SQUID and Josephson Devices By : Ya Y tin Singhal 1

Overview ► What is SQUID? ► Theory SQUID. Josephson effect : ► DC effect► AC effect► Inverse AC effect. Cooper pairs. ► Applications of SQUID and Josephson devices. 2

Facts about SQUID ► Superconducting quantum interference device is a mechanism used to measure extremely week signals. ► Detect change of 100 bil ion times weaker signal than that moves a compass needle. ► Have been used to measure the magnetic field in mouse brain to test whether there might be enough magnetism to attribute their navigational ability to an compass. ► Threshold for SQUID: 10-14 T► Magnetic field of heart:10-10 T► Magnetic field of brain: 10-13 T 3

SQUID 4

Material used for construction of SQUID ► SQUIDs are usual y fabricated from either a lead al oy (with 10% gold or indium) and/or niobium. ► often consisting of the tunnel barrier sandwiched between a base electrode of niobium and the top electrode of lead al oy. ► More recently developed "High Temperature" SQUIDS are made of a substance cal ed YBCO (chemical formula YBa2Cu3O7-x), 5

SQUID devices ► The great sensitivity of the SQUID devices is associated with measuring changes in magnetic field associated with one flux quantum. ► One of the discoveries associated with Josephson junctions was the flux is quantized in units 6

Josephson Junction ► A Josephson junction is a type of electronic circuit capable of switching at very high speeds when operated at temperatures approaching absolute zero. ► The ability of certain materials to conduct electric current with practically zero resistance. 7

Operation of junction ► Assume Hamiltonian for the system can be written as a sum of two Hamiltonians H = H0+ HT where: H0 = normal Hamiltonian for 2 isolated superconductors HT = tunneling Hamiltonian ► So, this is "right" for tunneling links only. 8

Josephson Equations ► Consider the very simple example of two, identical superconductors separated by a thin insulator. ► (Typical y about 1nm is sufficiently thin). ► Assume junction is sufficiently large in the x and y direction to ignore edge/boundary effects, and thick enough in z. ► The governing equations are: 9

DC Josephson Effect ► No Magnetic field: A current flows, nut no voltage drop, up until the critical current. Past the critical current, normal single electron tunneling is dominant. ► With magnetic field: Current is: 10

AC Josephson Effect ► With no magnetic field, static potential: Integrating equation for Ф : We can substitute into our other equation and get : From this, we get a time varying current with frequency 11

AC Josephson Effect ► With varying potential: Do similar analysis as static potential case. It turns out that this has dc component but when qVo/wħ = n (where n= integer) Dc current has spikes at regularly values of Vo Total current has steps at these points. 12

Inverse AC Josephson Effect ► If the phase takes the form φ(t) = φ0 + nωt + asin(ωt), the voltage and current wil be The DC components wil then be 13

Cooper pair ► Electron pairs coupling over range of hundreds of nanometers are called cooper pair. ► These coupled electron can take character of boson and condense into ground state. 14

Applications ► Magneto encephalography (MEG) Applications of MEG include mapping Somatosensory and motor cortices. Foetal Examination: SQUID are used to measure the minute magnetic fields generated by baby’s heart 15

Applications ► Further Applications of Josephson Devices Magnetic Sensors Gradiometers Oscil oscopes Decoders Analogue to Digital converters Samplers Oscil ators Microwave amplifiers Sensors for biomedical, scientific and defense purposes Digital circuit development for Integrated circuits Microprocessors Random Access Memories (RAM’s) 16

References ► http://en.wikipedia.org/wiki/SQUID► http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html► Steven T. Ruggiero, David A. Rudman, Superconducting Devices. New York: McGraw-Hil s, 1975. ► Barone A, Paterno G. Physics and Applications of the Josephson Effect. New York: John Wiley & Sons; 1982. ► http://whatis.techtarget.com/definition/0,,sid9_gci816722,00.html► http://en.wikipedia.org/wiki/Josephson_junction► http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html#c3► http://www.abdn.ac.uk/physics/case/squids.html 17

Questions? Comments? 18