squids.ppt 3556KB Jun 23 2011 10:31:44 AM

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SQUIDs

(Superconducting QUantum Interference Devices)

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Talk Outline

• Superconductivity

• Josephson Tunneling

• Phase of electron pairs

• DC SQUID

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Why SQUIDs?

• Can detect

extremely small

magnetic fields

• For example,

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Superconductivity

Below certain temperature:

•Meissner effect •Resistanceless

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Superconductivity Explained –

BCS Theory

• _{Electron –}_{Electron –} lattice

lattice

interaction interaction

• Cooper pairs • Energy Gap • Coherence • Flux

Quantization

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BCS Theory

• Electron – lattice

interaction

• _{Cooper pairs}_{Cooper pairs}

• Energy Gap • Coherence • Flux

Quantization

Two coupled electrons

with opposite momenta

and spins

Boson-like

Does not scatter -

resistanceless

Energetically favorable

in superconducting state

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Superconductivity Explained –

BCS Theory

interaction • Cooper pairs

• _{Energy Gap}_{Energy Gap}

• Coherence • Flux

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BCS Theory

interaction • Cooper pairs • Energy Gap

• _Coherence_Coherence

• Flux

Quantization

Can calculate phase and

amplitude at any point on

the wave

Coherence length

One wave equation

describes all Cooper pairs:

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Superconductivity Explained –

BCS Theory

interaction • Cooper pairs • Energy Gap • Coherence

• _Flux_Flux

Quantization Quantization

Magnetic flux around a

closed superconducting

current loop must be

quantized

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Josephson Tunneling

• Josephson Junction – small gap between two superconductors • Cooper pairs can tunnel

• Critical current

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Change in Phase

Due to:

• Current

• Magnetic field

• Josephson

Junctions

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DC SQUID

• Superconductor • Current loop

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How it works

Phase change due to

external magnetic field Current flow

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How it works

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Aiding Detection

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Lab Setup – Two Stage

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Characterisation

• I V curve

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Other Applications

Beyond X-Ray Astrophysics...

• Medical imaging (MEG)

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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