Effect of surface metallurgy on the penetration depth and rf breakdown field of superconducting niobium

The surface metallurgy of niobium is discussed, including factors such as impurity distributions at the surface and surface topography. Measurements of the penetration depth are presented both in the clean limit, where there is about a 20 A region of oxygen at the surface (after high temperature heat treatment) affecting the boundary condition on the GL order parameter; and in the dirty limit, where heating in an oxygen atmosphere near 10-6 torr can greatly affect the surface roughness. Auger measurements of the surface conditions in a 1 percent Zr in Nb alloy are also presented. A discussion is given of preliminary results in a special apparatus where rf breakdown experiments can be performed on small samples and correlated with the metallurgical factors mentioned above