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
, above which it is a normal
conductor. When a current is established
, it persists almost indefinitely. Magnetic field
s 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.