EXCESS NOISE IN YBA2CU3O7 THIN-FILM GRAIN-BOUNDARY JOSEPHSON-JUNCTIONS AND DEVICES

The subject of electronic noise in high-T-c superconducting Josephson devices and in their applications is considered. Several types of grai n boundary junctions, prepared in different ways by four separate inte rnational laboratories, are fully characterized in terms of their elec trical and noise properties at a range of temperatures, frequencies, m agnetic fields and in the presence of microwaves. Similar characteriza tion is carried out for multijunction Josephson flux-flow arrays, and bi-epitaxial SQUIDs. The theory of Likharev and Semenov for thermal no ise in low-T-c junctions is adopted as a reference against which exces s noise can be identified in high-T-c junctions. It is combined with t heoretical models of excess noise based on fluctuations of critical cu rrent and resistance to provide accurate fitting of the observed noise curves. It is shown that the normalized levels of critical current an d normal resistance fluctuations for all of the junction types examine d are remarkably close (typically around delta l(c)/l(c) = 1 x 10(-5) Hz(-1/2) and delta R(n)/R(n) = 4 x 10(-6) Hz(-1/2) at 100 Hz) and are nearly independent of temperature in the range T = 30-80 K. The freque ncy dependence is close to the universal 1/f law with some deviations due to trapping of charge carriers at discrete states in the tunnellin g barrier. Measurements of voltage noise levels and critical current f luctuations in multijunction flux-flow amplifiers, reported for the fi rst time, are consistent with levels in single junctions of the same t ype. Excess noise is studied as a function of external magnetic field, in a way that has not been previously described. A new interpretation of the magnetic field-dependent noise data indicates that absolute le vels of critical current fluctuations are nearly independent of applie d magnetic field. This observation does not appear to have been report ed elsewhere. It has important implications for the operation of grain boundary Josephson devices in real applications. Voltage noise in the presence of microwave irradiation, at operating points between Shapir o steps, is consistent with a modified form of the Likharev-Semenov eq uation which takes into account the known level of critical current fl uctuations as measured in field-free conditions. The implications of t hese results for further improvements in device performance are discus sed, and directions for further work are suggested.