Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene

TL;DR

This study uses transport spectroscopy to directly measure a small energy gap in ultra-clean bilayer graphene, revealing how it varies with electric and magnetic fields and mapping its ground states.

Contribution

It provides the first direct spectroscopic measurement of a gap in bilayer graphene and maps its dependence on external electric and magnetic fields.

Findings

Measured a ~2 meV gap at the charge neutrality point.

Gap closes with an electric field of ~13 mV/nm.

Gap increases monotonically with magnetic field.

Abstract

The flat bands in bilayer graphene(BLG) are sensitive to electric fields E\bot directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the possibility of a variety of interesting broken symmetry states, some characterized by spontaneous mass gaps, when the electron-density is at the carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in bilayer graphene are analogous[17,18] to the masses generated by broken symmetries in particle physics and give rise to large momentum-space Berry curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though recent experiments[20-23] have provided convincing evidence of strong electronic correlations near the CNP in BLG, the presence of gaps is difficult to establish because of the lack of direct spectroscopic measurements. Here we present transport measurements in ultra-clean double-gated BLG, using source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but increases monotonically with a magnetic field B, with an apparent particle-hole asymmetry above the gap, thus providing the first mapping of the ground states in BLG.