Correlated insulators and charge density wave states in chirally twisted triple bilayer graphene

Motivated by recent experimental observations of displacement-field-tuned correlated insulators at integer and half-integer fillings in chirally twisted triple bilayer graphene (CTTBG) [Phys.Rev.Lett.132.246501], we study the single-particle and interacting physics of CTTBG. We find that there are two inequivalent stacking orders, i.e., AB-AB-BC and AB-AB-AB, and both exhibit flat bands with nontrivial topology. We then use the Hartree–Fock approximation to calculate the rich phase diagram of CTTBG at all integer and half-integer fillings in both stacking orders and under the vertical displacement field. Under a small displacement field, the groundstates are flavor polarized states for AB-AB-BC stacking order and intervalley coherent states for AB-AB-AB stacking order at all integer and half-integer fillings. A larger displacement field will turn them into layer-polarized states. At half-integer fillings, the groundstates also exhibit charge density wave (CDW) order. For AB-AB-AB stacking, the groundstates at half-integer fillings are always 2 × 1 stripe state among a range of displacement fields. For AB-AB-BC stacking, the groundstates at half-integer fillings are also 2 × 1 stripe states under a small displacement field and a larger displacement will possibly favor further translation-symmetry-breaking, depending on filling and the direction of the displacement field. We demonstrate that the CDW states observed in the experiment can originate from the strong Coulomb interaction of the flat band electrons.