High-T-c gradiometric superconducting quantum interference device and its incorporation into a single-layer gradiometer

We describe a first-order gradiometric dc superconducting quantum interfere nce device (SQUID) and its incorporation into a first-order directly couple d single-layer gradiometer. The gradiometric SQUIDs were fabricated from a single layer of YBa2Cu3O7, with a silicon dioxide insulating layer and a go ld crossover structure. For several gradiometric SQUIDs, with estimated ind uctances of order 67 pH, we measured parasitic effective areas in the range 1-2 mu m(2), approximately two orders of magnitude lower than for conventi onal narrow linewidth SQUIDs of similar inductance. For a single-layer grad iometer incorporating a gradiometric SQUID, we measured a parasitic effecti ve area of 95 mu m(2). We demonstrate that for this device, the SQUID itsel f makes a negligible contribution to the overall parasitic effective area. We show that the improved balance leads to better performance in an unshiel ded environment. (C) 2000 American Institute of Physics. [S0003-6951(00)017 17-4].