Quantum spin Hall effect in the graphene zero energy Landau level - Part I

Shortly after the experimental discovery of graphene, it was predicted that Zeeman splitting of the graphene zero energy Landau level results in a quantum spin Hall phase, characterized by counterpropagating spin-filtered edge states. However, experimental realization of this state has been obscured by the existence of competing Coulomb interaction-driven insulating phases. We address this problem by fabricating monolayer graphene devices in which the Coulomb interaction is heavily screened by a proximal graphite gate. Despite the reduction in the strength of intralayer interactions, the resulting high mobility samples show all the usual signatures of Coulomb-driven symmetry breaking in high magnetic fields, with a strong insulating state developing at charge neutrality at fields of $\sim$1 Tesla. Unlike in conventional samples, however, we observe a continuous transition from this insulating state to a conducting state of order e$^2$/h as a function of in-plane field. Simultaneous high-sensitivity capacitance measurements reveal that the sample bulk remains gapped throughout the transition. The observation of finite conduction in the presence of a bulk insulator strongly implies that transport occurs via the edge states characteristic of the quantum spin Hall state.