Electric field tunable unconventional superconductivity in alternating twist magic-angle trilayer graphene

Abstract

We construct a van der Waals heterostructure consisting of three graphene layers stacked with alternating twisting angles $\pm\theta$. At the average twist angle $\theta\sim 1.56^{\circ}$, a theoretically predicted magic angle for the formation of flat electron bands, narrow conduction and valence moir\'e bands appear together with a linearly dispersing Dirac band. Upon doping the half-filled moir\'e valence band with holes, or the half-filled moir\'e conduction band with electrons, displacement field tunable superconductivity emerges, reaching a maximum critical temperature of 2.1 K at optimal doping and displacement field. By tuning the doping level and displacement fields, we find that superconducting regimes occur in conjunction with flavour polarization of moir\'e bands bounded by a van Hove singularity (vHS) at high displacement fields. This experimental observation is found to be inconsistent with a weak coupling description, suggesting that the observed moir\'e superconductivity has an unconventional nature.