Superconducting properties of niobium single crystals upon annealing in vacuum at different temperatures*

Niobium hydrides are known to be detrimental for superconducting applications. Niobium has a high affinity for hydrogen, and a significant amount of hydrogen can be accumulated during the production of niobium and fabrication devices such as RF cavities for accelerator and quantum information research. To degas hydrogen, vacuum annealing in the 600-1000 °C range is usually used. Here we use magneto-optical imaging, DC and RF susceptibility measurements to study the effects of annealing at 800°C, 1400°C, and near melting on the London penetration depth, superconducting transition temperature, upper critical field, and flux pinning. Although large hydrides (tens of micrometers) no longer appear already after 800 °C annealing, the pinning is still elevated, the transition temperature is suppressed, and the upper critical field is enhanced – all pointing to the significant presence of nanoscale defects (comparable with the coherence length ~ 20 nm). Only bringing the sample close to the melting point recovers Tc and significantly reduces pinning. This suggests that ultra-high-temperature annealing is needed to achieve the best performance of niobium devices.