Hc1of niobium stannide: 200 gauss or over 1000 gauss?

The microwave surface resistance of a type II superconductor is a very sensitive measure of the density of vortices in the region a few penetration depths from the sample surface. The surface resistance can be quite insensitive to the usual defect pinning and provides an excellent method for studying the often difficult to measure lower critical field Hc1. Various surface barrier effects exhibit themselves quite clearly. We have examined the low field properties of several superconductors including a series of Pb-In alloys. The values of Hc1we deduce from these measurements correlate very well with the values determined by earlier magnetization studies. We find that the low field microwave properties of highly stoichiometric, low flux pinning samples of Nb3Sn are qualitatively the same as those of the Pb-In alloys and other materials. The Hclwe determine for Nb3Sn is 1400 ± 100 gauss. This is well above the values for Hc1widely assumed. We believe that previous determinations are in error. The low ac loss and the lack of flux penetration in Nb3Sn, which has been interpreted in terms of a low Hcland a delayed flux entry by surface barriers, is actually largely due to the high Hc1in most cases. The favorable implications for practical applications of this high Hc1for Nb3Sn are discussed.