RF surface resistance of Nb3Sn, NbZr, and NbN thin films

The surface resistance of Nb3Sn films prepared by electron-beam co-deposition and magnetron sputtering has been measured at 8.6 GHz and from 1.5 K through Tc. We find that carefully controlled deposition temperatures of ≈900 C are necessary to obtain the lowest-loss materials. In both evaporated and sputtered samples with stoichiometric composition (Tc= 17.8 K), we see sharp transitions. In off-stoichiometric samples, however, the transition is lower and wider, yielding excessive losses. The residual losses, normal-state surface resistance, and the reduced gap values we obtain for our Nb3Sn samples are also discussed. The surface resistance of a NbZr alloy film and a NbN sample obtained from Hypres, Inc. are also reported.