type-II superconductor
/ type-TOO SOO-per-kun-DUK-ter /
Imagine a leakproof boat that, instead of either staying dry or sinking outright, learns a clever compromise: when the water gets high, it lets a few narrow columns of water pass straight through sealed tubes, staying afloat around them. By admitting the flood in tidy, isolated channels, the boat survives conditions that would swamp a simpler vessel.
A type-II superconductor does exactly this with magnetic field. Below a first critical field it expels the field completely, like a type-I material. But between a first and a much higher second critical field it enters a mixed state: the field threads through in thin quantized tubes called flux vortices, each carrying one quantum of magnetic flux, while the material between the vortices stays fully superconducting. Only when the field climbs past the second critical field, packing the vortices so densely they overlap, does superconductivity finally die.
This matters enormously: because they tolerate huge fields, type-II materials such as niobium-titanium alloys make every powerful superconducting magnet — in MRI machines, particle colliders, and fusion experiments. The subtlety is that the vortices can drift when a current pushes on them, and a moving vortex dissipates energy, mimicking resistance. To carry large lossless currents, real wires must pin the vortices in place with deliberate defects.
The magnets inside an MRI scanner are wound from niobium-titanium wire, a type-II superconductor that stays superconducting in fields of several tesla — strong enough to image the human body, and impossible for any type-I metal.
Type-II niobium-titanium tolerates several tesla, which is why it powers MRI and collider magnets.
Whether a material is type-I or type-II is decided by a competition of two length scales: if field penetrates deeper than the distance over which a Cooper pair stays correlated, the material favours admitting vortices and is type-II. Nearly all useful, high-field superconductors are type-II.