A superconductor is a pure metal that at low temperatures has negligible resistance to the flow of electric current. Each material has its own critical temperature, Tc, above which it is a normal conductor. When a current is established, it persists almost indefinitely. Magnetic fields can destroy the superconductivity, their strength depending on how far below the critical temperature the material is.

Generally, Tc has been < 20 K but in 1986-7 a class of materials with perovskite structures were discovered, where Tc ~ 90 K.

See Meissner effect.