A twisty trilayer
The discovery of superconductivity in bilayers consisting of graphene sheets twisted with respect to each other by just the right “magic” angle has inspired enormous interest in twisted materials. Hao et al. constructed twisted trilayer graphene, in which the middle layer is twisted with respect to the bottom layer and the top layer is roughly parallel to the bottom layer (see the Perspective by Yazdani). Such trilayers exhibited superconductivity that was tunable by an external electric field and consistent with having an unconventional nature.
Engineering moiré superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the average twist angle θ ~ 1.56°, a theoretically predicted “magic angle” for the formation of flat electron bands, we observed displacement field–tunable superconductivity with a maximum critical temperature of 2.1 kelvin. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moiré bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the observed moiré superconductivity has an unconventional nature.