Evidence for unconventional superconductivity in twisted bilayer graphene

Oh, Myungchul; Nuckolls, Kevin P.; Wong, Dillon; Lee, Ryan L.; Liu, Xiaomeng; Watanabe, Kenji; Taniguchi, Takashi; Yazdani, Ali

doi:10.1038/s41586-021-04121-x

Evidence for unconventional superconductivity in twisted bilayer graphene

Journal Article · Wed Oct 20 00:00:00 EDT 2021 · Nature (London)

DOI:https://doi.org/10.1038/s41586-021-04121-x· OSTI ID:1842831

^[1]; ^[2]; Wong, Dillon ^[2]; Lee, Ryan L. ^[2]; ^[2]; ^[3]; ^[3]; ^[2]

Princeton Univ., NJ (United States); Princeton University
Princeton Univ., NJ (United States)
National Institute for Materials Science (NIMS), Tsukuba (Japan)

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 screened5. This suggests that pairing in MATBG might be conventional in nature and a consequence of the large density of states of its flat bands. In this work, 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 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. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.

View Accepted Manuscript (DOE)

Research Organization:: Princeton Univ., NJ (United States)

Sponsoring Organization:: Gordon and Betty Moore Foundation; Japan Society for the Promotion of Science (JSPS); National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: FG02-07ER46419

OSTI ID:: 1842831

Journal Information:: Nature (London), Journal Name: Nature (London) Journal Issue: 7888 Vol. 600; ISSN 0028-0836

Publisher:: Nature Publishing GroupCopyright Statement

Country of Publication:: United States

Language:: English

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Data for : Evidence for unconventional superconductivity in twisted bilayer graphene Oh, Myungchul; Nuckolls, Kevin P.; Wong, Dillon Zenodo https://doi.org/10.5281/zenodo.5722484	dataset	January 2021

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