1. 1 SQUID and Josephson Devices By : Yatin Singhal

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

3. 3 Facts about SQUID ► Superconducting quantum interference device is a mechanism used to measure extremely week signals. ► Detect change of 100 billion 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

4. 4 SQUID

5. 5 Material used for construction of SQUID ► SQUIDs are usually fabricated from either a lead alloy (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 alloy. ► More recently developed "High Temperature" SQUIDS are made of a substance called YBCO (chemical formula YBa2Cu3O7-x), YBCO

6. 6 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

7. 7 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.

8. 8 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.

9. 9 Josephson Equations ► Consider the very simple example of two, identical superconductors separated by a thin insulator. ► (Typically 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:

10. 10 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:

11. 11 ► 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 AC Josephson Effect

12. 12 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.

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

14. 14 ► 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. Cooper pair

15. 15 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

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

17. 17 References ► http://en.wikipedia.org/wiki/SQUID http://en.wikipedia.org/wiki/SQUID ► http://hyperphysics.phy- astr.gsu.edu/hbase/solids/squid.html http://hyperphysics.phy- astr.gsu.edu/hbase/solids/squid.html http://hyperphysics.phy- astr.gsu.edu/hbase/solids/squid.html ► Steven T. Ruggiero, David A. Rudman, Superconducting Devices. New York: McGraw-Hills, 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://whatis.techtarget.com/definition/0,,sid9_gci816722, 00.html http://whatis.techtarget.com/definition/0,,sid9_gci816722, 00.html ► http://en.wikipedia.org/wiki/Josephson_junction http://en.wikipedia.org/wiki/Josephson_junction ► http://hyperphysics.phy- astr.gsu.edu/hbase/solids/squid.html#c3 http://hyperphysics.phy- astr.gsu.edu/hbase/solids/squid.html#c3 http://hyperphysics.phy- astr.gsu.edu/hbase/solids/squid.html#c3 ► http://www.abdn.ac.uk/physics/case/squids.html http://www.abdn.ac.uk/physics/case/squids.html

18. 18 Questions? Comments?