Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene

The flat bands in bilayer graphene (BLG) are sensitive to electric fields $E $directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics. Theories 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 in bilayer graphene are analogous to the masses generated by broken symmetries in particle physics and give rise to large momentum-space Berry curvatures accompanied by spontaneous quantum Hall effects. Though recent experiments have provided convincing evidence of strong electronic correlations near the CNP in BLG, the presence of gaps is controversial. Here we present transport measurements in ultra-clean double-gated BLG, using source-drain bias as a spectroscopic tool to resolve a gap of $\sim $2 meV at the CNP. The gap can be closed by an electric field $E\sim $ 15 mV/nm but increases monotonically with a magnetic field $B$, with an apparent particle-hole asymmetry above the gap, thus providing a spectroscopic mapping of the ground states in BLG.