On its own, a sheet of graphene is a semimetal—its electrons interact only weakly with each other. But as experimentalists discovered in 2018, the situation changes when two sheets of graphene are stacked together, with a slight
(~1°) rotation between them (Fig. 1). At this so-called magic twist angle and at low temperatures [1], the electrons become correlated, forming insulating or superconducting phases depending on the carrier density [2–7]. These phases appear to come from a twist-induced flattening of the electronic energy bands, which narrows the electrons’ range of kinetic energies relative to their interaction energy. Researchers are actively trying to understand the twist-induced phases—for example, whether the superconductivity is ordinary or something more exotic. In a trio of papers, three independent theoretical groups contribute to this effort. They show that the value of the superconducting transition temperature in twisted bilayer graphene (TBG) is much higher than expected because of the special geometry of its electronic wave functions [8–10]. This so-called geometric contribution would persist even if TBG’s energy bands could be made perfectly flat, a condition where superconductivity would disappear in most materials.
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