7.1 Background

Superconductors are materials which, when cooled below room temperature, exhibit a sudden drop of electrical resistance of the material to exactly zero at a temperature called as critical temperature, T_C. The phenomenon was discovered by Heike Kamerlingh Onnes in 1911when he was studying properties of mercury at liquid helium temperatures.

Most first generation superconductors were elemental metals and metallic alloys. While most pure elemental materials like tin, aluminum were called as type-I superconductors, metallic alloys like niobium nitride, niobium-titanium, and niobium-germanium alloys were classified as type-II superconductors. However, most of these are superconducting at temperatures below 30 K.

The onset of superconducting transition is also accompanied by abrupt changes in various physical properties such as heat capacity, which can be associated with a phase transition. For instance as shown in figure 1, while in the non-superconducting (normal) regime, the electronic heat capacity is proportional to the temperature i.e. C_v ∝ T, at T_C, it shows an abrupt jump and does not remain linear in the superconducting regime, varying as C_v ∝ T^-αT where α is a constant. The nature of phase transition depends upon the type of superconductor.

Superconductivity is also a quantum mechanical phenomenon like magnetism and a modern well accepted theory was developed by Bardeen-Cooper- Schrieffer³ in 1957 for which they won Physics Noble prize in 1972. They explained superconducting current as a superfluid of Cooper pairs, pairs of electrons interacting through the exchange of photons.

Another point to note is that while low resistivity is a necessary condition for a material to be a superconductor, it is not sufficient.