G ~ 4e²/h. We discuss possible mechanisms including the dynamics of the metallic edge states and the conduction and excitations of the bulk ferromagnet.