Negative electronic compressibility in charge islands in twisted bilayer graphene

By exploiting naturally occurring charge islands due to twist angle variations in twisted bilayer graphene, the authors measure here how strongly charge carriers interact. They discover negative capacitance contributions, which are indicative of correlation effects near band insulators at full filling on both flat and remote sides, and at integer partial fillings. The correlation strength remains constant across all bands, unaffected by a magnetic field. The reported results point to a possible Wigner crystal phase as the underlying mechanism.