Evidence for Dirac flat band superconductivity enabled by quantum geometry

Journal Article · Tue Feb 14 23:00:00 EST 2023 · Nature (London)

DOI:https://doi.org/10.1038/s41586-022-05576-2· OSTI ID:2345973

Tian, Haidong ^[1]; Gao, Xueshi ^[2]; Zhang, Yuxin ^[2]; Che, Shi ^[2]; Xu, Tianyi ^[3]; Cheung, Patrick ^[3]; Watanabe, Kenji ^[4]; Taniguchi, Takashi ^[5]; Randeria, Mohit ^[2]; Zhang, Fan ^[3]; Lau, Chun Ning ^[2]; Bockrath, Marc W. ^[2]

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1. The Ohio State Univ., Columbus, OH (United States); The Ohio State University
2. The Ohio State Univ., Columbus, OH (United States)
3. Univ. of Texas at Dallas, Richardson, TX (United States)
4. National Institute for Materials Science (NIMS), Tsukuba (Japan). Research Center for Functional Materials
5. National Institute for Materials Science (NIMS), Tsukuba (Japan). International Center for Materials Nanoarchitectonics

In a flat band superconductor, the charge carriers’ group velocity v_F is extremely slow. Superconductivity therein is particularly intriguing, being related to the long-standing mysteries of high-temperature superconductors and heavy-fermion systems. Yet the emergence of superconductivity in flat bands would appear paradoxical, as a small v_F in the conventional Bardeen–Cooper–Schrieffer theory implies vanishing coherence length, superfluid stiffness and critical current. Here, using twisted bilayer graphene, we explore the profound effect of vanishingly small velocity in a superconducting Dirac flat band system. Using Schwinger-limited non-linear transport studies we demonstrate an extremely slow normal state drift velocity v_n ≈ 1,000 m s^–1 for filling fraction ν between -1/2 and -3/4 of the moiré superlattice. In the superconducting state, the same velocity limit constitutes a new limiting mechanism for the critical current, analogous to a relativistic superfluid. Importantly, our measurement of superfluid stiffness, which controls the superconductor’s electrodynamic response, shows that it is not dominated by the kinetic energy but instead by the interaction-driven superconducting gap, consistent with recent theories on a quantum geometric contribution. Here we find evidence for small Cooper pairs, characteristic of the Bardeen–Cooper–Schrieffer to Bose–Einstein condensation crossover, with an unprecedented ratio of the superconducting transition temperature to the Fermi temperature exceeding unity and discuss how this arises for ultra-strong coupling superconductivity in ultra-flat Dirac bands.

• Research Organization: The Ohio State Univ., Columbus, OH (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division (CSGB); National Science Foundation (NSF); US Army Research Office (ARO); Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Japanese Society for the Promotion of Science (JSPS) (KAKENHI)
• Grant/Contract Number: SC0020187
• OSTI ID: 2345973
• Journal Information: Nature (London), Journal Name: Nature (London) Journal Issue: 7948 Vol. 614; ISSN 0028-0836
• Publisher: Nature Publishing Group
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
graphene
moiré materials
strongly correlated system
superconductivity