critical field
/ KRIT-ih-kul feeld /
Superconductivity is a delicate truce between the paired electrons and the world around them, and a magnetic field is its natural enemy. A weak field is simply pushed away and shrugged off. But turn the field up and up, and at some point it overwhelms the pairs' ability to keep it out — like a dam that holds firm until the water finally crests the top.
The critical field is the strength of applied magnetic field at which superconductivity is destroyed. A field exerts energy, and expelling it costs the superconductor energy too; once the field is strong enough that holding it out costs more than the pairing saves, the material gives up and returns to its normal state. In type-I materials there is a single such threshold; in type-II materials there are two — a lower field where vortices begin to enter and a much higher one where superconductivity finally collapses.
This matters because the critical field, together with the critical temperature and critical current, sets the boundary of where a superconductor can actually be used. The practical lesson is sobering: every superconducting device must operate well inside all three limits at once. A common slip is to treat the critical field as a fixed number, but it falls as the temperature rises, vanishing entirely right at the critical temperature.
Lead's critical field is only about 0.08 tesla, but the type-II compound niobium-tin keeps superconducting up to roughly 30 tesla — hundreds of times stronger — which is why high-field magnets are always made from type-II materials.
Critical fields range from a fraction of a tesla in type-I to tens of tesla in the best type-II materials.
A subtle point is that a superconductor carrying its own current creates a magnetic field around itself, so even with no external magnet a large enough current can produce a field that exceeds the critical field and quenches the superconductivity from the inside.