Magneto-optical study of the Effects of Annealing on Superconducting Properties of Niobium Single Crystals*

Niobium has a high affinity for hydrogen and can absorb it from solvents and even humid air. Upon cooling below 180 K, niobium hydrides precipitate. These precipitates form defects that decrease the performance of Nb-based devices used for applications in quantum computing and accelerators. In RF cavities, they lead to the so-called Q-disease. Vacuum annealing above 1073 K will remove hydrogen. Here, we use magneto-optical imaging to obtain spatially resolved profiles of the magnetic induction B(r) across the sample surface and deduce the effective critical current density. Although large (tens of micrometers) hydrides no longer appear after annealing at 1073 K when cooling below 180 K, the pinning remains elevated even after 1673 K annealing, implying that some, possibly nanoscale, hydrides still remain in the samples. Only bringing the sample close to the melting point significantly reduces pinning.