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-T
c
A15 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 Nb
3
Sn and V
3
Si 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.