An all-or-nothing first family
Recall from the Meissner guide that a superconductor pushes magnetic fields out of its body, and that if the field gets too strong — past its critical field — it gives up and turns back into an ordinary metal. The very first superconductors discovered, like Onnes's mercury and lead, do this in an all-or-nothing way. Below the critical field they are perfectly field-free inside; the instant the field exceeds it, superconductivity collapses completely everywhere at once.
These are called type-I superconductors. They are clean and simple, and they show the textbook Meissner effect perfectly. But there is a catch that makes them nearly useless for building strong magnets: their critical field is feeble. A modest magnet is enough to destroy their superconductivity. And since carrying a large current itself creates a magnetic field, a type-I wire can only carry a small current before its own field kills the very state it relies on.
A cleverer second family
Then a second family was found that strikes a compromise — and that compromise changed technology. These are type-II superconductors. At weak fields they behave like the first family, expelling the field entirely. But instead of collapsing all at once when the field grows, they make a deal: above a first, lower threshold they let the field poke through in a controlled, piecemeal way, while staying superconducting everywhere in between.
Picture an umbrella in a heavy rain. A type-I umbrella keeps you bone dry until the wind rips it inside out, after which you are soaked all over. A type-II umbrella, faced with the same wind, lets a few drops through tiny pinholes — you get a little wet in a few spots — but the rest of you stays dry and the umbrella keeps working through far worse weather. The pinholes are the price of survival.
Whirlpools of magnetic field
Those pinholes have a name and a beautiful structure. Each one is a flux vortex: a thin tube where the magnetic field is allowed to pass straight through the material, surrounded by a tiny whirlpool of circulating superconducting current — like the swirl of water spiraling down a drain. The current swirls around the tube precisely to wall off the field, keeping it pinned inside the narrow core and out of everywhere else.
And here the flux quantization from earlier returns with full force: every vortex carries exactly the same fixed packet of magnetic field, never more, never less. Stronger field simply means more vortices, packed closer together, each identical to the last. At low fields they spread out into a neat triangular grid you can actually photograph — a tidy lattice of whirlpools threading the metal.
TYPE-I: field | superconducting?
weak | yes, fully field-free inside
> Hc | NO (collapses everywhere at once)
TYPE-II: field | superconducting?
weak | yes, fully field-free inside
medium | yes, but field pierces as vortices
> Hc2 | NO (vortices overlap; state is gone)Why vortices must be pinned down
Admitting vortices is what lets type-II superconductors survive enormous fields — but it brings a new problem. The largest steady current a superconductor can carry without losing its zero resistance is called its critical current. In a type-II material, the limit on that current is set by the vortices. When current flows past a vortex, it pushes on it sideways, and if the vortices start to move, that motion drags energy out of the current and resistance creeps back. A superconductor with wandering vortices is no longer perfectly lossless.
The fix is to nail the vortices in place. Engineers deliberately sprinkle a wire with tiny flaws and impurities, which act like little hooks that snag each vortex and hold it still. This is called vortex pinning. A well-pinned wire can carry a huge current in a powerful field and still show no resistance, because the vortices are stuck and cannot drain energy. The whole art of making superconducting magnets is, to a surprising degree, the art of pinning vortices well.
Why the difference at all
What decides which family a material belongs to? It comes down to two tiny length scales fighting it out. One is the coherence length from the last guide — roughly the size of a Cooper pair, the smallest patch over which the superconducting state can vary. The other is how deeply a magnetic field can seep in from the surface before the screening currents stop it. When the field seeps in over a longer distance than the coherence length, the material finds it worthwhile to riddle itself with vortices, and it is type-II; when the reverse holds, it is type-I.
You do not need the arithmetic to keep the punchline: type-I gives up all at once at a low field, while type-II tolerates field by letting it through as a lattice of quantized vortices, surviving to fields hundreds of times higher. Almost every superconductor that does real work — every powerful magnet you will meet in the next guide — is a type-II material.