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Evidence for unconventional superconductivity in twisted trilayer graphene

Kim, Hyunjin and Choi, Youngjoon and Lewandowski, Cyprian and Thomson, Alex and Zhang, Yiran and Polski, Robert and Watanabe, Kenji and Taniguchi, Takashi and Alicea, Jason and Nadj-Perge, Stevan (2022) Evidence for unconventional superconductivity in twisted trilayer graphene. Nature, 606 (7914). pp. 494-500. ISSN 0028-0836. doi:10.1038/s41586-022-04715-z. https://resolver.caltech.edu/CaltechAUTHORS:20220621-285198100

[img] PDF (Supplementary Information Sections 1–5, including Supplementary Figs. 1–6 and Supplementary References) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 1: Spectroscopy of twisted bilayer and twisted trilayer graphene) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 2: Comparison between spectra on ABA and AAA sites at finite fields) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 3: Distinguishing dispersive band LLs and Dirac band LLs) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 4: Spectroscopy near v = −2) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 5: Spectral features around CNP and their comparison to the superconducting (SC) gap at v   ≈    −2.1) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 6: Andreev reflection signal from point contact spectroscopy of MATTG) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 7: Additional datasets showing magnetic field and temperature dependence of spectroscopic gap in the −3 < v < −2 range) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 8: Spectroscopic gap in the +2 < v < +3 range) - Supplemental Material
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[img] Image (JPEG) (Extended Data Fig. 9: Normalization of tunnelling conductance and fitting) - Supplemental Material
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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.


Item Type:Article
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https://doi.org/10.1038/s41586-022-04715-zDOIArticle
https://rdcu.be/cP54KPublisherFree ReadCube access
ORCID:
AuthorORCID
Kim, Hyunjin0000-0001-9886-0487
Choi, Youngjoon0000-0001-9783-5992
Lewandowski, Cyprian0000-0002-6944-9805
Thomson, Alex0000-0002-9938-5048
Zhang, Yiran0000-0002-8477-0074
Polski, Robert0000-0003-0887-8099
Watanabe, Kenji0000-0003-3701-8119
Taniguchi, Takashi0000-0002-1467-3105
Alicea, Jason0000-0001-9979-3423
Nadj-Perge, Stevan0000-0002-2916-360X
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.
Group:Institute for Quantum Information and Matter, Walter Burke Institute for Theoretical Physics
Funders:
Funding AgencyGrant Number
Office of Naval Research (ONR)N142112635
NSFDMR-1753306
Army Research Office (ARO)W911NF17-1-0323
Department of Energy (DOE)DE-SC0019166
NSFDMR-2005129
Alfred P. Sloan FoundationUNSPECIFIED
Institute for Quantum Information and Matter (IQIM)UNSPECIFIED
Gordon and Betty Moore FoundationGBMF1250
Gordon and Betty Moore FoundationGBMF8682
Walter Burke Institute for Theoretical Physics, CaltechUNSPECIFIED
Kwanjeong Graduate FellowshipUNSPECIFIED
Issue or Number:7914
DOI:10.1038/s41586-022-04715-z
Record Number:CaltechAUTHORS:20220621-285198100
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20220621-285198100
Official Citation:Kim, H., Choi, Y., Lewandowski, C. et al. Evidence for unconventional superconductivity in twisted trilayer graphene. Nature 606, 494–500 (2022). https://doi.org/10.1038/s41586-022-04715-z
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115208
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:21 Jun 2022 17:26
Last Modified:16 Aug 2022 18:49

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