Evidence for Spatially Separated Flat Bands in a Graphene Double Moiré System

The experimental advances in realizing two-dimensional moiré materials such as twisted bilayer graphene (TBG) has opened up new opportunities for flat band engineering. Correlated insulators and superconducting states have been demonstrated in TBGs when the twist angle is close to the magic angle. We demonstrate here a double moiré material in a twisted quadlayer graphene structure, where the twist angles between the first and second layers, as well as the third and fourth layers are small (1.2°-1.6°), while the angle between the second and third layer is large. The transport properties indicate the presence of two spatially separated moiré bands in the top and bottom small-angle TBGs formed by the top two layers and bottom two layers of the quadlayer, respectively. We observe clear resistance maxima when the top and bottom moiré bands are at charge neutrality and full-filling of the moiré Brillouin zone, controlled largely by the top and bottom gates, respectively. While neither top or bottom individual TBG shows correlated insulating states at half-filling of the moiré Brillouin zone, a finding consistent with their respective twist being away from the magic angle, surprisingly the double moiré structure exhibits correlated insulating states when both TBGs are at half moiré band fillings. The realization of spatially separated, independently tunable flat bands in this double moiré system provides insight into the evolution of the correlations between electrons when two moiré energy bands are closely spaced.