A Diamond Microscope Watched a Superconductor Repel a Magnetic Field

Lanthanum hydride is not a material you can hold in your hand. To make it, you squeeze a speck of lanthanum and hydrogen between the polished tips of two diamonds until the pressure passes a million atmospheres, and it survives only as long as the vise stays shut. Sealed inside that space is the boldest surviving claim in superconductivity: a material that shows signs of carrying current with no resistance at around 250 K, roughly minus 23 °C.

That claim has been hard to test, for reasons that have nothing to do with theory. On September 11, 2026, a team at the Institute of Physics of the Chinese Academy of Sciences reported in the Proceedings of the National Academy of Sciences that it had finally got a magnetic sensor in there beside the sample. Yang Chen led the work with Gang-Qin Liu and eight colleagues, and what the sensor saw was a magnetic field being pushed out.

A superconductor announces itself twice. Its electrical resistance falls to zero, and it also pushes a magnetic field out of its interior, which is the Meissner effect. Resistance is the easier of the two to measure through a diamond anvil, and it is the weaker evidence, because a drop in resistance can have other causes. The magnetic signature is the one the field has learned to insist on. High-pressure superconductivity has carried two bolder claims than lanthanum hydride's in other materials and from other groups, and both were retracted. That history is why lanthanum hydride's is called the highest transition temperature claimed for any superconductor whose claim still stands.

Measuring that expulsion is the hard part. Magnetic evidence for lanthanum hydride has been gathered before, by Minkov and colleagues, who trapped magnetic flux in hydrogen-rich samples. Those measurements average over a whole sample, and a sample squeezed that hard is a single grain. They cannot say where in the grain the transition happens.

The alternative is to build the sensor into the diamond itself. Nitrogen-vacancy centers, atom-scale compasses made from defects in the crystal, sit in the anvil face against the sample and report the magnetic field where each of them sits. Their limit has been pressure: published work had taken them to about 130 gigapascals, well short of what lanthanum hydride needs.

The team extended that working pressure to nearly 180 gigapascals, using a gaseous medium to transmit the pressure to the sample. Two numbers are easy to run together here and should not be. The sensors can now survive that pressure; the superconductor itself was measured at 155 gigapascals.

At that pressure the sensors picked up magnetic field screening and the Meissner effect at around 240 K, consistent with the record transition temperature of LaH10, the hydrogen-packed form of lanthanum hydride that the claim rests on. The map then did something a bulk measurement cannot. One sample had been heat-treated too little, and the sensors resolved it point by point as uneven, with a transition near 220 K. Independent X-ray measurements suggest that lower temperature could come from a phase with slightly less hydrogen in it.

The authors are careful about what this settles: their own summary is that the work provides experimental evidence for superconductivity in lanthanum hydride. Not proof, not confirmation. On the published record it is the first time the Meissner effect in lanthanum hydride has been imaged with diamond quantum sensors, and the first spatially resolved magnetic map of its transition. Diamond sensors had imaged the Meissner effect in a hydride once before, in cerium hydride rather than lanthanum. That was an independent group in 2024, and this paper cites it. This one is peer-reviewed, and it reports a single laboratory's measurements on its own samples.

The authors' second conclusion is the more practical one: at these pressures the limiting factor is the sample rather than the physics, and improving samples means being able to see which part of one is working. That is what the work hands the field: an instrument, not a verdict on lanthanum hydride. A sample can now be checked, while it is still under pressure, for where it superconducts and where it does not. That kind of feedback is what makes a material better instead of just more argued over.