Thermal Effects at Superconducting Point Contacts

Abstract

In this paper we have calculated $I-V$ curves of superconducting weak-link constriction junctions by assuming that there is a region of normal material which tends to spread with increasing power levels. The causes for the spreading of the normal resistance are twofold. One is the increase of the current-density distribution and the other is the increase of localized Joule heating at the contact as the total current is increased. The resultant rise in temperature of the link above the bath temperature, over the range of the $I-V$ characteristic, is found to be significant. Using material constants that are representative of bulk Nb, we found that the calculated $I-V$ characteristic is very similar to several experimentally observed Nb point-contact curves. The spreading normal-resistance analysis has suggested a model to explain the $I-V$ characteristic of a superconductor-normal-metal ($S-N$) point-contact system. A calculation has indicated that large excess temperatures are also present at the contacts when biased in the millivolt region. These findings have prompted us to review several published experiments with $S-N$ contacts.