Derek Lee and Dimitri Vvedensky

Twisted bilayer graphene and beyond
Graphene has remarkable electronic and mechanical properties that promise a wide range of applications. More than one of these sheets of carbon atoms can bind by van der Waals forces to form a coherent lattice for electronic motion giving rise to new properties. More recently, experiments have shown that bilayer graphene has yielded remarkable properties. A small twist of 1.1 between the two layers turns the semimetal into a superconductor with a critical temperature of 1.7K. A Mott insulator can also be found where Coulomb repulsion causes a trac jam of electrons and the system becomes electrically insulating.

It is believed that the twist gives rise to narrow electronic bands near the Fermi level. This means that the kinetic energy of the electrons is suppressed, and Coulomb interaction becomes important. In fact, this is reminiscent of the cuprate superconductors which also exhibit Mott insulating antiferromagnetic state near a superconducting phase. In that case, Coulomb interactions dominate the properties of a single band of electrons on the CuO planes.

The overarching goal of the project is to explore graphene physics in a new avenue recently made possible in experiments. Our initial stage to study multilayer graphene would open up new avenues of research.

One question that relates back to the correlated electron physics is whether the number and the nature of narrow bands near the Fermi level can be engineered. This can give rise to new analogues of correlated physics found in other electronic materials where multiple orbitals in d-band and f-band electrons give rise to a rich variety of phenomena.

Another possible direction is to study whether bilayer graphene physics can be reproduced in other fields, for instance, using optical lattices in cold atoms and micro-pillar polariton lattices. For instance, strong spin-orbit coupling, which is so small that it is undetectable in graphene, can be simulated in these systems that is hard to achieve in carbon.

It is hard to predict the direction of research in a very fast moving field. However, it is clear that there will be intense experimental attention in these systems in the next few years and plenty of challenges for theorists to address.