Evidence for Dirac flat band superconductivity enabled by quantum geometry

• Published in: Nature (London) Vol. 614; no. 7948; pp. 440 - 444
• Main Authors: Tian, Haidong, Gao, Xueshi, Zhang, Yuxin, Che, Shi, Xu, Tianyi, Cheung, Patrick, Watanabe, Kenji, Taniguchi, Takashi, Randeria, Mohit, Zhang, Fan, Lau, Chun Ning, Bockrath, Marc W
• Format: Journal Article
• Language: English
• Published: England Nature Publishing Group 16.02.2023
• Subjects: BCS theory; Coherence length; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Cooper pairs; Critical current (superconductivity); Current carriers; Fermions; Fluids; Graphene; Group velocity; High temperature; Kinetic energy; Magnetic fields; moiré materials; Stiffness; strongly correlated system; Superconductivity; Superfluidity; Superlattices; Transition temperature; Transition temperatures; Velocity
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-022-05576-2

In a flat band superconductor, the charge carriers' group velocity v 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 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  ≈ 1,000 m s 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 . 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.