WASHER RESONANCES OF DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE COUPLED TO MULTITURN INPUT COIL

Properties of washer resonances in dc superconducting quantum interference devices (SQUIDs) coupled to a multiturn input coil have been studied using an expanded model of the coupled SQUID. The condition of resistances required to suppress the washer resonance is clarified. In the coupled SQUID, junction shunt resistances should be determined by taking into account this condition in addition to the conventional condition beta(c) < 1, where beta(c) is the McCumber parameter. In the case of small junction capacitances, it is shown that the washer resonance limits the value of junction shunt resistances, and prevents the use of large resistances given by beta(c) < 1. In order to solve this problem, a new washer geometry of the SQUID is proposed. From the experiment using the expanded model of the new washer, it is shown that the new washer can control the washer resonances, and enables the use of large resistance in the coupled SQUID. Therefore, the new washer will be useful for the improvement of the performance of the coupled SQUID.