Are superconductors really superconducting?

The most striking difference between the behaviour of the copper oxide high-temperature superconductors and previous low-temperature type II superconductors is the much more gradual decrease in electrical resistance with temperature in the latter, in the presence of a magnetic field. This raises the question of whether a type II superconductor has strictly zero resistivity, when cooled in a magnetic field. Theoretical and experimental evidence now suggests that as the temperature is lowered, there is a sharp phase transition to a truly superconducting, impurity-dominated phase containing a disordered, frozen arrangement of magnetic flux vortices. © 1992 Nature Publishing Group.