Superconducting materials are characterized by the loss of resistance and two parameters: a critical temperature T and a critical magnetic field which brings the 
superconductor to its normal state.  In 1911, H. Kamerlingh Onnes discovered the superconductivity of mercury at a temperature below 4 K. Later, 
other substances with superconductivity at temperatures up to 30 K were found. Superconductors prevent the penetration of the external magnetic field 
into the sample when the magnetic field strength is less than the critical value. This effect was called the Meissner effect.
High-temperature superconductivity was discovered in the 1980s. Of the known compounds, the highest critical temperature T = 135 K belongs to 
HgBaCaCuO.
Low-temperature superconductivity has found a theoretical explanation in the model of Bardeen, Cooper, and Schrieffer (BCS theory).
The physical basis of the model is the phenomenon of Cooper pairing of electrons. Since a pair of electrons carries an integer spin, the correlated states of the electrons can form a Bose–Einstein condensate.
An equivalent formalism was developed by Bogoliubov
and Valatin 
Cooper pairing of nucleons takes place in ordinary nuclei. The effect manifests itself in the Bethe–Weizsacker mass formula, the last pairing term 
of which describes the correlation energy of two nucleons. Because of the pairing, the binding energy of even-even nuclei systematically exceeds the binding energy of odd-even and 
odd-odd nuclei.