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

By P. Maher, C. Dean, A. Young, Takashi Taniguchi, Kenji Watanabe, Kenneth L. Shepard, James C. Hone, Philip Kim. Nature Physics, 2012

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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.

Subfields: Atomic and Molecular Physics, and Optics, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/4fadbdf34f901cad3da6b6289cdcb74b19fdb81c