LC-resonant voltage response of superconducting quantum interference filters

The voltage vs. magnetic field relation V(B) of superconducting interferometers consisting of two or more Josephson junctions is the basic characteristic of their operation as highly sensitive magnetometers and amplifiers. The conversion efficiency /spl part/V//spl part/B of interferometers containing two Josephson junctions, i.e. dc SQUIDs, decreases significantly with increasing loop inductance L. Therefore, the range of SQUID loop sizes for which the SQUID operates properly is very restricted. However, for sufficient coupling between the SQUID and the signal coil a large value of L is desirable. In this work theoretical and experimental results on the voltage response function of quantum interference filters (SQIFs) are presented. A SQIF consists of N Josephson junctions connected in parallel to form a 1D array. The array loop sizes are chosen such that the voltage response becomes a unique function around B=0. It is shown that for McCumber parameters /spl beta//sub C/>0.5 and sufficiently large array inductances the V(B) relation of SQIFs shows LC-resonances that lead to two pronounced minima in the response function. In the LC-resonant operation mode the conversion efficiency of SQIFs is not degraded even for large array inductances. By this, a very strong coupling to signal coils can be achieved. The experimental results agree very well with the theoretical predictions on which basis the SQIF has been fabricated.