Infrared Spectroscopy of Phase Transitions in the Lowest Landau Levels of Bilayer Graphene

We perform infrared magnetospectroscopy of Landau level (LL) transitions in dual-gated bilayer graphene. At ν=4 when the zeroth LL (octet) is filled, two resonances are observed indicating the opening of a gap. At ν=0 when the octet is half-filled, multiple resonances disperse nonmonotonically with increasing displacement field, D, perpendicular to the sheet, showing a phase transition at modest displacement fields from a canted antiferromagnet (CAFM) to the layer-polarized state, with a gap that opens linearly in D. When D=0 and ν is varied, resonances at ±ν show an electron-hole asymmetry with multiple line splittings as the octet is progressively filled. The ν=4 data show good agreement with predictions from a mean-field Hartree-Fock calculation when accounting for multiple tight-binding terms in a four-band model of bilayer graphene. However, even by incorporating a valley interaction anisotropy tuned to the CAFM ground state, only partial agreement is found at ν=0. Our results suggest additional physics is required to understand bilayer graphene at half-filling.