squids.ppt 3556KB Jun 23 2011 10:31:44 AM

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

ns !

o
n
Pho

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

Due to B field

Current flow

Due to junctions

Voltage
change

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