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, T
c, 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.