Evidence for a spin phase transition at charge neutrality in bilayer graphene

Abstract

A quantum phase transition from an antiferromagnetic to a ferromagnetic state is now measured in graphene bilayers. This observation supports the idea that bilayer graphene can sustain counter-propagating spin-polarized edge modes in analogy to the quantum spin Hall effect seen in topological insulators. The quantum spin Hall effect is characterized by spin-polarized counter-propagating edge states1,2,3. It has been predicted that this edge state configuration could occur in graphene when spin-split electron- and hole-like Landau levels are forced to cross at the edge of the sample4,5,6. In particular, a quantum-spin-Hall analogue has been predicted in bilayer graphene with a Landau level filling factor ν = 0 if the ground state is a spin ferromagnet7. Previous studies have demonstrated that the bilayer ν = 0 state is an insulator in a perpendicular magnetic field8,9,10,11,12,13,14, although the exact nature of this state has not been identified. Here we present measurements of the ν = 0 state in a dual-gated bilayer graphene device in a tilted magnetic field. We map out a full phase diagram of the ν = 0 state as a function of experimentally tunable in-plane magnetic field and perpendicular electric field. At large in-plane magnetic field we observe a quantum phase transition to a metallic state with conductance of the order of 4e2/h, consistent with predictions for the ferromagnet.