Superconductivity is a state characterised by zero electrical resistance, perfect diamagnetism and long range quantum order. The route to the discovery of superconductivity began in April 1911 during low temperature electrical resistance tests on pure mercury by Heike Kamerlingh Onnes, where it was found that the resistivity decreased to ‘practically zero’. In May 1911 he noted the abrupt nature of that decrease, and the fact that it occurred at 4.2 K. At first labelled ‘supraconductivity’, Kamerlingh Onnes soon settled on naming the state ‘superconductivity’. The temperature at which the material transitions from normal to superconducting state is called the critical temperature, Tc, as shown in the resistivity graph of Figure.
Figure Temperature dependence of a normal metal and a superconductor
The lack of electrical resistivity would describe a perfect conductor, but superconductors also display unique magnetic properties, as they are capable of perfect diamagnetism in weak applied fields. This property was discovered in 1933 by Meissner and Ochsenfeld, many years after the initial discovery of superconductivity, and named the Meissner Effect. Perfect diamagnetism describes the ability of a material to completely exclude magnetic flux; an absence of magnetic permeability.
Superconductors can be classified in accordance with their critical temperature as;
(i) Low-temperature superconductors, or LTS: those whose critical temperature is below 77K.
(ii) High-temperature superconductors, or HTS: those whose critical temperature is above 77K.