1. SQUIDs
(Superconducting QUantum Interference Devices)
By Agnieszka Czeszumska

2. Talk Outline
• Superconductivity
• Josephson Tunneling
• Phase of electron pairs
• DC SQUID
• Laboratory Setup
• Applications

3. Why SQUIDs?
• Can detect
extremely small
magnetic fields
• For
example, read
out TES device

4. Superconductivity
Below certain temperature:
•Meissner effect•Resistanceless
supercurrent

5. Superconductivity Explained –
BCS Theory
• Electron –
lattice
interaction
• Cooper pairs
• Energy Gap
• Coherence
• Flux
Quantization

6. 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

7. Superconductivity Explained –
BCS Theory
• Electron –
lattice
interaction
• Cooper pairs
• Energy Gap
• Coherence
• Flux
Quantization

8. 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:

9. 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

10. Josephson Tunneling
• Josephson Junction – small gap
between two superconductors
• Cooper pairs can tunnel
• Critical current
supercurrent Phase difference
across the junction

11. Change in Phase
Due to:
• Current
• Magnetic field
• Josephson
Junctions
= P

12. DC SQUID
• Superconductor
• Current loop
• Two Josephson
Junctions

13. How it works
Phase change due to
external magnetic field
Current flow
Voltage
change
Due to B field Due to junctions Must be quantized

14. How it works

15. Aiding Detection
• Gradiometer, Magnetometer, Voltometer

16. Lab Setup – Two Stage

17. Characterisation
• I V curve
• V Phi curve

18. Other Applications
Beyond X-Ray
Astrophysics...
• Medical imaging
(MEG)
• Geology

19. 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
• Special thanks to Emily Barrentine for help and resources