Superconductivity a presentation

A presentaion about theory of Superconductivity.

1. A presentation about SUPERCONDUCTIVITY By Aqeel A. Al- Taie May, 2016

2. Outline: Definition of Superconductivity Discovery The Meissner Effect BCS Theory Properties of Superconductivity Applications of Superconductivity Conclusions

3. Definition of Superconductivity Superconductivity is a phenomenon occurring in certain materials generally at very low temperatures , characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field . In simple words For some materials, the resistivity vanishes at some low temperature: they become superconducting ,such materials are called superconductors .

4. Discovery Superconductivity was discovered on 1911 by Heike Kamerlingh Onnes , who was studying the resistance of solid mercury at cryogenic temperatures using the recently-discovered liquid helium as a refrigerant . At the temperature of 4.2 K, he observed that the resistance suddenly disappeared. In subsequent decades, superconductivity was found in several other materials.

5. Critical Temperature The temperature at which electrical resistance is zero is called the Critical temperature ( T c ) T he cooling of the material can be achieved using liquid nitrogen or liquid helium for even more lower temperature.

6. The Meissner Effect The limit of external magnetic field strength at which a superconductor can exclude the field is known as the critical field strength, H c . In 1920 Meissner discovered not only did superconductors exhibit zero resistance but also spontaneous expel all magnetic flux when cooled through the superconducting transition , that is they are also perfect dimagnets . We call this the Meissner effect .

7. Normal state Superconducting state H H

8. BCS THEORY The complete microscopic theory of superconductivity was finally proposed in 1957 by Bardeen , Cooper , and Schrieffer . This BCS theory explained the superconducting current as a super fluid of Cooper pairs , pairs of electrons interacting through the exchange of phonons. For this work, the authors were awarded the Nobel Prize in 1972.

9. BCS – COOPER PAIRS A Cooper pair is the name given to electrons that are bound together in a certain manner first described by Leon Cooper . In normal superconductors, the attraction is due to the electron interaction. The Cooper Pair state forms the basis of the BCS theory of superconductivity .

10. Formation of cooper pairs Cooper pairs are formed by an attractive force between electrons from the exchange of phonon. The energy of phonon is usually less than 0.1eV

11. When a metal is cooled to the critical temperature, electrons in the metal form Cooper Pairs. Cooper Pairs are electrons which exchange phonons and become bound together. Bound electrons behave like bosons. Their wavefunctions don’t obey Pauli exclusion rule and thus they can all occupy the same quantum state. The BCS theory of Superconductivity states that bound photons have slightly lower energy, which prevents lattice collisions and thus eliminates resistance. As long as kT < binding energy, then a current can flow without dissipation.

12. PROPERTIES : Josephson effect Specific heat Super fluidity Thermal conductivity Isotope effect Field intensity Persistent current

13. Josephson e ffect When two superconductors are joined by a thin ,insulating layer ,it is easier for the electron pairs to pass from one superconductor to another without resistance . This is called the J osephson effect This effect has applications for superfast electrical switches that can be used to make small ,high speed computers.

14. Specific heat A finite jump in the specific heat is observed at the critical temperature. In a superconducting phase, the electron resistance changes with the jump , while the energy undergoes a continuous variation . When the substance is cooled its specific heat typically decreases but at the critical temperature, it increases suddenly.

15. Super fluidity This phenomenon was first observed in Helium at a temp below 2.17 K. helium at this temp flow quite freely without any friction , through any gaps and even small capillary tubes. Once it is set in motion it will keep on flowing forever-if there are no external forces acting upon it. Unlike all chemicals helium does not solidify when cooled down near absolute zero.

16. Thermal conductivity In an ideal superconductor , there is a marked drop in the thermal conductivity when superconductivity sets in . In non ideal superconductors an increase in thermal conductivity on becoming super conducting has been observed in few specimens.

17. Isotope effect It has been observed that critical temperature varies with isotopic mass. i.e., critical temperature is inversely proportional to the square root of mass of the isotope

18. Field intensity Removal of the superconductivity state does not only occur by raising the temperature , but also by subjecting a material to a magnetic field . The critical value of magnetic field for the destruction of superconductivity , H c is function of temperature , at T= T c , H c =0 . With only small deviations, the critical field H c varies with the temperature according to the parabolic law, H c =H [ 1 – ( T/ T c ) 2 ]

19. The magnetic field which causes a superconductor to become normal from superconducting state is not necessarily an external applied field , it may rise as a result of electric current flow in the conductor. In a long superconductor wire of radius r, the super- conductivity may be destroyed when a current I exceeds a critical current value I c , which at the surface of wire produce a critical field H is given by I c = 2 *3.14* rH c called Silsbee’s rule

20. Persistent current Superconductors are having property to maintain a current without application of voltage .

21. Applications of Superconductivity Some of them are: Magnetic levitation(Maglev) Magnetic Field Detection (SQUID) Power Transmission Superconducting Generators

22. Conlusions That superconductivity is one of the important discoveries in the field of modern physics . The mentioned properties predicts that the superconductivity can be classified into different types. The advantage of superconductors is their capability of carrying enormous amount of power without loss under critical temperature, so superconductors can save a lot of energy.

23. References Concepts of modern Physics- Arthur Beiser . Introduction to superconductivity-Michael Tinkham . www.superconductors.org www.chavaenergy.com

24. Thanks for Listening