Crossover from BEC-like to BCS-like behavior and paramagnetic Meissner effect in LaAlO3/KTaO3(111) interface superconductivity.

Superconductivity has been discovered in a variety of heterostructures, yet the general principles governing interface superconductivity remain elusive. Superfluid density, or superfluid stiffness, is a key parameter for characterizing the superconducting state, and its relation to the transition temperature Tc is central to understanding the superconducting mechanism. Measuring the superfluid stiffness of interface superconductors is, however, technically challenging because the signal arises from an extremely small volume near the interface. Here, we overcome this challenge by developing a highly sensitive two-coil mutual inductance technique compatible with a dilution refrigerator, enabling systematic measurements of the superfluid stiffness ρs in LaAlO3/KTaO3(111) (LAO/KTO(111)) heterostructures. Under electrostatic gating, the diamagnetic transition temperature TcMI first increases in proportion to ρs, then saturates beyond a threshold and becomes independent of ρs, signaling a crossover from Bose-Einstein-condensate-like to Bardeen-Cooper-Schrieffer-like behavior. We propose that these results arise from emergent granular superconductivity, an interpretation further supported by the observation of a paramagnetic Meissner effect under small applied magnetic fields.