SQUID and Josephson Devices

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SQUID and Josephson Devices

• By
• Yatin Singhal

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Overview

• What is SQUID?
• Theory
• SQUID.
• Josephson effect
• DC effect
• AC effect
• Inverse AC effect.
• Cooper pairs.
• Applications of SQUID and Josephson devices.

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Facts about SQUID

• Superconducting quantum interference device is a
  mechanism used to measure extremely week signals.
• Detect change of 100 billion times weaker signal
  than that moves a compass needle.
• Have been used to measure the magnetic field in
  mouse brain to test whether there might be enough
  magnetism to attribute their navigational ability
  to an compass.
• Threshold for SQUID 10-14 T
• Magnetic field of heart10-10 T
• Magnetic field of brain 10-13 T

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SQUID
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Material used for construction of SQUID

• SQUIDs are usually fabricated from either a lead
  alloy (with 10 gold or indium) and/or niobium.
• often consisting of the tunnel barrier sandwiched
  between a base electrode of niobium and the top
  electrode of lead alloy.
• More recently developed "High Temperature" SQUIDS
  are made of a substance called YBCO (chemical
  formula YBa2Cu3O7-x),

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SQUID devices

• The great sensitivity of the SQUID devices is
  associated with measuring changes in magnetic
  field associated with one flux quantum.
• One of the discoveries associated with Josephson
  junctions was the flux is quantized in units

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Josephson Junction

• A Josephson junction is a type of electronic
  circuit capable of switching at very high speeds
  when operated at temperatures approaching
  absolute zero.
• The ability of certain materials to conduct
  electric current with practically zero
  resistance.

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Operation of junction

• Assume Hamiltonian for the system can be written
  as a sum of two Hamiltonians
• H H0 HT
• where
• H0 normal Hamiltonian for 2 isolated
  superconductors
• HT tunneling Hamiltonian
• So, this is "right" for tunneling links only.

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Josephson Equations

• Consider the very simple example of two,
  identical superconductors separated by a thin
  insulator.
• (Typically about 1nm is sufficiently thin).
• Assume junction is sufficiently large in the x
  and y direction to ignore edge/boundary effects,
  and thick enough in z.
• The governing equations are

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DC Josephson Effect

• No Magnetic field
• A current flows, nut no voltage drop, up until
  the critical current.
• Past the critical current, normal single electron
  tunneling is dominant.
• With magnetic field
• Current is

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AC Josephson Effect

• With no magnetic field, static potential
• Integrating equation for ?
• We can substitute into our other equation and get
  
• From this, we get a time varying current with
  frequency

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AC Josephson Effect

• With varying potential
• Do similar analysis as static potential case.
• It turns out that this has dc component
• but when qVo/wh n (where n integer)
• Dc current has spikes at
• regularly values of Vo
• Total current has steps at these
• points.

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Inverse AC Josephson Effect

• If the phase takes the form
• f(t) f0 n?t asin(?t),
• the voltage and current will be
• The DC components will then be

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Cooper pair

• Electron pairs coupling over range of hundreds of
  nanometers are called cooper pair.
• These coupled
• electron can take character of boson and
  condense into ground state.

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Applications

• Magneto encephalography (MEG)
• Applications of MEG include mapping Somatosensory
  and motor cortices.
• Foetal Examination SQUID are used to measure the
  minute magnetic fields generated by babys heart

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Applications

• Further Applications of Josephson Devices
• Magnetic Sensors
• Gradiometers
• Oscilloscopes
• Decoders
• Analogue to Digital converters
• Samplers
• Oscillators
• Microwave amplifiers
• Sensors for biomedical, scientific and defense
  purposes
• Digital circuit development for Integrated
  circuits
• Microprocessors
• Random Access Memories (RAMs)

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References

• http//en.wikipedia.org/wiki/SQUID
• http//hyperphysics.phy-astr.gsu.edu/hbase/solids/
  squid.html
• Steven T. Ruggiero, David A. Rudman,
  Superconducting Devices. New York McGraw-Hills,
  1975.
• Barone A, Paterno G. Physics and Applications of
  the Josephson Effect. New York John Wiley
  Sons 1982.
• http//whatis.techtarget.com/definition/0,,sid9_gc
  i816722,00.html
• http//en.wikipedia.org/wiki/Josephson_junction
• http//hyperphysics.phy-astr.gsu.edu/hbase/solids/
  squid.htmlc3
• http//www.abdn.ac.uk/physics/case/squids.html

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Questions?

• Comments?