SQUIDs
SQUIDs
(Superconducting QUantum Interference Devices)
Talk Outline
Talk Outline
• Superconductivity
• Josephson Tunneling
• Phase of electron pairs
• DC SQUID
Why SQUIDs?
Why SQUIDs?
• Can detect
extremely small
magnetic fields
• For example,
Superconductivity
Superconductivity
Below certain temperature:
•Meissner effect •Resistanceless
Superconductivity Explained –
Superconductivity Explained –
BCS Theory
BCS Theory
• Electron – Electron – lattice
lattice
interaction interaction
• Cooper pairs • Energy Gap • Coherence • Flux
Quantization
Superconductivity Explained –
Superconductivity Explained –
BCS Theory
BCS 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 spins
and spins
Boson-like
Boson-like
Does not scatter -
Does not scatter -
resistanceless
resistanceless
Energetically favorable
Energetically favorable
in superconducting state
Superconductivity Explained –
Superconductivity Explained –
BCS Theory
BCS Theory
• Electron – lattice
interaction • Cooper pairs
• Energy GapEnergy Gap
• Coherence • Flux
Superconductivity Explained –
Superconductivity Explained –
BCS Theory
BCS 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 wave
the wave
Coherence length
Coherence length
One wave equation
One wave equation
describes all Cooper pairs:
Superconductivity Explained –
Superconductivity Explained –
BCS Theory
BCS Theory
• Electron – lattice
interaction • Cooper pairs • Energy Gap • Coherence
• Flux Flux
Quantization Quantization
Magnetic flux around a
Magnetic flux around a
closed superconducting
closed superconducting
current loop must be
current loop must be
quantized
quantized
Josephson Tunneling
Josephson Tunneling
• Josephson Junction – small gap between two superconductors • Cooper pairs can tunnel
• Critical current
Change in Phase
Change in Phase
Due to:
• Current
• Magnetic field
• Josephson
Junctions
DC SQUID
DC SQUID
• Superconductor • Current loop
How it works
How it works
Phase change due to
external magnetic field Current flow
How it works
Aiding Detection
Aiding Detection
Lab Setup – Two Stage
Characterisation
Characterisation
• I V curve
Other Applications
Other Applications
Beyond X-Ray Astrophysics...
• Medical imaging (MEG)
References
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
