Unconventional superconductivity in magic-angle graphene superlattices

Abstract

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has puzzled physicists for decades. Such difficulties have stimulated new research paradigms, such as ultracold atom lattices for simulating quantum materials. Here we report on the realization of intrinsic unconventional superconductivity in a two-dimensional superlattice created by stacking two graphene sheets with a small twist angle. For angles near 1.1°, the first ‘magic’ angle, twisted bilayer graphene exhibits ultraflat bands near charge neutrality, which lead to correlated insulating states at half-filling. Upon electrostatic doping away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature T_c up to 1.7 kelvin. The temperature–density phase diagram shows similarities with that of the cuprates, including superconducting domes. Moreover, quantum oscillations indicate small Fermi surfaces near the correlated insulating phase, in analogy with underdoped cuprates. Its relatively high T_c, given such a small Fermi surface (corresponding to a record-low two-dimensional carrier density of about 10¹¹ per square centimetre), puts twisted bilayer graphene among the strongest coupling superconductors, in a regime close to the crossover between the Bardeen–Cooper–Schrieffer regime and a Bose–Einstein condensate (BCS–BEC). These results establish twisted bilayer graphene as the first purely carbon-based two-dimensional superconductor, providing a highly tunable platform with which to investigate strongly correlated phenomena, which could lead to insights into the physics of high-T_c superconductors and quantum spin liquids.