Most of metal’s electrical resistance decreases in low temperature . Lattice vibration (thermal vibration of atoms) invites electrical resistance and scatters conduction electrons.

That is why metals has electrical resistance. Electrical resistance decreases in lower temperature because the lattice vibration is weakened by decreasing temperature. However, some of the metal completely lost electrical resistance at a temperature (superconducting transition temperature: T_c). This is the superconductivity!!

This phenomena was discovered by Kamerlingh Onnes (University of Leiden, Holland) in 1911[1]. At that time, many discussed electrical resistance at low temperature. Therefore He measured electrical resistance in Hg. He found that electrical resistance suddenly decreased to zero around 4.2 K (Fig.1). Firstly, he doubted experimental mistake to the surprising result. However, with the re-examination, this phenomena was determined essential phenomenon and named “SUPERCONDUCTIVITY”. Today, about 2000 types of superconductor are discovered.

[1]  H. Kamerlingh Onnes, Akad. Van Wetenschappen 14, 113 818 (1911)