High-transition-temperature superconducting quantum interference devices

The advent of high-T-c superconductors gave great impetus to the developmen t of thin-film superconducting quantum interference devices (SQUIDs) for op eration at temperatures up to the boiling point of liquid nitrogen, 77 K. T he spectral density of the white flux noise can be calculated analytically for rf SQUIDs and by computer simulation for de SQUIDs; however, observed n oise spectral densities are typically an order of magnitude higher. Low-fre quency lif noise from thermally activated vortex motion is a much bigger is sue in high-T-c SQUIDs at 77 K than in low-T-c SQUIDs because of the low fl ux-pinning energies in high-T-c superconductors. The magnitude of the noise depends strongly on the quality of the thin films, and much effort has bee n expended to improve techniques for depositing YBa2Cu3O7-x (YBCO) on latti ce-matched single-crystal substrates. Substantial effort has also been inve sted in the development of new types of Josephson junctions, of which grain -boundary junctions are the most widely used in SQUIDs. Appropriate electro nic read-out schemes largely eliminate lif noise from fluctuations in the j unction critical current in both rf and dc SQUIDs. Typical levels of white flux noise are a few mu Phi(0) Hz(-1/2) (Phi(0) is the flux quantum). Magne tometers-consisting of a superconducting flux transformer coupled to a SQUI D-achieve a white magnetic-field noise as low as 10 fT Hz(-1/)2, increasing to typically 30 fT Hz(-1/2) at 1 Hz. When these devices are operated in an unshielded environment, it is important to minimize the motion of trapped vortices and induced supercurrents, which can greatly increase the 1/f nois e. The ambient noise is far greater than the intrinsic noise of the devices , but can be substantially reduced by various gradiometer configurations. T here is now considerable effort to apply high-T-c SQUIDs in magnetocardiolo gy, nondestructive evaluation, microscopy, and geophysics. [S0034-6861(99)0 0403-1].