Condensed Matter Seminar Series

Mathias S. Scheurer

University of Innsbruck

Superconductivity, its relation to the correlated insulators, and electric control of nematicity in moiré graphene

Twisted bilayer graphene and related systems provide an exciting playground for correlated many-body physics. They are characterized by rich phase diagrams, featuring correlated insulating behavior, around integer filling fractions, surrounded by superconducting domes. Signs of additional ordering tendencies, such as high-temperature orders leading to “Dirac revivals” and nematicity, have also emerged. In this talk, I will review some of our recent efforts to explore the complex physics of twisted multilayer graphene systems. In particular, we will discuss the possibility that superconductivity and the correlated insulators of twisted-bilayer graphene are connected by Wess-Zumino-Witten terms. We classify the different possible microscopic realizations of this novel type of mechanism for superconductivity (and insulating behavior), in the presence and absence of additional high-temperature orders, which we classify as well. This leads to constraints on the phase diagram. In the second part of the talk, we will discuss evidence for nematic order based on STM data on twisted double-bilayer graphene and analyze implications for the microscopic form of the nematic order parameter. Finally, we will argue that twisted double-bilayer graphene allows for an unprecedented tunability of the orientation of the nematic director via electric fields.