Evidence for unconventional superconductivity in twisted trilayer graphene
Abstract
Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity. 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. 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 bilayers.
Additional Information
© 2022 Nature Publishing Group. Received 20 September 2021; Accepted 01 April 2022; Published 15 June 2022. We acknowledge discussions with F. von Oppen, G. Refael, Y. Peng and A. Yazdani. This work was primarily supported by the Office of Naval Research (grant number N142112635); the National Science Foundation (grant number DMR-1753306); and the Army Research Office under grant award W911NF17-1-0323. Nanofabrication efforts were in part supported by Department of Energy DOE-QIS programme (DE-SC0019166) and National Science Foundation (grant number DMR-2005129). S.N.-P. acknowledges support from the Sloan Foundation. J.A. and S.N.-P. also acknowledge support from the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation through grant GBMF1250; C.L. acknowledges support from the Gordon and Betty Moore Foundation's EPiQS Initiative, grant GBMF8682. A.T. and J.A. are grateful for the support of the Walter Burke Institute for Theoretical Physics at Caltech. H.K. and Y.C. acknowledge support from the Kwanjeong fellowship. Data availability: The data that support the findings of this study are available from the corresponding author on reasonable request. These authors contributed equally: Hyunjin Kim, Youngjoon Choi. Contributions: H.K. and Y.C. fabricated samples with the help of Y.Z. and R.P., and performed STM measurements. H.K., Y.C. and S.N.-P. analysed the data. C.L. and A.T. provided the theoretical analysis supervised by J.A. S.N.-P. supervised the project. H.K., Y.C., C.L., A.T., J.A. and S.N.-P. wrote the manuscript with input from the other authors. The authors declare no competing interests. Peer review information: Nature thanks Iván Brihuega and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.Attached Files
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Additional details
- Eprint ID
- 115208
- DOI
- 10.1038/s41586-022-04715-z
- Resolver ID
- CaltechAUTHORS:20220621-285198100
- Office of Naval Research (ONR)
- N142112635
- NSF
- DMR-1753306
- Army Research Office (ARO)
- W911NF17-1-0323
- Department of Energy (DOE)
- DE-SC0019166
- NSF
- DMR-2005129
- Alfred P. Sloan Foundation
- Institute for Quantum Information and Matter (IQIM)
- Gordon and Betty Moore Foundation
- GBMF1250
- Gordon and Betty Moore Foundation
- GBMF8682
- Walter Burke Institute for Theoretical Physics, Caltech
- Kwanjeong Graduate Fellowship
- Created
-
2022-06-21Created from EPrint's datestamp field
- Updated
-
2023-03-15Created from EPrint's last_modified field
- Caltech groups
- Institute for Quantum Information and Matter, Walter Burke Institute for Theoretical Physics