A.C. properties of Nb-NbxOy-Pb Josephson tunnel junctions for 2e/H measurements

In order to improve the Italian voltage standard via the Josephson effect at IEN some results have been obtained in the study of the a.c. properties of Nb- -NbxOy-Pb junctions. Our devices, characterized by a very high geometrical definition, usually exhibit a resonant frequency of about 10.5 GHz with a length in the microwave direction of 0.52 mm. A fine adjustment (better than 0.5%) of the resonant frequency of a single junction can be achieved after testing at 4.2 K by means of a photolithographic procedure. This technique makes easier to couple junctions in series with the microwave signal. Some properties of the radiated junctions have been studied by means of "Q" measurements obtained by both the analysis of self-resonant modes behavior and the junction response to a microwave signal. Q values as high as 130 at 4.2 K has been evaluated. The results, also discussed in comparison with data derived from Auger spectroscopy, give interesting information about the influence of the dielectric barrier and its metallic interfaces on the coupling to the microwave. Measurements of 2e/h at 5 mV level have been carried out successfully.