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 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 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. Thesemore »
- Authors:
-
- Princeton Univ., NJ (United States)
- National Institute for Materials Science (NIMS), Tsukuba (Japan)
- Publication Date:
- Research Org.:
- Princeton Univ., NJ (United States)
- Sponsoring Org.:
- USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF); Gordon and Betty Moore Foundation; Japan Society for the Promotion of Science (JSPS)
- OSTI Identifier:
- 1842831
- Grant/Contract Number:
- FG02-07ER46419; NSF-DMR-2011750; NSF-DMR-1904442; PHY-1607611; GBMF9469; GBMF9616; JPMXP0112101001; 19H05790; JP20H00354
- Resource Type:
- Accepted Manuscript
- Journal Name:
- Nature (London)
- Additional Journal Information:
- Journal Name: Nature (London); Journal Volume: 600; Journal Issue: 7888; Journal ID: ISSN 0028-0836
- Publisher:
- Nature Publishing Group
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; superconductivity; correlated insulators; magic-angle twisted bilayer graphene
Citation Formats
Oh, Myungchul, Nuckolls, Kevin P., Wong, Dillon, Lee, Ryan L., Liu, Xiaomeng, Watanabe, Kenji, Taniguchi, Takashi, and Yazdani, Ali. Evidence for unconventional superconductivity in twisted bilayer graphene. United States: N. p., 2021.
Web. doi:10.1038/s41586-021-04121-x.
Oh, Myungchul, Nuckolls, Kevin P., Wong, Dillon, Lee, Ryan L., Liu, Xiaomeng, Watanabe, Kenji, Taniguchi, Takashi, & Yazdani, Ali. Evidence for unconventional superconductivity in twisted bilayer graphene. United States. https://doi.org/10.1038/s41586-021-04121-x
Oh, Myungchul, Nuckolls, Kevin P., Wong, Dillon, Lee, Ryan L., Liu, Xiaomeng, Watanabe, Kenji, Taniguchi, Takashi, and Yazdani, Ali. Wed .
"Evidence for unconventional superconductivity in twisted bilayer graphene". United States. https://doi.org/10.1038/s41586-021-04121-x. https://www.osti.gov/servlets/purl/1842831.
@article{osti_1842831,
title = {Evidence for unconventional superconductivity in twisted bilayer graphene},
author = {Oh, Myungchul and Nuckolls, Kevin P. and Wong, Dillon and Lee, Ryan L. and Liu, Xiaomeng and Watanabe, Kenji and Taniguchi, Takashi and Yazdani, Ali},
abstractNote = {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 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.},
doi = {10.1038/s41586-021-04121-x},
journal = {Nature (London)},
number = 7888,
volume = 600,
place = {United States},
year = {Wed Oct 20 00:00:00 EDT 2021},
month = {Wed Oct 20 00:00:00 EDT 2021}
}
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