Design of stable thin-film Josephson tunnel junctions for the maintenance of voltage standards

The behaviour of Josephson tunnel junctions is theoretically investigated in order to determine the junction preparation parameters which allow one to obtain a junction, which, when subjected to microwave radiation produces a convenient constant-voltage current step structure at high bias voltages. Major junction characteristics, like geometrical dimensions, normal tunnel resistance RN, critical current Io, are taken into account, as well as various coupling conditions between junction and microwave radiation. Stable Nb-oxide-Pb junctions having various electrical characteristics for rather different geometrical dimensions have been prepared and tested. The experimental results are in good agreement with our calculations.