Inhomogeneities in superconducting niobium surfaces

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Abstract
Inhomogeneities in superconducting Nb have considerable influence on ac losses in cables; they also give bad rf results. To study inhomogeneities near Nb surfaces, the magnetic induction profile B (x) and hence the critical current density jc(x) were determined as a function of distance x from the surface in samples which were machined, annealed, and chemically polished or electro‐polished and anodized. With identical samples the magnetization and 18‐kHz penetration depths were also measured as a function of dc magnetic field, applied parallel to the surface. These measurements on Nb samples, which were all exposed to air, indicate that two superconducting phases exist near the surface. One of them has an Hc2 much higher (50%) and the other somewhat higher (10%) than Hc2 in the bulk. In addition, strong pinning exists near the surface as evidenced by jc higher than 106 A/cm2. These effects were found, although to a somewhat lesser extent, even after UHV annealing. The high Hc2 values and strong pinning near the surface, even above the bulk Hc2, can be explained by the presence of inhomogeneities there.