On the temperature and frequency dependence of the surface resistance of superconducting Nb

Surface resistances R(T,f)of superconducting Nb show Δ-values well below the bulk value and a frequency dependence deviating from homogeneous Nb. So, surface resistances in various modes between 12 and 18 GHz have been measured for the first time. As results, Δ/kT
c
is increasing from 1.83 to 1.87 with frequency, and
R_{sc}(4.2 K) \propto f^{1.63 \pm 0.02}
has been fitted to the data. This with
f
improving agreement with what is anticipated from the BCS theory of homogeneous Nb can either be explained by rf induced superconductivity or by h.f being larger than the smearing of the density of states by weak superconducting spots.