THE EFFECTIVE DISSIPATION IN NB ALOX/NB JOSEPHSON TUNNEL-JUNCTIONS BYRETURN CURRENT MEASUREMENTS/

Measurements of temperature dependence of the return current in high q uality Nb/AlOx/Nb Josephson junctions are presented. From the experime ntal data, we obtain the effective resistance, i.e., the effective dis sipation, for the retrapping process, according to the generalized res istively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the meas ured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, whic h includes the quasiparticle and the quasiparticle-pair interference t erms, is responsible for the return process. The measurements have bee n performed on various samples, in a wide range of critical current de nsities from 50 to 2250 A/cm(2), covering different damping regimes an d spanning over the high and low temperature limits. Junctions with lo w critical current density show ideal dissipation which makes the retu rn current scale with temperature according to the BCS exponential beh avior without flattening out effects. This result may be relevant for the possible use of Nb/AlOx/Nb junctions in macroscopic quantum cohere nce experiments, which strongly require a very low dissipation. (C) 19 97 American Institute of Physics.