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Evidence for unconventional superconductivity in twisted bilayer graphene

Abstract

The emergence of superconductivity and correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors1,2,3,4, as described by the Bardeen–Cooper–Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened5,6. This suggests that pairing in MATBG might be conventional in nature and a consequence of the large density of states of its flat bands. Here we combine tunnelling and Andreev reflection spectroscopy with a scanning tunnelling microscope to observe several key experimental signatures of unconventional superconductivity in MATBG. We show that the tunnelling spectra below the transition temperature Tc are inconsistent with those of a conventional s-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunnelling gap ΔT, which far exceeds the mean-field BCS ratio (with 2ΔT/kBTc ~ 25), and the gap ΔAR extracted from Andreev reflection spectroscopy (2ΔAR/kBTc ~ 6). The tunnelling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase. Moreover, the pseudogap and superconductivity are both absent when MATBG is aligned with hexagonal boron nitride. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.

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Fig. 1: STS of the tunnelling gap of superconducting MATBG.
Fig. 2: PCS and Andreev reflection for MATBG.
Fig. 3: Tunnelling and Andreev reflection spectra curve fits.
Fig. 4: Pseudogap regime and phase diagram of hole-doped MATBG.
Fig. 5: DT-STS and DT-PCS on non-superconducting MATBG aligned to hBN.

Data availability

The data that support the findings of this study are available at https://doi.org/10.5281/zenodo.5722484.

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Acknowledgements

We thank P. Jarillo-Herrero, A. H. MacDonald and S. A. Kivelson for helpful discussions. We thank C.-L. Chiu, G. Farahi and H. Ding for helpful technical discussions. This work was primarily supported by the Gordon and Betty Moore Foundation’s EPiQS initiative grant GBMF9469 and DOE-BES grant DE-FG02-07ER46419 to A.Y. Other support for the experimental work was provided by NSF-MRSEC through the Princeton Center for Complex Materials NSF-DMR- 2011750 and NSF-DMR-1904442, ExxonMobil through the Andlinger Center for Energy and the Environment at Princeton, and the Princeton Catalysis Initiative. A.Y. acknowledges the hospitality of the Aspen Center for Physics, which is supported by the National Science Foundation grant PHY-1607611, and Trinity College, Cambridge, UK, where part of this work was carried out with the support of, in part, a QuantEmX grant from ICAM and the Gordon and Betty Moore Foundation through the grant GBMF9616. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, grant JPMXP0112101001, and JSPS KAKENHI grants 19H05790 and JP20H00354.

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Contributions

M.O., K.P.N., D.W. and A.Y. designed the experiment. M.O., D.W. and K.P.N. fabricated the devices used for the study. M.O., K.P.N., D.W. and R.L.L. carried out STM/STS and PCS measurements, with invaluable input from X.L. on the latter. M.O., D.W. and K.P.N. performed the data analysis. K.W. and T.T. synthesized the hBN crystals. All authors discussed the results and contributed to the writing of the manuscript.

Corresponding author

Correspondence to Ali Yazdani.

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Peer review information Nature thanks Dante Kennes and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

This file contains Supplementary Sections A–L, including text and data, tables and Supplementary Figs. 1–16. See contents page for details.

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Oh, M., Nuckolls, K.P., Wong, D. et al. Evidence for unconventional superconductivity in twisted bilayer graphene. Nature (2021). https://doi.org/10.1038/s41586-021-04121-x

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