In year 1911, Kammerling Ones observed the sudden disappearance of electrical resistance when pure mercury was cooled below a particular temperature. In mercury, transition from normal state to superconducting state was observed at a temperature 4.2K called critical transition temperature ‘Tc’. Since then scientists and researchers are trying to understand the basic principle phenomenon responsible for such transitions. The basic concept was given by Bardeen, Cooper & Schreiffer in 1957 which is known as BCS- theory. Looking into the potential applications of superconducting materials with higher critical transition temperatures, continuous efforts have been made to raise the transition temperature ‘Tc. Although many metals, alloys and oxides exhibited superconductivity but till 1985, highest Tc of 23.2K was achieved in Nb3Ge. In 1987, Y-Ba-Cu-O oxide was observed to exhibit a Tc of 90K. Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O and Hg-Ba-Ca-Cu-O were discovered later with more higher Tc, the maximum value of Tc being 154K.
In 1933, it was observed by Meissner and Ochsenfeld that superconductors in the superconducting state expel all magnetic field from its interior and behave like perfect diamagnetic materials. The superconducting state is vanished when the magnetic field strength is increased beyond a particular magnetic field, called the critical magnetic field ‘Hc’. The superconductivity of a material can also be destroyed by passing sufficient electrical current through it, called critical current ‘Ic’.
A new class of oxide superconductors was discovered in 1966 when superconductivity was found in SrTiO3 at 0.3K. In 1973, a higher Tc of 13.7K was reported in Li-Ti-O system. In 1986, a research for High Tc superconducting materials started in a big way soon after the discovery of superconductivity in La 2-xBax CuO4-y at 38K by Bednorz and Muller. Later Superconductivity in the mixed valence compound BaPb1-xBixO3, was discovered which has perovskite structure. One may expect still higher Tc’s in other metallic oxides if the electron phonon interaction and the carrier density could be further enhanced. The strong phonon-electron interaction in oxides may result into polaron formation and mixed valence states. The conditions of polaron formation- mixed valence state was achieved in La2-xMxCuO4-y (M= Ba, Sr &Ca). The YBa2Cu3O7-y also show polaron formation and mixed valence states and thus higher transition temperatures. As of year 2019, the highest critical transition temperature is 250K which has been observed in LaH10 at a very high pressure.
Dr. Veer Singh
Department of Applied Science and Humanities