SQUIDs (Superconducting QUantum Interference Devices)
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
Superconductivity
Below certain temperature:
•Resistanceless supercurrent
•Meissner effect
Superconductivity Explained –
BCS Theory
• Electron – lattice interaction
• Cooper pairs
• Energy Gap
• Coherence
• Flux
Quantization
Superconductivity Explained –
BCS Theory
• Electron – lattice interaction
• 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
Superconductivity Explained –
BCS Theory
• Electron – lattice interaction
• Cooper pairs
• Energy Gap
• Coherence
• Flux
Quantization
Superconductivity Explained –
BCS Theory
• Electron – lattice interaction
• Cooper pairs
• Energy Gap
• Coherence
• Flux
Quantization
Can calculate phase and amplitude at any point on the wave
Coherence length
One wave equation describes all Cooper pairs:
Superconductivity Explained –
BCS Theory
• Electron – lattice interaction
• Cooper pairs
• Energy Gap
• Coherence
• Flux
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
= P
Due to:
• Current
• Magnetic field
• Josephson
Junctions
DC SQUID
• Superconductor
• Current loop
• Two Josephson
Junctions
How it works
Phase change due to external 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.phyastr.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
