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Wednesday, April 6, 2022 1:30pm to 2:30pm

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In a flat band superconductor, the charge carriers’ group velocity vf is extremely slow, quenching their kinetic energy. The emergence of superconductivity thus appears paradoxical, as conventional BCS theory implies a vanishing coherence length, superfluid stiffness, and critical current. Here, using twisted bilayer graphene (tBLG), we explore the profound effect of vanishingly small vf in a Dirac superconducting flat band system. Using Schwinger-limited non-linear transport studies we demonstrate an extremely slow vf ~ 1000 m/s for filling fraction 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 pairs, characteristic of the BCS to Bose-Einstein condensation (BEC) crossover, with a surprising ratio of the superconducting transition temperature to the Fermi temperature exceeding unity, indicating very strong coupling superconductivity in ultra-flat Dirac bands.

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