Electric field tunable correlated insulating states and spin-polarized phase transitions in twisted bilayer-bilayer graphene

Understanding how strongly correlated materials behave has been a challenge for the last decades. The recent discovery of superconductivity and correlated insulators in magic angle twisted bilayer graphene (MAGTB) has paved the way to realizing strongly correlated phases of matter using twisted van der Waals heterostructures. We report here on a new correlated platform, twisted bilayer-bilayer graphene (TBBG), which consists of two sheets of Bernal stacked bilayer graphene rotated by a small angle. This system exhibits tunable correlated insulating states at different commensurate fillings. In particular, these insulating states can be switched on and off if an electric displacement field is applied across the sample. The magnetic field response of the correlated insulators in TBBG points towards evidence of spin-polarized ground states. Besides, TBBG shows multiple flat bands near charge neutrality for small twist angles. This results into different correlated states that can be tuned by electric field and are located at half-filling of each flat band. This work contributes to the study of twist-angle and electric-field dependent correlated phases of matter.