superconductivity The electrical resistivity of some materials drops to extremely low value, ~ 10-12 times of that of normal value, when its temperature is reduced below a critical temperature, Tc . The critical temperature is different for different material. Below the critical temperature the material is said to have made a transition to superconducting state. The phenomenon was first discovered by Onnes in 1911, in mercury at Tc = 4.15K. Over two dozen elements and thousands of alloys and compounds exhibit superconductivity. A note worthy feature is that copper, silver gold, and alkali metals do not show superconductivity. Also the ferromagnetic elements such as iron cobalt nickel do not show superconductivity.
Meissner effect: When a material makes a transition to the superconducting state it expels all magnetic flux from the interior of the material sample (see fig. s6). Since,
B = m 0 ( H+M)= m 0(1+c )H
= 0 for superconductor at T< Tc
Susceptibility,c is therefore,
c
= -1A superconductor therefore behaves like a perfectly diamagnetic substance below the critical temperature Tc.
Critical magnetic fields: When the magnetic field applied to superconductor exceeds critical field, Bc the flux penetrates into the material and superconductivity disappears. Superconductors show two types of behavior in external magnetic field. Depending on their behavior, superconductors are classified as class I and class II. In fig. s7-a the variation of critical field Bc with temperature is shown for class I superconductors. Class II (hard) superconductors are characterized by two critical fields (fig. s7-b). Above the critical field Bc1 the flux penetrates the superconductor in thin filaments but the superconducting current still persists. At the second critical field Bc2, the flux penetrates the sample completely and the sample goes to normal state.
High frequency properties: The ac resistance of a superconductor increases with frequency. At high frequency beyond 1011 Hz superconductivity disappears completely.
BCS theory of superconductivity: Burdeen, Cooper and Schrieffer proposed a theory of superconductivity. They suggested that the electron-phonon-electron interaction can produce a small attractive force between the oppositely spinning electrons, neutralizing the Coulomb repulsion between them. All the electrons in the conductor pair up, which are known as Cooper pairs. A normal current flowing in a conductor dissipates energy. Each flowing electron collides with other electrons and also with the ions of the solid. Below the critical temperature the Cooper pairs are all moving with same momentum. They do not transfer energy to the lattice as their speed is less than the speed of sound in the medium. Also there is no loss of energy by collision between the electrons. Therefore the superconducting current can flow indefinitely.
High Tc superconductors: Muller and Bednorz discovered in 1985, that ceramics formed by Ba-La-Cu-O become superconducting at 35K. The transition temperature was found to be at 90K when the lanthanide was replaced by yttrium. This opened up possibility of attaining superconductivity above the liquid nitrogen temperature (77K). Since then the Tc value is rising steadily with the discovery of new ceramics. Much developmental work is however needed before these materials can be fully exploited. No satisfactory theory has yet been developed for high Tc superconductors.