RF surface resistance of high-Tcsuperconducting A15 thin films

A calorimetric apparatus for measuring the surface resistance of thin film superconductors has been developed and applied to the study of high-TcA15 materials. The apparatus is capable of measurement at 8.6 GHz over a temperature range of 1.5-20K. The samples were deposited using electron-beam co-evaporation on sapphire substrates. The effective magnetic loss tangent of the sapphire substrates has been observed to have a value as low as 4.2×10^-8. Surface resistance data are presented for the A15´s Nb3Sn and V3Si and for the elements Nb and Sn. Structure in the surface resistance of A15´s indicate material inhomogeneities that have been linked to temperature variations during deposition. Use of an improved style of substrate holder has greatly reduced this structure.