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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The data that support the findings of this study are available at https://doi.org/10.5281/zenodo.5722484.
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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.
The authors declare no competing interests.
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Oh, M., Nuckolls, K.P., Wong, D. et al. Evidence for unconventional superconductivity in twisted bilayer graphene. Nature 600, 240–245 (2021). https://doi.org/10.1038/s41586-021-04121-x
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