Illustrated Physics
physics, drawn
Concept

Intermediate state — where it appears

The mixture of normal and superconducting regions that a type-I superconductor breaks into when its shape forces part of a field through it. It appears between zero and the critical field, as stripes or tubes whose spacing is set by the energy of the walls between them.

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

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 Flux leaving a plate in stripes and spreading into a uniform field. A cross-section through a superconducting plate in the intermediate state near its top surface (below the line), with a normal fraction of 0.40: normal laminae (shaded) each carry the critical field straight up, the superconducting laminae between them carry none. Lengths are in units of the period of the pattern. Above the surface the field lines (solid) are computed from the stray field of the stripes: they fan out from each normal lamina and close up over the superconducting ones, and within about a third of a period the field is uniform at 0.40 of the critical field — the applied field, which is all a distant observer measures. The fanning field stores energy, and it stores more the wider the stripes are, because a wider pattern's non-uniformity reaches further out. That is half of what sets the period.

The stripes a superconductor makes of a field

A superconductor expels a magnetic field, but only if the field it pushes aside stays below the critical field everywhere at its surface — and for anything but a long rod held along the field, the field it pushes aside piles up at its edges. A flat plate laid across a field cannot expel any of it. It splits instead into interleaved bands of normal metal carrying exactly the critical field and superconducting metal carrying none, in the proportion flux demands, and the bands have a definite width. Walls between the two cost energy and favour few, broad bands; the field fanning out of the surface favours many narrow ones. The compromise is the geometric mean of the plate's thickness and a microscopic length.

electromagnetism · Superconductivity

Named alongside it

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

Critical fieldSuperconductivityAmpere lawCritical currentDemagnetising factorDomain wallMagnetic domainsMagnetic fluxMeissner effectPenetration depthSurface energyType II superconductor

All concepts