Arian Lalezari 22 September 2004 Modeling in Applied Math SQUIDs Arian Lalezari 22 September 2004 Modeling in Applied Math

Arian Lalezari: SQUIDS Overview History Physics: Components Fabrication Implementation MEG Other Applications Future 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS History Brian David Josephson Born 4 January 1940 Graduate work with superconductors and quantum tunneling at Cambridge in 1962 Verification of “Josephson Junction” in 1963 Nobel Prize in 1973 First commercial sales in 1974 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS What is a SQUID? A SQUID is a Superconducting Quantum Interference Device Construction is two Josephson Junctions in parallel, creating a loop through which one measures magnetic flux 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Physics “Josephson Junction” Analogy: water pipe Superconductor: water pipe Insulating barrier: pinch DC current: water flow Voltage: water pressure If the material is not superconducting, no current flows until there is adequate voltage Like a blockage Not useful for this application hose pinch superconductor thin insulating barrier 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Physics If the material is superconducting, small amounts of current can flow without generating voltage at the resistive barrier This violates Ohm’s Law from classical physics: V = IR Phenomenon is called “quantum tunneling” If currents get too big, tunneling cannot take place, and the barrier becomes resistive So where is this all going? 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Physics Key Ideas: Magnetic flux can only be produced in quanta (F0) Flux through a loop may be a fractional part of a generated quantum By Faraday’s Law, a loop in a changing magnetic field experiences an induced current In SQUIDs, the barriers of the Josephson Junction are designed not to fail (become resistive) until the the induced current change is equal to what is generated by one quantum change in flux (DF=F0) 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Physics: Large Fields Each flux quantum that is let through “resets” the system when the quantum is allowed to pass through the loop This returns the setup to equilibrium Systems to track the number of times this occurs can measure magnetic flux at the quantum level Reset counting means no dependence on the presence of a DC magnetic field 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Physics: Small Fields If DF is less than one quantum, the flux cannot pass Around the SQUID loop, current must stay in phase (Df = n*2p) Used alone, SQUIDs can “trap” magnetic flux If we supply and measure a supplemental current so that the phase quantum requirement is met, we can estimate the percentage of a flux quantum through the loop 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Sensitivity This means SQUIDs can theoretically be made more precise than a flux quantum! In practice, this requires amplification of the signal obtained from the SQUID 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Fabrication Concerns SQUIDs have edge lengths on the order of 10 – 100mm (1e-5m – 1e-4m) Challenging, but not impossible Traditional SQUIDs must be kept at about 4°K (-269°C) to stay superconductive Much more challenging: liquid Helium 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Implementation: MEG Helmet with dozens of SQUIDs To bathe each SQUID in liquid Helium, the majority of the heavily insulated helmet enclosure is filled with liquid Helium 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Implementation: MEG Each SQUID returns a time-domain signal reflecting changes in magnetic flux This visualization maps the head to a circle, and shows short time samples from each SQUID 4/17/2019 Arian Lalezari: SQUIDS

Implementation: Microscope Large flux application Scanning SQUID Microscope scan of a small region of a floppy disk, and a diagonal cross-section 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Future High-Temperature Semiconductors Semiconducting behavior at much higher temperatures (e.g. above 77°K boiling point of liquid Nitrogen) Fetal heartbeats Better back-mapping techniques for data 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS Questions? 4/17/2019 Arian Lalezari: SQUIDS

Arian Lalezari: SQUIDS References Fundamentals of superconductors: http://www.physnet.uni-hamburg.de/home/vms/reimer/htc/pt3.html Basic Introduction to SQUIDs: http://www.abdn.ac.uk/physics/case/squids.html Fancy cross-referenced site for Josephson Junctions/Josephson: http://en.wikipedia.org/wiki/Josephson_junction http://en.wikipedia.org/wiki/B._D._Josephson SQUID sensitivity and other ramifications of Josephson’s work: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid2.html Understanding a SQUID magnetometer: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html#c1 Some exciting applications of SQUIDs: http://www.lanl.gov/quarterly/q_spring03/squid_text.shtml Relative strengths of pertinent magnetic fields http://www.physics.union.edu/newmanj/2000/SQUIDs.htm The 1973 Nobel Prize in physics http://nobelprize.org/physics/laureates/1973/ Critical overview of SQUIDs http://homepages.nildram.co.uk/~phekda/richdawe/squid/popular/ Research Applications http://boojum.hut.fi/triennial/neuromagnetic.html Technical overview of SQUIDs: http://www.finoag.com/fitm/squid.html http://www.cmp.liv.ac.uk/frink/thesis/thesis/node47.html 4/17/2019 Arian Lalezari: SQUIDS