Exciton Mechanism of Superconductivity
A large number of metals and alloys become superconducting when cooled below a certain critical temperature, Tc which is typically of the order of a few degrees Kelvin.1 In this state the electrical resistivity is immeasurably small and currents can be carried without dissipation of energy. However, a magnetic field which is normally excluded from the interior of a superconductor, penetrates when its value exceeds a certain critical value He driving the metal into the normal state. Two types of superconductors occur: the type I, in which the superconductivity is quenched at a field Hc and the type II in which field penetration occurs at Hcl but superconductivity remains up to a higher critical field Hc2. Practical applications of superconductivity are based on the use of type II superconductors because, while He is typically less than about 1000 gauss, Hc2 can be much higher, exceeding 200 Kgauss. Superconducting magnets have been built using the alloys Nb3Sn and NbTi which produce fields of the order of 150 Kgauss and 80 Kgauss, respectively. These magnets offer something totally new to the area of electromotive machinery for the field energy and electromagnetic forces produced are orders of magnitude greater than those attainable with iron cored magnets. Large scale application of superconducting technology in high speed transportation, motors, generators and ship propulsion is expected in the next few decades. However, in all these applications sophisticated refrigeration is needed to maintain the metal in the superconducting state. We shall examine therefore, those factors which limit the value of Tc, the superconducting transition temperature and what prospects exist for substantially raising Tc.
KeywordsSuperconducting State Coulomb Repulsion Superconducting Transition Temperature Exciton Band Debye Frequency
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