Field-induced insulating states in a graphene superlattice
We report on high-field magnetotransport (B up to 35 T) on a gated superlattice based on single-layer graphene aligned on top of hexagonal boron nitride. The large-period moiré modulation (≈15 nm) enables us to access the Hofstadter spectrum in the vicinity of and above one flux quantum per superlattice unit cell (Φ/Φ0=1 at B=22 T). We thereby reveal, in addition to the spin-valley antiferromagnet at ν=0, two insulating states developing in positive and negative effective magnetic fields from the main ν=1 and ν=−2 quantum Hall states, respectively. We investigate the field dependence of the energy gaps associated with these insulating states, which we quantify from the temperature-activated peak resistance. Referring to a simple model of local Landau quantization of third-generation Dirac fermions arising at Φ/Φ0=1, we describe the different microscopic origins of the insulating states and experimentally determine the energy-momentum dispersion of the emergent gapped Dirac quasiparticles.