Experiments on Josephson Junctions Driven by a Biharmonic rf Source

The amplitude of the steps at the reference voltages V n = nf(h/2e) in the I-V characteristic of a Josephson junction RF biased at a frequency f is an important parameter in designing devices for voltage standards. In fact the stability of the lock between the junction phase and the applied microwave field, which is a crucial problem for voltage standard application, is strongly dependent on the step amplitude1. It is well-known that in the simplified model of a voltage biased junction, where the spatial dependence of the phase is neglected, the expected amplitude ?I n of the nth step is a fraction of the critical current I c given by the Bessel function of the first kind of integer order n, as in the equation2: