Voltage induced variations of the tunnel barrier in Nb/Pb junctions

It has been recently shown that, in Nb/Pb junctions having the tunnel barriers grown by r.f. plasma oxidation, large bias voltages induce at room temperature permanent but reversible changes of the I-V characteristics and in particular changes of the Rnn values. In this paper similar variations of thermally oxidated junctions are reported, and a more simple behaviour has been found. Different IV characteristics, corresponding to different values of Run, have been drawn at 77 K on both r.f. and thermal junctions. The voltage dependences of both the conductance and its logarithmic derivative have been studied. A preliminary evaluation of the induced variations of the tunnel barrier parameters is also performed.