Microwave Properties of Nb_3Sn Films on Sapphire Substrates

摘要

We have prepared Nb_3Sn films on sapphire by a combination of two well-approved processes. First, Nb precursor films are sputtered onto the substrates which are then converted into Nb_3Sn by Sn-vapour diffusion at 1150 °C. The polycrystalline films show grain sizes around 2μm. Measurements of the temperature dependent surface resistance R. revealed a critical temperature T_c = 18.0K and an energy gap Δ/kT_c up to 2.3, reflecting strong coupling. At 4.2K and 19 (87)GHz, R_s dropped below the sensitivity limit of the measurement systems of 15 (500)μΩ. Small penetration depths and large mean free paths indicated the weak quasiparticle scattering. The transport properties are comparable to Nb_3Sn layers on bulk Nb, also prepared by tin vapour diffusion. Above microwave fields B_s approx= 15mT, R_S(B_S) increases strongly. R_s measurements in superposed dc magnetic field revealed no clear evidence for magnetically limited power handling. Significant influence from the substrate material were concluded from analogous observations with Nb films, hinting for thermal effects.