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Superconducting Quantum Interference Devices SQUIDs

Overview. BackgroundSQUID TheorySQUID Uses. SQUID Background. The SQUID is an extremely sensitive magnetic field detector.Can detect fields on the order of 10-15 TEarth's magnetic field: 10-4 THeart's magnetic field: 10-11 TBrain's magnetic field: 10-15 T. SQUID Background. Invented in 1964 by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau of Ford Research Labs.Two years after the Josephson effect was postulated in 1962.One year after the first Josephson Junction24

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Superconducting Quantum Interference Devices SQUIDs

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    1. Superconducting Quantum Interference Devices (SQUIDs) By Leyan Lo

    2. Overview Background SQUID Theory SQUID Uses

    3. SQUID Background The SQUID is an extremely sensitive magnetic field detector. Can detect fields on the order of 10-15 T Earth’s magnetic field: 10-4 T Heart’s magnetic field: 10-11 T Brain’s magnetic field: 10-15 T

    4. SQUID Background Invented in 1964 by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau of Ford Research Labs. Two years after the Josephson effect was postulated in 1962. One year after the first Josephson Junction was made by John Rowell and Philip Anderson at Bell Labs in 1963.

    5. SQUID Background Two kinds of SQUID: DC and RF. RF SQUIDs have only one Josephson and are cheaper to produce, but are not as sensitive. DC SQUIDs have two or more junctions and are much more sensitive. (We will only focus on these types).

    6. Cooper Pairs A superconductor is a many body system made of electrons bound in pairs, called “Cooper pairs”. These pairs can be described by the wavefunction: According to the Bardeen, Cooper and Schrieffer (BCS) theory Charge 2e, mass 2m_e Spread between e is larger than spread between pairs!According to the Bardeen, Cooper and Schrieffer (BCS) theory Charge 2e, mass 2m_e Spread between e is larger than spread between pairs!

    7. Flux Quantization The current density for a superconducting ring is approximately zero.

    8. Flux Quantization Introduce the vector potential A into the Schroedinger equation:

    9. Flux Quantization Integrate over the curve G: This is called the “Flux quantum”

    10. Screening Current A superconducting ring will always have an integer multiple of this ??0. The ring will generate a screening current to satisfy this property:

    11. Screening Current But how can we measure this screening current? This is a periodic function: V = I R Will not work in this case because there is no resistance in a superconductor!

    12. Josephson Junction Cooper pairs can tunnel across a thin barrier separating two superconductors up until a critical current value: DT is the temperature dependent energy gap ~3meV R_n is the resistance of the junctions when the two electrodes are in normal state Silicon Oxide layer ~ 3um thickDT is the temperature dependent energy gap ~3meV R_n is the resistance of the junctions when the two electrodes are in normal state Silicon Oxide layer ~ 3um thick

    13. Bias Current Remember, our task was to measure the screening current! Inject a bias current to ride the “knee” of the curve.

    14. Periodic Relationship Periodic relationship between voltage and flux: Introduce Phase Locked Loop for a direct relationship.

    15. Gradiometer A two coil system allows the SQUID to measure the derivate of the B-field. This system ignores plane waves emitted from distant sources, and focuses attention to local sources.

    16. Photograph of a dc-SQUID 10 x 10 mm2 A Lot of Loops

    17. SQUIDS in the Body Biomagnetism is one of the most promising applications of SQUIDs Today it is a new field of research where interdisciplinary collaboration takes place by physicists, mathematicians, physiologists and psychologists.

    18. SQUIDs in the Body In 1791, Galvani discovered animal electricity In 1887, English physiologist Waller measured electric potentials in the heart. It wasn’t until 1969 when the heart’s magnetic field could be observed by Baule and McFee with the SQUID.

    19. SQUIDs in the Body Nowadays, SQUIDs are able to detect fields in the brain, which are 10,000 weaker than those from the heart. This is called Magnetoencephalography (MEG)

    20. SQUIDs in the Field Portable SQUID vector system developed in Japan Could be used to detect geological activity

    21. SQUIDs as accelerometers SQUID sensors can be used to sense small displacements in objects under acceleration.

    22. SQUIDs Searching for Gravitational Waves The search for gravitational waves began in the 1960’s. Two types of detectors: Michelson interferometers light paths altered by GW Bar detectors large “bells” rung by GW

    23. AURIGA Resonant bar detector near Padova, Italy 3m, 2.3 ton Aluminum mass Q ~ 4x106 @ 100mK Resonance is at 920Hz

    24. Conclusion SQUIDs are cool (literally!) There are many applications for SQUIDs in various fields: MEG Geology Gravitational Waves The field is still young

    25. References Barone, Antonio, ed. Principles and Applications of Superconducting Quantum Interference Devices. Singapore: World Scientific, 1992. Hull, John R., and Thomas M. Mulcahy. "Gravimeter Using High-Temperature Superconducting Bearing." IEEE (1999). 29 Jan. 2007 <http://ieeexplore.ieee.org>. Kirtley, J. R., et al. “Design and applications of a scanning SQUID microscope.” Journal of Research & Development (1995). 29 Jan. 2007 <http://www.neiu.edu>. Machitani, Y., et al. “Vector HTS-SQUID System for ULF Magnetic Field Monitoring.” IEEE (2003). 29 Jan. 2007 <http://ieeexplore.ieee.org>. Singh, Manbir, et al. "Neuromagnetic Localization Using Magnetic Resonance Images." IEEE (1990). 29 Jan. 2007 <http://ieeexplore.ieee.org>.

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