SQUIDS: Theory and Practice

Abstract

DC Superconducting QUantum Interference Devices (SQUIDs) incorporating two resistively shunted tunnel junctions are routinely fabricated from thin films of low-transition-temperature (Tc) superconductors. An integrated superconducting input coil couples the SQUID to the signal source. Typical dc SQUIDs operating at 4.2K have a magnetic flux noise of 10-6Φ0Hz-1/2 corresponding to a noise energy of 10-32 JHz-1 at frequencies f above the I/f noise knee, which may be below 1Hz (Φ0≡h/2e is the flux quantum). Recently, the performance of thin-film rf SQUIDs, which involve a single junction, has improved significantly, and the sensitivity of a device operated at 3GHz approaches that of dc SQUIDs. In the last year, there have been dramatic improvements in the performance of both dc and rf SQUIDs made from high-Tc thin films, and noise energies of about 10-30 JHz-1 at frequencies down to a few Hz have been achieved at 77K. Multilayer thin-film flux transformers are now available. Instruments based on low-Tc SQUIDs include magnetometers, magnetic gradiometers, voltmeters, susceptometers, amplifiers, and displacement sensors; their applications vary from neuromagnetism and magnetotelluric sounding to the detection of gravity waves and magnetic resonance.