IMPURITY NOISE IN CURRENT-BIASED SUPERCONDUCTING WEAK LINKS

Impurities in the vicinity of a weak link, modeled as a ballistic supe rconducting quantum point contact, fluctuate in rime and generate low- frequency voltage noise. Adopting the RSJ-model giant fluctuations are found for bias currents (I-B) slightly exceeding the critical current (I-C). This enhanced noise level is caused by switching of a mode-qua ntized super current - in the absence of quantization noise is vanishi ngly small. Minimal noise is always attained at I-B = root 2I(C)(T). I n the presence of many impurities the noise spectrum is of the form S( omega) proportional to omega(-1+beta). The order of magnitude of the e xponent beta is universally found to be 1/ln (Gamma(max)/Gamma(min)) w here Gamma(max) and Gamma(min) are the maximum and minimum rates of tr ansitions, respectively, of the impurities.