Superconducting Quantum Interference Devices SQUIDs

1
Superconducting Quantum Interference Devices
(SQUIDs)

• By Leyan Lo

2
Overview

• Background
• SQUID Theory
• SQUID Uses

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SQUID Background

• The SQUID is an extremely sensitive magnetic
  field detector.
• Can detect fields on the order of 10-15 T
• Earths magnetic field 10-4 T
• Hearts magnetic field 10-11 T
• Brains magnetic field 10-15 T

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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.

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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).

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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

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Flux Quantization

• The current density for a superconducting ring is
  approximately zero.

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Flux Quantization

• Introduce the vector potential A into the
  Schroedinger equation

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Flux Quantization

• Integrate over the curve G
• This is called theFlux quantum

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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

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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!

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Josephson Junction

• Cooper pairs can tunnel across a thin barrier
  separating two superconductors up until a
  critical current value

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Bias Current

• Remember, our task was to measure the screening
  current!
• Inject a bias current to ride the knee of the
  curve.

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Periodic Relationship

• Periodic relationship between voltage and flux
• Introduce Phase Locked Loop for a direct
  relationship.

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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.

http//www.aston.ac.uk/
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A Lot of Loops

• Photograph of adc-SQUID
• 10 x 10 mm2

Barone, A.
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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.

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SQUIDs in the Body

• In 1791, Galvani discovered animal electricity
• In 1887, English physiologist Waller measured
  electric potentials in the heart.
• It wasnt until 1969 when the hearts magnetic
  field could be observed by Baule and McFee with
  the SQUID.

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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)

Singh, Manbir, et al. 1990
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SQUIDs in the Field

• Portable SQUID vector system developed in Japan
• Could be usedto detectgeologicalactivity

Machitani, Y., et al. 2003
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SQUIDs as accelerometers

• SQUID sensors can be used to sense small
  displacements in objects under acceleration.

Hull, John R., and Thomas M. Mulcahy 1999
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SQUIDs Searching for Gravitational Waves

• The search for gravitational waves began in the
  1960s.
• Two types of detectors
• Michelson interferometers
• light paths altered by GW
• Bar detectors
• large bells rung by GW

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AURIGA

• Resonant bar detector near Padova, Italy
• 3m, 2.3 ton Aluminum mass
• Q 4x106 _at_ 100mK
• Resonance is at 920Hz

http//www.auriga.lnl.infn.it/
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Conclusion

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

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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 lthttp//ieeexplore.ieee.
  orggt.
• Kirtley, J. R., et al. Design and applications
  of a scanning SQUID microscope. Journal of
  Research Development (1995). 29 Jan. 2007
  lthttp//www.neiu.edugt.
• Machitani, Y., et al. Vector HTS-SQUID System
  for ULF Magnetic Field Monitoring. IEEE (2003).
  29 Jan. 2007 lthttp//ieeexplore.ieee.orggt.
• Singh, Manbir, et al. "Neuromagnetic Localization
  Using Magnetic Resonance Images." IEEE (1990). 29
  Jan. 2007 lthttp//ieeexplore.ieee.orggt.