The Lock

Heike Kamerlingh Onnes chilled mercury with liquid helium to 4.2 kelvin and watched its resistance fall off a cliff. Not to small. To nothing. A current started in a superconducting loop will keep circling for years with no battery attached.

YBCO does the same trick at 93 kelvin. That sounds cold until you remember liquid nitrogen sits at 77 and costs about as much as milk. Here comes the nitrogen.

Walther Meissner and Robert Ochsenfeld found that a superconductor doesn't just carry current perfectly. It refuses to let a magnetic field inside.

Watch the field lines. They used to run straight through the puck. Now they bend underneath it and pile up. Pushing a field out of a material takes force, and the force on the puck points up.

If that were the whole story, the puck would skate off like a hockey puck on ice. Ceramics like YBCO are messier, and the mess helps. The field forces its way through in thin threads, each carrying exactly one quantum of magnetic flux, and defects in the crystal nail those threads in place.

So the puck is locked in all three dimensions, at whatever height and tilt it had when it went cold. It can still slide along the track, because the field looks the same all the way around. Give it a push.

Same pins, other direction. Flip the track and the puck hangs underneath it at the same distance, still circling. Gravity is pulling. The pinned flux is pulling harder.

Someone will call this levitation. It's closer to the puck being nailed to the air.

Every mark is a record. The climb is slow until 1986, when two IBM researchers in Zurich found superconductivity in a ceramic nobody had bothered to check, and the record leapt from 23 to 133 kelvin in seven years.

Then it stalled. The hydrides got hotter, but only when crushed to pressures found at the edge of the Earth's core. Two loud room-temperature claims, in 2020 and 2023, fell apart under scrutiny. At ordinary pressure the record has sat near 133 kelvin since the early nineties.

Neodanium superconducts up to 371 kelvin, 98 °C, at ordinary pressure. Set it on the track at room temperature and it locks. No nitrogen. No flask. No fog.

Every superconducting magnet ever built, in every MRI scanner and particle accelerator, has needed a cryogenic plant to keep it alive. With Dn they need a shelf. That missing fog is the story of the next hundred years.