Superconductivity and Optical Fibers
Introduction of superconductivity:
Superconductivity is a phenomenon observed in several materials that demonstrate no resistance to the ﬂow of an electric current when cooled to temperatures ranging from near absolute zero (0K) to liquid nitrogen temperatures (77K). The temperature below which electrical resistance is zero is called the critical temperature Tc and this temperature is a characteristic of the material. Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity does not occur in noble metals like gold and silver, nor in pure samples of ferromagnetic metals. Another striking property of superconductors is that they are perfectly diamagnetic. Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, who was studying the resistance of solid mercury at cryogenic temperatures using liquid helium as a refrigerant. He got 1913 Physics Nobel Prize for his work.
Temperature dependence of resistivity in superconducting materials:
The resistivity of metals is attributed to the scattering of conduction electrons. The scattering of electrons takes place because of two reasons: one due collisions of conduction electrons with the vibrating lattice ions and the other is caused by scattering of electrons by the impurities present in the metal.
The resistivity due to scattering of electrons by the lattice vibrations called phonons is denoted by ρp. This increases with temperature. It arises even in a pure conductor and hence called the ideal resistivity. Whereas the resistivity of metals caused by scattering of electrons with the impurities is denoted by ρi. This is independent of temperature and present even at absolute zero of temperature and hence called residual resistivity. Therefore, the total resistivity of a metal can be written as the sum of the two resistivities. This is called Matthiessen’s rule. Mathematically,
ρ = ρp ρi
However, some metals show a remarkable behavior. They lose their electrical resistance completely below a certain temperature, called critical temperature Tc. Below the critical temperature these superconducting materials can carry large amounts of electrical current for long periods of time without loosing energy as ohmic heat. The variation of electrical resistance versus temperature of ordinary metals and superconductors is shown the graph.