Magic-Angle Twisted Trilayer Graphene: Part 2

Moiré superlattices have recently become a playground for exploring correlated physics and superconductivity. Despite the presence of correlated effects in several moiré systems, magic-angle twisted bilayer graphene has remained the only one with robust superconductivity. Here we present a new moiré superconductor, magic-angle twisted trilayer graphene with dramatically richer properties and tunability. By exploring the entire phase space as a function of carrier density, electric and magnetic fields, and temperature, we determine the system’s tunable phase boundaries and reveal the intimate connection between the superconducting state and the broken symmetry phase at two carriers per moiré unit cell. The suppression and bounding of superconductivity at the Van Hove singularities is difficult to reconcile with the weak-coupling BCS theory. More strikingly, we can tune the system to be in the ultra-strong coupling regime close to the two-dimensional BCS-BEC crossover, where the Ginzburg-Landau coherence length reaches the average inter-particle distance and T_BKT/T_F ratios are in excess of 0.1. Our system establishes a new generation of moiré platform where we can investigate correlated states, strong coupling superconductivity, and more, with unprecedented tunability.