Diffusion Caps Superconductivity

01The evidence

• Hydrogenation of metal superhydrides at extreme conditions is diffusion-driven and transient, with the hydrogen distribution evolving on experimental timescales rather than reaching equilibrium.Nature Communications, 2025
• The same transient hydrogenation behaviour has been presented as a design consideration for high-pressure synthesis, implying that what is measured depends on how the sample was made and how long it was held.MRS Spring 2026

02The tension

Electron-phonon calculations predict that several hydride families should superconduct above the current record, and some predict room temperature at pressures already reached. Measured values have plateaued. The usual response is that the right structure has not been stabilised yet, which is unfalsifiable as stated.

03The theory

The superconducting phase in these materials is a hydride with a specific hydrogen sublattice. That sublattice is mobile. At the temperatures and pressures involved, hydrogen can rearrange, and the arrangement that superconducts may be a configuration that persists only while the sample is held in a particular state.

If mobility is high enough, the phase has a lifetime rather than an equilibrium existence. The measured transition temperature is then a property of the transient, bounded by how long the configuration survives. That creates a ceiling that is kinetic, not electronic.

This explains the plateau without requiring the coupling calculations to be wrong. It also explains why different groups report different values for nominally the same material: they are measuring at different points in the transient.

The discriminating experiment is to hold the sample at fixed pressure and temperature and watch the transition temperature drift. If it drifts downward on a timescale of hours, the phase is transient and the ceiling is kinetic. If it is stable indefinitely, diffusion is not the limit.

04What this is not

05Predictions

• P1Transition temperature should decline measurably over hours at fixed pressure and temperature, at a rate that increases with temperature.
• P2Deuterium substitution should shift the effective ceiling more than the mass-scaling prediction, because diffusion changes more than phonon frequencies.
• P3Rapid quenching should suppress the transition temperature while slow annealing should raise it.

06What would refute it

A transition temperature stable to within a kelvin over days at fixed conditions would remove the kinetic ceiling and return the problem to structure prediction.

07What it would change

It would redirect synthesis toward suppressing mobility rather than optimising coupling, and reframe the hydride record as a kinetic boundary.