Intrinsic noise characteristics of NbN SQUID’s

Abstract

Thin-film NbN SQUID’s with granular weak links have been fabricated and their rf I-V characteristics measured at 23 MHz. Three different temperature variations of the critical currents were observed. A Josephson-like temperature dependence for those devices with strongly coupled grains and a bulk-mean-field-like dependence for those with the weakest coupled grains were found. An intermediate case where the critical current follows the bulk-mean-field theory at low critical currents and switches over to Josephson-like for larger currents was also observed. The noise properties of these devices were deduced from the measurement of the step rise parameter a. The data were in good agreement with the theory of Kurkijarvi over a wide range of temperature and critical currents for those devices with a bulk-mean-field-like critical current dependence and a low critical current density (∼104 A/cm2). Other devices were in agreement over a much smaller region of temperature and critical current. These devices are rugged, stable, and show very little change in their characteristics after extended storage at room temperature.