Abstract
Two graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. Here we study a related but different system with reduced symmetry - twisted double bilayer graphene (TDBG), consisting of two Bernal stacked bilayer graphenes, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field. We construct a phase diagram as a function of twist angle and displacement field, incorporating interactions via a Hartree-Fock approximation. At half-filling, ferromagnetic insulators are stabilized with valley Chern number {C}_{{rm{v}}}=pm 2. Upon doping, ferromagnetic fluctuations are argued to lead to spin-triplet superconductivity from pairing between opposite valleys. We highlight a novel orbital effect arising from in-plane fields plays an important role in interpreting experiments. Combined with recent experimental findings, our results establish TDBG as a tunable platform to realize rare phases in conventional solids.
Highlights
Two graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states
Motivated by recent experimental report[20], we study a related system—twisted double-bilayer graphene (TDBG)—which consists of a pair of bilayer-graphene sheets, twisted with respect to one another with AB–AB-stacking structure
Due to the absence of C2 rotation symmetry, twisted double bilayer graphene (TDBG) has a lower symmetry compared with twisted bilayer graphene (TBG), which simplifies the problem by removing the band touching at the Dirac points, leading to a low energy effective description involving one rather than two narrow bands per spin and valley
Summary
Two graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. The recent discovery of correlated insulating states and superconductivity in twisted bilayer graphene (TBG)[1,2,3,4] has opened a new window to exploring strong correlation effects in systems whose doping can be tuned, enabling the exploration of a rich range of interaction-driven phenomena. Motivated by recent experimental report[20], we study a related system—twisted double-bilayer graphene (TDBG)—which consists of a pair of bilayer-graphene sheets, twisted with respect to one another with AB–AB-stacking structure. We perform an accurate calculation of the single-particle band structure to identify ranges of displacement field and twist angle for which a single band is isolated and relatively flat. We identify a hitherto-neglected in-plane orbital effect which is used to explain the experimentally observed deviation of the in-plane g factor from 220, as well as the effect of in-plane field on superconducting Tc
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
