Low-noise high-T/sub c/ DC SQUIDs at 77 K

Low-noise DC superconducting quantum interference devices (SQUIDs) have been fabricated using a reproducible superconductor-normal metal-superconductor (SNS) step-edge technique. The superconductor is YBa/sub 2/Cu/sub 3/O/sub 7-x/ and the normal metal is either Ag or an Ag-Au alloy. At 77 K, the energy sensitivity for the SQUIDs with the smallest inductance, L=12 pH, is below 9*10/sup -30/ J/Hz at 1 Hz. The energy sensitivity of SQUIDs with L=70 pH is 5*10/sup -29/ J/Hz at 1 Hz. These measured noise values are sufficient for many practical applications. The larger SQUID inductance is also adequate to permit coupling to multiturn input coils. Thin-film input coils with as many as 30 turns have been developed. Critical current densities for the multiturn coils routinely exceed 10/sup 6/ A/cm/sup 2/ at 77 K.<
>