2000 | OriginalPaper | Buchkapitel
Low- and High-Tc Squids and Some Applications
verfasst von : John Clarke
Erschienen in: Applications of Superconductivity
Verlag: Springer Netherlands
Enthalten in: Professional Book Archive
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DC Superconducting QUantum Interference Devices are routinely fabricated from Nb films with an integrated input coil to couple in magnetic flux from the signal source. Typical dc SQUIDs operated at 4.2 K have a magnetic flux noise of 10−6 Φ0 Hz−1/2, where Φ0 is the flux quantum, corresponding to a magnetic noise energy of 10−32 JHz−1. With the aid of a superconducting flux transformer the magnetic field noise can approach 1 fT Hz−1/2. High-Tc dc SQUIDs fabricated from thin films of YBa2Cu3O7-x (YBCO) have achieved noise energies below 10−30 JHz−1 at 77 K and, with multilayer flux transformers, a magnetic field noise below 10 fT Hz−1/2 at frequencies down to a few Hz. An rf SQUID made from YBCO and operated at 77 K at frequencies around 1 GHz has achieved a magnetic field noise of 16 fT Hz−1/2 at frequencies above 100 Hz. A recent theory for rf SQUIDs predicts noise levels in good agreement with experiment. Two issues relevant to the operation of high-Tc SQUIDs in an unshielded environment are the increase in low-frequency noise arising from the entry of flux vortices into the YBCO films and the realization of long-baseline gradiometers. The applications of low-Tc SQUIDs to nuclear magnetic and nuclear quadrupole resonance, and of high-Tc SQUIDs to scanning SQUID microscopy and geophysics are discussed.