Critical current and SQUID operation of granular films

The range of operation of common thin film SQUIDS is severely limited due to the high critical current density of usual superconductors. Orders of magnitudes are as follows: thin film SQUIDS with cross sections of the order of 10-9 cm2 will typically operate only within 50 mK to 100 mK from Tsub(c). Since much smaller cross sections are difficult to achieve, other geometries such as the Notary's proximity effect bridge have been proposed to reduce the critical current density. Another solution is proposed: the use of low critical current density granular superconducting films. The critical current of granular Al-Al2O3 thin film bridges of about 1000 A in thickness and 10 μm to 20 μm in width has been measured. (Auth.)