Tunable spin-polarized Mott insulator in twisted bilayer-bilayer graphene

Twisted bilayer graphene, forming a flatband at the magic angle, has become a new platform to study correlated electron states. Superconductivity and Mott insulator phase have been demonstrated in this system. However, the flatband only occurs at a specific twist angle, which is both hard to achieve and lacks tunablity to enable other emergent phenomena. Here we report experiments on twisted bilayer-bilayer graphene, where two Bernal-stacked bilayer graphene are placed together with a small twist angle. Due to the tunable band structure of bilayer graphene with displacement fields, we can achieve flatband structure in a large range of twist angles. At each twist angle, we observed Mott insulator phase at 1/2 filling in a specific range of displacement fields. Under a parallel magnetic field, additional insulating states emerge at 1/4 and 3/4 filling while the 1/2 filled insulating state become more robust. This suggest the 1/2 insulator is a spin-polarized state.