Condensed Matter Theory Forum - Dr Alex Thompson

Despite launching the recent interest in moiré heterostructure, superconductivity has remained relatively elusive and, until recently, it had been confined only to twisted bilayer graphene (TBG). The special status of TBG has been simultaneously downgraded and reinforced by the observation of superconductivity in three distinct twisted graphene systems possessing three, four and five layers: this apparent abundance of superconducting moiré systems is belied by the fact that the predicted electronic structure of the newly constructed multilayer devices is nearly identical to that of TBG by design. Flat bands are realized at the magic angle, but now overlap energetically with highly dispersive bands originating from the additional layers. Nevertheless, while TBG and its multilayer family clearly share many features, experiments do not support a minimalistic interpretation of these systems as effective carbon copies of TBG. First, I will discuss the weak and strong pairing superconducting regimes seen in the three layer system, but absent from TBG. Interestingly, spectroscopic measurements suggest a nodal pairing symmetry, implying that a BEC-BCS transition occurs as opposed to the crossover familiar from s-wave systems. I will end by addressing the yet more striking distinctions between TBG and the four and five layer systems and by outlining a number of potential explanations.