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 superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened. This suggests that pairing in MATBG might be conventional in nature and a consequence of the large density of states (DOS) of its flat bands. Here we combine tunnelling and Andreev reflection spectroscopy with the scanning tunnelling microscope (STM) to observe several key experimental signatures for 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 (hBN). These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.
Magic-angle graphene is an incredible multi-functional material, easily tuned amongst a diverse set of quantum phases by changing its temperature, magnetic field, and electronic density. Here, researchers have uncovered essential signatures of its unconventional superconducting phase (yellow), which conducts electricity with zero resistance and zero energy loss, and its previously unknown pseudogap regime (blue), a seemingly necessary precursor to superconductivity.
M. Oh, K. P. Nuckolls, D. Wong, R. L. Lee, X. Liu, K. Watanabe, T. Taniguchi and A. Yazdani,
"Evidence for unconventional superconductivity in twisted bilayer graphene," Nature 600, 240-245 (2021). Free link.
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Video ▶: Graduate student and co-author, Kevin Nuckolls, explains the team's research results.