Evidence for a Spin Phase Transition at {\nu}=0 in Bilayer Graphene

TL;DR

This study investigates the phase transition at filling factor ν=0 in bilayer graphene, revealing a transition from an insulating to a metallic state under certain magnetic and electric field conditions, supporting the existence of a spin ferromagnetic phase.

Contribution

The paper provides experimental evidence for a spin phase transition at ν=0 in bilayer graphene, mapping out a phase diagram using tilted magnetic fields and electric fields.

Findings

Observation of a transition to a metallic state at large in-plane magnetic fields.

Mapping of the phase diagram as a function of magnetic and electric fields.

Support for the spin ferromagnet hypothesis at ν=0.

Abstract

The most celebrated property of the quantum spin Hall effect is the presence of spin-polarized counter-propagating edge states. This novel edge state configuration has also been predicted to occur in graphene when spin-split electron- and hole-like Landau levels are forced to cross at the edge of the sample. In particular, a quantum spin Hall analogue has been predicted at {\nu}=0 in bilayer graphene if the ground state is a spin ferromagnet. Previous studies have demonstrated that the bilayer {\nu}=0 state is an insulator in a perpendicular magnetic field, though the exact nature of this state has not been identified. Here we present measurements of the {\nu}=0 state in a dual-gated bilayer graphene device in tilted magnetic field. The application of an in-plane magnetic field and perpendicular electric field allows us to map out a full phase diagram of the {\nu}=0 state as a function of experimentally tunable parameters. At large in-plane magnetic field we observe a quantum phase transition to a metallic state with conductance of order 4e^2/h, consistent with predictions for the ferromagnet.