Equilibrium states in multijunction superconducting quantum interferometers

Multijunction superconducting quantum interferometers can be created spontaneously by intrinsic junctions in granular high-T-c materials, and as a by-product of step-edge technology used to fabricate high-T-c thin film de superconducting quantum interference devices. The properties of such systems are relatively little known. The theory predicts multivalued dependence of critical current I-c on applied magnetic flux Phi(e) and the appearance of different stationary `'phase states'' related to different possible relationships between the phases of superconducting order parameter in individual junctions. We report on the measurements performed on a (2X2) quantum interferometer, comprising a series array of two underdamped all-niobium tunnel junctions in each parallel arm. Experimental results closely follow the theoretical predictions and reveal several instability regions, in which noise generation takes place. These findings can possibly be related to the random telegraph noise observed in high-T-c materials.