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 Voltage change Phase change due to external magnetic field 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, 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