squids squids (superconducting quantum interference devices) by agnieszka czeszumska

SQUIDsSQUIDs(Superconducting QUantum Interference Devices)

By Agnieszka Czeszumska

Talk OutlineTalk Outline

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

Why SQUIDs?Why SQUIDs?

• Can detect extremely small magnetic fields

• For example, read out TES device

SuperconductivitySuperconductivityBelow certain temperature:

•Meissner effect•Resistanceless supercurrent

Superconductivity Explained – Superconductivity Explained – BCS TheoryBCS Theory

• Electron – Electron – lattice lattice interactioninteraction

• Cooper pairs• Energy Gap• Coherence• Flux

Quantization

Phonons!

Superconductivity Explained – Superconductivity Explained – BCS TheoryBCS Theory

• Electron – lattice interaction

• Cooper pairsCooper pairs• Energy Gap• Coherence• Flux

Quantization

Two coupled electrons Two coupled electrons with opposite momenta with opposite momenta and spinsand spins

Boson-likeBoson-like

Does not scatter - Does not scatter - resistancelessresistanceless

Energetically favorable Energetically favorable in superconducting statein superconducting state

Superconductivity Explained – Superconductivity Explained – BCS TheoryBCS Theory

• Electron – lattice interaction

• Cooper pairs• Energy GapEnergy Gap• Coherence• Flux

Quantization

Superconductivity Explained – Superconductivity Explained – BCS TheoryBCS Theory

• Electron – lattice interaction

• Cooper pairs• Energy Gap• CoherenceCoherence• Flux

Quantization

Can calculate phase and Can calculate phase and amplitude at any point on amplitude at any point on the wavethe wave

Coherence lengthCoherence length

One wave equation One wave equation describes all Cooper pairs:describes all Cooper pairs:

Superconductivity Explained – Superconductivity Explained – BCS TheoryBCS Theory

• Electron – lattice interaction

• Cooper pairs• Energy Gap• Coherence• Flux Flux

QuantizationQuantization

Magnetic flux around a Magnetic flux around a closed superconducting closed superconducting current loop must be current loop must be quantizedquantized

One fluxon

Josephson TunnelingJosephson Tunneling

• Josephson Junction – small gap between two superconductors

• Cooper pairs can tunnel• Critical current

supercurrent Phase difference across the junction

Change in PhaseChange in Phase

Due to:• Current• Magnetic field• Josephson

Junctions

= P

DC SQUIDDC SQUID• Superconductor• Current loop• Two Josephson

Junctions

How it worksHow it worksPhase change due toexternal magnetic field

Current flow Voltage change

Due to B fieldDue to B field Due to junctionsDue to junctions Must be quantizedMust be quantized

How it worksHow it works

Aiding DetectionAiding Detection

• Gradiometer, Magnetometer, Voltometer

Lab Setup – Two StageLab Setup – Two Stage

CharacterisationCharacterisation

• I V curve• V Phi curve

Other ApplicationsOther Applications

Beyond X-Ray Astrophysics...

• Medical imaging (MEG)

• Geology

ReferencesReferences• 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