Origin of Mott insulating behavior and superconductivity in twisted bilayer graphene*

A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat mini-band that arises a magic twist angle. However, the nature of the Mott insulator, origin of superconductivity and an effective low energy model remain to be determined. We will present a phenomenological picture of the Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry, and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hunds coupling. We will discuss consequences of this picture for superconducting states obtained on doping the valley ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future.