Jenks, William G.; Thomas, Ian M.; Wikswo, John P.,Jr.
Encyclopedia of Applied Physics 1997, Vol 19, pp 457-468
The Superconducting QUantum Interference Device, or SQUID, is an extremely sensitive magnetic flux-to-voltage transducer. The SQUID may be the most sensitive detector of any physical quantity, with an energy resolution that approaches the quantum limit. The extreme sensitivity of the SQUID has caused it to be incorporated into a great variety of systems. SQUIDs have been used to measure cortical activity in the human brain and to search for gravity waves. This article should serve as a starting point to any engineer or physicist who wishes to understand SQUIDs. The principles of operation, the methods of manufacture and the applications of SQUIDs are each discussed in turn.
The fundamental component of the SQUID is the Josephson junction, essentially two superconductors weakly coupled through a small insulating gap or constriction. The Josephson junction has unique electrical/magnetic properties and when incorporated into a superconducting loop forms a SQUID. The distinction is made between rf (one junction in the loop) and dc (two junctions) SQUIDs. The flux-locked loop is used in both cases to make the flux-to-voltage transduction linear. Typically a superconducting pick-up coil is used to funnel flux into the SQUID loop to increase the SQUID's voltage response.
The fabrication of conventional SQUIDs from low-Tc metallic superconductors and the newer high-Tc SQUIDs is discussed. The low-Tc SQUID are made principally of niobium by sputtering and photolithography. They require a liquid Helium bath for cooling. YBa2Cu3O7-x is the dominant material for high-Tc SQUIDs which may be formed by pulsed-laser deposition. High-Tc SQUIDs can operate when cooled by a liquid nitrogen bath. The high-Tc SQUIDs are cheaper to operate but at present are noisier than the low-Tc SQUIDs.
The SQUID is but one part of the SQUID system, which may be configured to sense one component of the magnetic field, first and higher order derivatives of the magnetic field, or even displacement of magnetic objects. A superconducting pick-up coil couples the SQUID to the ambient magnetic field and its design is determined by the planned applications. Biomagnetic applications in magnetoencephalography, magnetocardiography and elsewhere are discussed. Other applications included in the text are geomagnetism, nondestructive testing, radio frequency amplification and the measurement of fundamental constants.
No attempt is made to completely cover any particular aspect of SQUID construction and use, rather the reader should gain an overview of the field and may refer to the bibliography for more comprehensive works in any particular sub-field. © 1997 VCH Publishers Inc.
The following reprint from Encyclopedia of Applied Physics , Volume 19, Jenks, William G.; Thomas, Ian M.; Wikswo, John P.,Jr. , "SQUIDS", pp 457-468 , Copyright 1997 is made available by permission of the publisher VCH Publishers Inc.
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