Title: 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 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. Thesemore » findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.« less

Authors:

Oh, Myungchul  ^[1];

• Search DOE PAGES for author "Oh, Myungchul"
• Search DOE PAGES for ORCID "0000-0003-0477-1390"

Nuckolls, Kevin P.  ^[1];

• Search DOE PAGES for author "Nuckolls, Kevin P."
• Search DOE PAGES for ORCID "0000-0002-1078-7113"

Wong, Dillon ^[1]; Lee, Ryan L. ^[1]; Liu, Xiaomeng  ^[1];

• Search DOE PAGES for author "Liu, Xiaomeng"
• Search DOE PAGES for ORCID "0000-0002-0039-7090"

Watanabe, Kenji  ^[2];

• Search DOE PAGES for author "Watanabe, Kenji"
• Search DOE PAGES for ORCID "0000-0003-3701-8119"

Taniguchi, Takashi  ^[2];

• Search DOE PAGES for author "Taniguchi, Takashi"
• Search DOE PAGES for ORCID "0000-0002-1467-3105"

Yazdani, Ali  ^[1]

• Search DOE PAGES for author "Yazdani, Ali"
• Search DOE PAGES for ORCID "0000-0003-4996-8904"

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+ Show Author Affiliations

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

Publication Date:

Wed Oct 20 00:00:00 EDT 2021

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