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

The quantum spin Hall effect is characterized by spin-polarized counter-propagating edge states(1-3). 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 sample(4-6). In particular, a quantum-spin-Hall analogue has been predicted in bilayer graphene with a Landau level filling factor is v = 0 if the ground state is a spin ferromagnet(7). Previous studies have demonstrated that the bilayer v = 0 state is an insulator in a perpendicular magnetic field(8-14), although the exact nature of this state has not been identified. Here we present measurements of the is = 0 state in a dual-gated bilayer graphene device in a tilted magnetic field. We map out a full phase diagram of the is v = 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 4e(2)/h, consistent with predictions for the ferromagnet.