Local spectroscopy of a gate-reversible Chern insulator in twisted monolayer-bilayer graphene

Stacking two-dimensional van der Waals materials is a novel approach to realize Chern insulators featuring topologically protected edge states that arise from non-zero total Chern number (C_tot) for occupied bands. Signatures of correlated insulating states and quantized Hall conductance were recently detected in twisted monolayer-bilayer graphene (tMBLG) – a graphene monolayer rotationally misaligned with respect to a Bernal-stacked bilayer. We have fabricated gate-tunable tMBLG devices and characterized their local electronic structure using scanning tunneling microscopy and spectroscopy. We observe charge gaps in the local density of states at ½- and ¾-filling of the tMBLG conduction flat band, consistent with the emergence of symmetry-breaking insulating phases. In an out-of-plane magnetic field the ¾-filling correlation gap evolves into two separate gaps at different electron filling levels that follow the Středa formula with C_tot = 2 and C_tot = -‍2, thus signifying gate-controlled switching of the spontaneous valley polarization. Our observations are consistent with reported transport studies and can be understood theoretically as resulting from a competition between bulk and edge contributions to the overall magnetization.