On Wednesday, a paper was published by Nature that describes a mixture of elements that can superconduct at room temperature. The work follows a general trend to find new ways to cram hydrogen into a mixture of other atoms using extreme pressure. This trend produced a number of high-temperature superconductors in previous research, although characterizing them was difficult due to the pressure involved. However, this new chemical superconducts at much lower pressures than previous versions, which should make it easier for others to copy the work.
However, the laboratory that produced the chemical had one of its previous papers on high-temperature superconductivity retracted due to a lack of detail regarding one of its key measurements. So it’s a fair bet that many other researchers will try to copy it.
Low pressure environment
The kind of superconductivity involved here requires electrons to cooperate with each other, forming what are called Cooper pairs. One of the things that encourages the formation of Cooper pairs is a high-frequency vibration (called a phonon) among the atomic nuclei that these electrons are associated with. It is easier to arrange with light nuclei, and hydrogen is the lightest thing there is. So finding ways to pack more hydrogen into a chemical is thought to be a viable path toward producing higher-temperature superconductors.
However, the safest way to do it involves extreme pressure. These pressures can cause hydrogen to enter the crystal structure of metals or to form hydrogen-rich chemicals that are unstable at lower pressures. Both of these approaches have resulted in chemicals with very high critical temperatures, the highest point at which they will support superconductivity. However, while these have approached room temperature, the pressures required were several Gigapascals – with each Gigapascal being nearly 10,000 times the atmospheric pressure at sea level.
In essence, this involves trading impractical temperatures for impractical pressures.
However, the hope was that we could use these chemicals to identify the general principles that produce this kind of hydrogen-rich superconductivity, and then use them to identify other chemicals that show similar behavior under conditions that are much easier to maintain.
That’s what happens in the new magazine. The research team zeroed in on lutetium based on the fact that the occupancy of its electron orbitals should provide a few more electrons that could potentially participate in the formation of Cooper pairs, possibly facilitating superconductivity. And they added trace amounts of nitrogen in the hope that doping the material would enable the chemical to adopt a configuration that helps stabilize it, potentially lowering the pressure needed.
Out of the blue
It was clear that something happened to the lutetium/nitrogen/hydrogen mixture before any measurements were made. At ambient conditions, lutetium turned blue upon addition of the two gases, probably due to hydrogen infiltrating the metal. But as the pressure increased to thousands of atmospheres, the mixture turned a dramatic pink, which was found to be associated with the mixture becoming metallic. Continuing to increase the pressure to over 30,000 times atmospheric pressure, it lost its metallic properties and became a deeper red color.
Superconductivity was possible throughout the range from 3,000 to 30,000 times atmospheric pressure. So the researchers worked through this pressure range to find the pressure that supports the highest critical temperature. The peak was found to be at approximately 10,000 times atmospheric pressure.
That temperature was 294 K. That means about 21° C, or 70° F, which, for most of us, is room temperature.
Superconductivity also changes the magnetic properties of the material, and a large part of the paper is taken up with a discussion of measuring the sample’s magnetic properties. This is not an easy thing to do, given how small the sample is and that it is sandwiched within all the hardware necessary to crush the sample under extreme pressure.
A lot of work also went into figuring out what the material is. It almost certainly has some hydrogen and nitrogen incorporated into the metal, but it is unclear how much, since any excess of the two gases could simply be excluded from the sample. The researchers tried to do crystallography on it, but the results are somewhat ambiguous. The signal from hydrogen (one atomic weight) is swamped by the signal from lutetium (atomic weight 175), and it is possible for hydrogen to move around the material.
So while they identify where hydrogen power be in the material, it is not clear how many of these sites were actually occupied. And this will make it challenging to extract larger principles from the behavior of this material.
Can we believe it?
Hanging over all this is the retraction of the paper describing some of the same lab’s previous measurements. That retraction was made by the editors of Nature over the scientists’ objections. It was withdrawn due to problems with the data involved in the magnetic measurements, but was undoubtedly hastened by the fact that no one could confirm the magnetic behavior because they were unable to produce the chemical described in the earlier paper.
In light of that, the opposite response here would be to distrust the current work. But it is also reasonable to expect that all the peer reviewers of the new paper had the same reluctance, so it is likely that the new paper was reviewed very carefully.
But most importantly, if this work can be reproduced, it is likely that many people will do so relatively quickly. That’s because it requires far less complicated hardware to create. As long as a lab has decent air conditioning, it should be trivial to keep a sample at the temperatures reported here. And the required pressure can be achieved with far less complicated equipment than you need to hit the Gigapascals required by previous materials of this nature.
As a result, this material should be available to many more laboratories than could previously work on hydrogen-rich superconductors. So if these results are real, we should see reports of the results being reproduced very soon.
Nature, 2023. DOI: 10.1038/s41586-023-05742-0 (About DOIs).