EXPERIMENTAL INVESTIGATION OF THE THRESHOLD CURVES OF A 4-JUNCTION SQUID

We have studied experimentally the variation of critical current with control current of superconducting quantum interference devices (SQUID s). We used an integrated SQUID consisting of four Josephson junctions with nominally equal critical currents. One current (the bias current ) is fed directly to the four junctions in parallel, while a second cu rrent (the control current) is coupled inductively into the SQUID loop s. The fabricated SQUID functions properly as designed and exhibits th e threshold curves expected from calculation. By suitable design of th e SQUID, we have realized for the first time threshold curves with mor e than two visible minor lobes. This was previously predicted but not observed.