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: Resistanceless
supercurrent
Meissner effect

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

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
Voltage change
Current flow
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

20. 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,
Thank you Mark and Prof. McCammon for answering my random questions
Special thanks to Emily Barrentine for help and resources