Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene

October 2, 2018

Fig. 5. Localization of the other set of constructed Wannier functions.*

In the last few months, researchers have discovered that twisted bilayer graphene—two sheets of graphene layered at a slight angle to one another—can behave as either a superconductor or a type of insulator. An exciting aspect of this realization is that the behavior is selectable: By simply changing an applied voltage, it is possible to go from insulator to superconductor and back.

In a recent article in Physical review X, 2018 Harvard Physics PhD graduate Hoi Chun Po, grad student Liujun Zhou, Prof. Ashvin Vishwanath, and Prof. Senthil Todadri (MIT) provide fundamental aspects of a theory describing how these systems work.

In particular, the authors look at three elements related to this dual nature of twisted bilayer graphene. First, they point out key properties of the electronic band structure that make this problem unique and place surprising constraints on how these systems are modeled. Second, they calculate Wannier functions—orthogonal functions commonly used to describe molecular orbitals in a crystal—as a first step toward incorporating the effects of electron correlation effects. Finally, they provide a simple theory of the insulating and superconducting phases that relies on an unusual form of broken symmetry ordering, and, further, discuss the observed phenomenology in this light.

This work reveals important topological aspects to the band structure and raises fascinating challenges for future theories to incorporate both the topology and the strong Coulomb interactions.

*See H.C. Po, L. Zou, A. Vishwanath, and T. Senthil, "Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene," Phys. Rev. X 8 (2018) DOI:10.1103/PhysRevX.8.031089.