The nontrivial effects of annealing on superconducting properties of Nb single crystals

Abstract

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy methods. Pieces of the same crystal boule were studied before and after the annealing at 800 C, 1400 C, and near the melting point of niobium (2477 C). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 C. However, the overall superconducting properties of the annealed samples did not improve and, in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 C, measured, annealed at 1700 C, and measured again. The STM revealed a ``dirty'' superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 C. The clean gap was recovered after annealing at 1700 C. This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. 
Our results indicate that vacuum annealing at least up to 1400 C, while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from $\Gamma=0.2$ to about $\Gamma=0.4$ after annealing at 1400 C. These results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency (SRF) cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.