Superconductivity

For metals in the usual temperature range around 298 K, resistance decreases with T, or conductivity increases with lowering T. In 1911, Kimberling ones showed that when Hg is cooled to T 4.2K, the resistance to electron flow drops to below 10^-5 ohms and the metal behaves as a "superconductor". In the superconducting state, electrons flow without any resistance offered by lattice sites or lattice vibrations. Unusual magnetic and entropic behaviour is also exhibited. In the first insightful theory of superconductivity, Bardeen, Cooper and Schrieffer (1957) proposed a theory wherein ordered pairs of electrons are formed at low temperatures. Although the ordering is caused by the interaction of electrons with lattice ions, the "ordered pairs" move freely in the lattice. Considerable efforts have been put in to find high temperature superconductors and nonstoichiometric materials of Pervoskite structures, such as Y Ba₂Cu₃ O_7-x with x 0.1 have shown superconductivity for T T_c = 90 K.