Illustrated Physics
physics, drawn
Concept

Type ii superconductor — where it appears

A superconductor that lets magnetic flux in as quantised vortices above a lower critical field instead of losing superconductivity outright. Pinning those vortices on defects lets it carry large currents in strong fields, which is why magnet wire is made of alloys like niobium–titanium.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

How a field enters a superconductor that pins it. The magnetic field inside a slab of type-II superconductor, against position across its thickness, as an applied field is raised from zero, in units of the full-penetration field H = Jc a. Each line is the profile at an applied field of 0.5 H, 1.0 H, 1.5 H, 2.0 H. The field enters from both faces as a wedge whose slope is the most current the pinned vortices can carry, Jc; at 0.5 H it reaches halfway to the middle, at H* it just reaches the middle, and beyond that the whole slab carries Jc and the profile rides up with the applied field. A type-I superconductor would show no field inside at all, and a normal metal the applied field everywhere.

The field a superconductor keeps after all

A superconductor is famous for throwing magnetic field out, and the experiment that showed it is the one that made superconductivity a state of matter. Most superconductors that are actually used do something closer to the opposite: they let the field in as threads, grip the threads where they are, and keep them when the field is taken away. The result is a slope of field inside the material that is as steep as the grip allows and no steeper, which is all that is needed to explain why such a superconductor has a hysteresis loop, heats up in an alternating field, can hang a magnet underneath itself, and can be made into a magnet stronger than any of iron.

electromagnetism · Superconductivity A current limited by the wire's circumference. The largest supercurrent a wire can carry before the field of its own current destroys superconductivity at its surface, Ic = 2πaHc, against the wire's diameter, for lead, mercury and tin at 1.5 K, on logarithmic axes; dashed, a niobium–titanium wire carrying 3,000 A/mm² in a field of 5 tesla, which grows with the cross-section rather than the circumference. A lead wire 1 mm across carries 192 A — 244 A/mm² — and one 1 cm across only ten times as much, at a tenth of the density. Below 81 micrometres lead carries more than niobium–titanium; above it niobium–titanium wins by a factor that grows with the wire, and it does so in a field of 5 tesla, where lead cannot superconduct at all.

The current a superconductor stops with its own field

A superconducting wire has no resistance, so it seems it should carry any current at all. It carries a definite amount and no more, and the amount is set not by the metal's ability to conduct but by the field the current itself makes: when that field at the wire's surface reaches the critical field, superconductivity fails there. The rule makes the largest current proportional to the wire's radius rather than its area, makes a thick wire worse per square millimetre than a thin one, and explains why the first superconducting magnet, wound in 1913, did not work.

electromagnetism · Superconductivity

Named alongside it

The objects these essays reach for when they reach for this one.

Meissner effectSuperconductivityAmpere lawCritical currentCritical fieldCritical stateEddy currentFlux freezingFlux quantumHysteresisIntermediate stateMagnetisation

All concepts