Evidence for unconventional superconductivity in twisted bilayer graphene

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^1–4, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened^5,6. 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 T_c 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 Δ_T, which far exceeds the mean-field BCS ratio (with 2Δ_T/k_BT_c ~ 25), and the gap Δ_AR extracted from Andreev reflection spectroscopy (2Δ_AR/k_BT_c ~ 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.