Nontrivial behavior of hydrides in the superconducting state of high-purity niobium used for high-Q resonant cavities

The formation of niobium hydrides is a known issue for superconducting applications of high purity niobium, for example, in high-quality factor resonant cavities. The hydrides precipitation occurs roughly below 150 K upon the first cooldown, and the final structure of the precipitates depends on the material's thermal cycling history, making it challenging to study. A wide array of experimental probes was employed over the years for their studies, yet the microscopic behavior of the hydrides in the superconducting state is not established. It has always been assumed that these non-superconducting precipitates are detrimental to applications. Here we report direct visualization of hydrides-rich phase that forms well-outlined "boomerang" shape regions tens of micrometers in scale. Surprisingly, these regions show a much higher critical current density compared to the surrounding areas. We suggest that in this case, nanoscale hydride precipitates act as strong pinning centers and stabilize type-II behavior of low-kappa high-purity niobium. A more systematic investigation is planned to fully understand this physics and, perhaps, find regimes where hydrides would be not detrimental to superconducting applications that benefit from enhanced pinning.