Frequency dependence of the quality factor of Nb-NbxOy-Pb Josephson tunnel junctions

In the context of Josephson devices optimization for the Italian Voltage Standard manteinance, the frequency dependence of the quality factor (Q) of Nb-NbxOy-Pb tunnel junctions has been carefully measured at various temperatures in the frequency range 10-160 GHz. The Q was determined via the measure of the amplitude of Fiske resonant modes in the junction. The different frequencies were obtained by changing the lenght of the junction after the liquid helium test through a photolitographic process. The results show that for our junctions, for current densities of 1-10 Amp/cm²,the only relevant contributions to the Q are the losses associated with the surface impedance of the superconducting films. The data show good agreement with the BCS theory if, as already pointed out by other authors, a small residual term is added to the conductivity.