Squids

SQUIDs(Superconducting QUantum Interference Devices)

By Agnieszka Czeszumska

Talk Outline

• Superconductivity• Josephson Tunneling• Phase of electron pairs• DC SQUID• Laboratory Setup• Applications

Why SQUIDs?

• Can detect extremely small magnetic fields

• For example, read out TES device

SuperconductivityBelow certain temperature:

• Meissner effect• Resistanceless supercurrent

Superconductivity Explained – BCS Theory

• Electron – lattice interaction

• Cooper pairs• Energy Gap• Coherence• Flux

Quantization

Phonons!




Quantization

Two coupled electrons with opposite momenta and spins

Boson-like

Does not scatter - resistanceless

Energetically favorable in superconducting state




Quantization




Quantization

Can calculate phase and amplitude at any point on the wave

Coherence length

One wave equation describes all Cooper pairs:




Quantization

Magnetic flux around a closed superconducting current loop must be quantized

One fluxon

Josephson Tunneling

• Josephson Junction – small gap between two superconductors

• Cooper pairs can tunnel• Critical current

supercurrent Phase difference across the junction

Change in Phase

Due to:• Current• Magnetic field• Josephson

Junctions

= P

DC SQUID• Superconductor• Current loop• Two Josephson

Junctions

How it worksPhase change due toexternal magnetic field

Current flow Voltage change

Due to B field Due to junctions Must be quantized

How it works

Aiding Detection• Gradiometer, Magnetometer, Voltometer

Lab Setup – Two Stage

Characterisation

• I V curve• V Phi curve

Other Applications

Beyond X-Ray Astrophysics...

• Medical imaging (MEG)

• Geology

References• A.C. Rose-Innes and E.H. Rhoderick, „Introduction to

Superconductivity”, 2nd ed., 1978• Kittel, Charles, „Introduction to Solid State Physics”, 6th ed.,

1986• Clarke, John, „SQUIDs”, Scientific American (August 1994)• STAR Cryoelectronics, Manual• SQUID Magnetometry,

http://www.cmp.liv.ac.uk/frink/thesis/thesis/node41.html• http://hyperphysics.phy-astr.gsu.edu/hbase/solids/

supcon.html#c1• Thank you Mark and Prof. McCammon for answering my

random questions• Special thanks to Emily Barrentine for help and resources

Technology

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