A research team led by John Clarke has succeeded in effectively eliminating "excess" low-frequency noise, noise resulting from the earth's magnetic field, in superconducting quantum interference devices (SQUIDs) made from high-Tc superconducting materials. This noise had prevented wider application of these devices for sensing very small magnetic fields in, for example, geophysical measurements or medical studies of magnetic activity in the heart or the brain, in unshielded environments.

A SQUID magnetometer monitors magnetic fields. To enhance its sensitivity it is coupled to a superconducting "flux transformer" with a "pick-up loop." Application of a magnetic field induces a supercurrent in the pick-up loop which in turn produces a magnetic flux in the SQUID. As with any measuring device, a critical issue in increasing sensitivity is the reduction of noise- intrinsic noise, which is inherent in the device itself, and "excess" noise induced by static, ambient magnetic fields. Over the last several years, researchers have been able to reduce the intrinsic noise in SQUID magnetometers fabricated from the high-transition temperature (Tc) superconductor YBa2Cu3O7-delta (YBCO) to a level at which these devices are sufficiently sensitive for a wide variety of applications including geophysical prospecting, nondestructive evaluation of materials, and magnetocardiology. Current state-of-the-art devices can detect magnetic fields of 30 femtotesla (fT) and less. The next step was to eliminate "excess" noise.

The standard SQUID design consists of a washer-shaped element of superconducting material (see figure). However, a disadvantage of this design is that ambient magnetic fields penetrate the superconducting material, leading to the generation of "excess" noise above and beyond the intrinsic noise. The ambient magnetic field of the earth, which is approximately 50 microtesla (mT), is large enough to produce a significant increase in the noise and interfere with the sensitive measurements.

Clarke and his group succeeded in eliminating this noise by modifying the structure of the SQUID. When a strip of superconducting film is cooled in an ambient magnetic field, that field is excluded, but only up to a characteristic "critical value." The narrower the strip, the higher the critical value. The Clarke group fabricated a series of new SQUID designs in which "slots" or "holes" are punched in the washer creating narrower structures (see figure). They predicted that at the SQUID operating temperature of 77K, the Earth's magnetic field would be excluded; it would thread through the holes or slots rather than penetrate the superconducting film. Experiments showed that these new designs were in fact able to eliminate the excess low-frequency noise. For example, in the presence of a 50 mT ambient field, the noise of a device with an 8-slot washer is 10 times less than in the device with the solid washer in a 20 mT field (see figure). In fact, the new design is effective in eliminating noise resulting from fields as large as 130 mT (see figure), over twice the magnitude of the Earth's magnetic field. Devices based on this new design will thus be able to perform sensitive magnetic measurements in a variety of applications in unshielded environments.

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