Originally it was very difficult to explain the origin of superconductivity as then, it was not even possible to explain the zero resistance at 0 K putting the explanation of high temperature zero resistance out of question. Initial theories proposed a qualitative two conduction mechanisms i.e. a normal fluid of electron and another superconducting fluid of electrons. However, the theory did not have any theoretical or analytical basis and was discarded.

An appropriate theory of superconductivity was proposed by J. Bardeen, L.H.  Cooper and J.R. Schreiffer in 1957 at University of Illinois (USA) and for which they were awarded the Nobel Prize in Physics in 1972. Following this work, the theory has been called as BCS theory.

BCS theory relies on quantum mechanical calculations which show that in the superconducting state below T_c, there exists an energetically favoured ordered state formed by the electrons and is called as Cooper Pairs.  While earlier it was thought that in the superconducting state, the electrons do not interact with the lattice atoms destructively. In the contrast, the BCS Theory supports the interaction of electrons with the atoms but in a constructive manner leading the formation of Cooper Pairs.

The theory makes an important assumption that there exists an attractive force between the electrons in typical type‑I superconductors, which is due to Coulombic attraction between the electron and the lattice. It is based on the understanding that due to negative charge an electron in the lattice, there is a build of slight positive charges around it which in turn, attracts another electron and these two electrons are known as a Cooper Pair, as shown schematically in Figure 7.4. The pair is stable only when the binding energy of this pair is energy to keep them together is smaller than that from the thermal vibrations of the lattice which would attempt to break them apart. This is why it is necessary that superconductivity is essentially a low temperature phenomenon.

7.4.1 Experimental Validation

The experiments were performed on the basis that if electrical conduction in mercury was purely electronic, it should have no dependence on the nuclear masses. The observed dependence of the superconducting critical temperature, T_C, upon the isotopic mass was the first direct evidence for interaction between the electrons and the lattice (see Fgure 7.5). These experiments supported the BCS theory based on electron-lattice interaction leading to the formation of Cooper pairs. The effect is clearly observed in the case of Type I superconductors while rather weakly in Type II superconductors. At the end, one can observe that it is quite remarkable that an electrical phenomenon such as the transition to zero electrical resistance should be associated with the purely mechanical process of the lattice.