Condensed Matter Seminar: Liujun Zou, Harvard University - “Moire is different: Correlated insulating behavior and superconductivity in twisted bilayer graphene”

A remarkable recent experiment has observed correlated insulator and proximate superconductor phases in twisted bilayer graphene (TBG) when electrons partly fill a nearly flat mini-band that arises near a magic twist angle. However, the nature of the correlated insulator, origin of superconductivity and an effective low energy model remain to be determined. I will first present our understanding of the experimental phenomena in terms of a correlated insulator with intervalley coherence that spontaneously breaks the U(1) valley symmetry. Then I will present effective models to describe the magic-angle TBG. Along the way, I will identify interesting topology of the nearly flat bands that are key to understanding the strong correlation physics and constrain any tight-binding description. In particular, I will show that the nearly flat bands of TBG features fragile topology, a new type of band topology.