squids squids (superconducting quantum interference devices) by agnieszka czeszumska
TRANSCRIPT
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