Evidence for unconventional superconductivity in twisted trilayer graphene

Hyunjin Kim, Youngjoon Choi, Cyprian Lewandowski, Alex Thomson, Yiran Zhang, Robert Polski, Kenji Watanabe, Takashi Taniguchi, Jason Alicea and Stevan Nadj-Perge ()
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Hyunjin Kim: California Institute of Technology
Youngjoon Choi: California Institute of Technology
Cyprian Lewandowski: Institute for Quantum Information and Matter, California Institute of Technology
Alex Thomson: Institute for Quantum Information and Matter, California Institute of Technology
Yiran Zhang: California Institute of Technology
Robert Polski: California Institute of Technology
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Jason Alicea: Institute for Quantum Information and Matter, California Institute of Technology
Stevan Nadj-Perge: California Institute of Technology

Nature, 2022, vol. 606, issue 7914, 494-500

Abstract: Abstract Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity1,2. 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 bands3,4. Most notably in a density window spanning two to three holes per moiré 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 moiré structures beyond twisted bilayers5.

Date: 2022
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DOI: 10.1038/s41586-022-04715-z

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