Evidence for unconventional superconductivity in twisted trilayer graphene

Magic-angle twisted trilayer graphene (MATTG) has emerged as a moire material that exhibits strong electronic correlations and unconventional superconductivity(1,2). However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands(3,4). Most notably in a density window spanning two to three holes per moire unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen-Cooper-Schrieffer superconductor to a Bose-Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moire structures beyond twisted bilayers(5).

Automated summary

Magic-angle twisted trilayer graphene (MATTG) is a moire material that exhibits strong electronic correlations and unconventional superconductivity. In this study, high-resolution scanning tunnelling microscopy and spectroscopy are used to investigate MATTG. Extensive regions of atomic reconstruction with mirror-symmetric stacking are observed, showing symmetry-breaking electronic transitions and doping-dependent band-structure deformations. Superconductivity is observed as pronounced dips in the tunnelling conductance at the Fermi level, accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed conductance evolution with doping suggests a transition from a gapped superconductor to a nodal superconductor, and the presence of peak-dip-hump structures indicates strong coupling to bosonic modes of MATTG.