Discovered by Heike Kamerlingh Onnes on April 8, 1911, when the electrical resistance of solid mercury cooled in liquid helium vanished abruptly at 4.2 K.
Phenomenological Ginzburg-Landau theory (1950) and Alexei Abrikosov's analysis divide superconductors into Type I (complete flux expulsion) and Type II (mixed vortex state), while BCS theory (1957) explains conventional pairing.
In 1986 Georg Bednorz and Karl Alexander Müller discovered high-Tc cuprate superconductivity above 35 K in LBCO, followed in 1987 by YBCO at 92–93 K above liquid nitrogen's 77 K boiling point.