How do high-temperature superconductors work?

Superconductivity, the total disappearance of electrical resistance, was discovered in 1911 and explained in 1957 by BCS theory, one of the triumphs of twentieth-century physics: electrons pair up via vibrations of the crystal lattice and flow without friction. That mechanism caps out at low temperatures, and for decades nature obeyed the cap. Then in 1986, Bednorz and Müller found a copper-oxide ceramic superconducting at temperatures the theory could not reach, and within a year relatives were superconducting above the boiling point of liquid nitrogen. The discovery won the fastest Nobel in the prize’s history. The mechanism behind it is still argued about.

Why it matters

Room-temperature superconductivity at ordinary pressure would rank among the most transformative technologies imaginable: lossless power grids, cheap magnetic levitation, compact fusion magnets, desktop MRI. Whether it is possible depends on understanding why the cuprates work, since blind materials search is slow and the parameter space is vast. The problem is also a benchmark for theory itself: the cuprates are the flagship of strongly correlated electron systems, materials where electrons cannot be treated one at a time, and physics lacks general methods for them. Cracking the cuprates likely means cracking that wider class.

What has been tried

The electrons in cuprates clearly still pair; magnetic flux measurements show charge moving in units of two. The dispute is over the glue. Lattice vibrations appear too weak at these temperatures, so leading candidates include magnetic spin fluctuations, since superconductivity in these materials always appears next to magnetic phases, and more radical proposals in which no conventional glue exists and pairing emerges directly from electron repulsion. The materials complicate every test by being strange even when not superconducting: above the transition they show a pseudogap phase and a strange metal regime whose resistivity defies standard theory. Iron-based superconductors, discovered in 2008, gave a second family to compare. Hydrogen-rich compounds crushed to immense pressures superconduct near room temperature, a real advance that confirms conventional theory under exotic conditions rather than explaining the cuprates. High-profile claims of ambient-condition superconductivity have so far ended in retraction.

Where the edge is

After nearly forty years there is no consensus mechanism for cuprate pairing, no accepted theory of the strange metal, and no predictive recipe for raising the transition temperature. Simulation of the relevant models remains beyond classical computers, which has made these materials a target for quantum computing.

What would count as an answer

A theory that predicts transition temperatures from composition and structure, confirmed by a designed material that works as calculated. The field’s dream deliverable follows: a superconductor for the world as it is, no cryogenics required.