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

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 T_c 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 ρ_s in LaAlO₃/KTaO₃(111) (LAO/KTO(111)) heterostructures. Under electrostatic gating, the diamagnetic transition temperature T_c^MI first increases in proportion to ρ_s, then saturates beyond a threshold and becomes independent of ρ_s, 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.