Evidence for Dirac flat band superconductivity enabled by quantum geometry

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(1) and heavy-fermion systems(2). 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(3-7), we explore the profound effect of vanishingly small velocity in a superconducting Dirac flat band system(8-13). Using Schwinger-limited non-linear transport studies(14,15) we demonstrate an extremely slow normal state drift velocity v(n)1,000 m s(-1) for filling fraction upsilon between -1/2 and -3/4 of the moire superlattice. In the superconducting state, the same velocity limit constitutes a new limiting mechanism for the critical current, analogous to a relativistic superfluid(16). 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(8-12). We find evidence for small Cooper pairs, characteristic of the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation crossover(17-19), 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.

Automated summary

In a flat band superconductor, the charge carriers' velocity is extremely slow, leading to peculiar superconducting behavior. Using twisted bilayer graphene, the researchers investigate the effect of the slow velocity on the superconducting state. They find evidence for small Cooper pairs and discuss the unusual nature of ultra-strong coupling superconductivity in ultra-flat Dirac bands.