How a paper on room-temperature superconductors is creating controversy
University of Rochester Assistant Professor Ranga Dias. J. Adam Fenster /University of Rochester
A new study led by Ranga Dias at the University of Rochester, reported discovering room-temperature superconductivity in nitrogen-doped lutetium hydride at roughly a thousand atmospheres of pressure. The discvery has been turning heads but not for the right reasons
VIJAY B. SHENOY
At every power plant, a portion of the electricity generated is lost during transmission because the wires and cables that carry the current have electrical resistance. We can mitigate this to a large extent if we use a material that does not resist the flow of current. Physicists discovered such materials a century ago — they are called superconductors. They have since realised that superconductors can exhibit truly quantum phenomena that have the potential to enable revolutionary technologies, such as enabling efficient quantum computers.
‘Holy grail’
All the materials we now know to be superconductors become that way in special circumstances. Outside those circumstances, they resist the flow of current. For example, aluminium becomes superconducting at a devilishly cold temperature of less than –250°C. Physicists and engineers have been toiling to find materials that superconduct electricity in ambient conditions, that is at one or a few atmospheres of pressure and at room temperature. Given their potential, finding such materials is one of the holy grails of physics and materials science.
The theory that explains why some materials become superconductors in some conditions suggests that hydrogen and materials based on it could hold great promise in this pursuit. And just as predicted, in 2019, scientists in Germany found lanthanum hydride (LaH10) to be a superconductor at –20° C, but under more than a million atmospheres of pressure — pressure that is only realised at the centre of the earth.
This is where a new study, published in Nature on March 8, becomes relevant. Researchers in the U.S., led by Ranga Dias at the University of Rochester, reported discovering room-temperature superconductivity in nitrogen-doped lutetium hydride at roughly a thousand atmospheres of pressure, which on the face of it is a great advance.
The method
The key is the choice of the material. The authors specifically suggest that the presence of nitrogen is what led to the findings. They found a way to push some nitrogen into the crystal of lutetium hydride by developing a high-pressure synthesis process. Superconductivity in the material is brought about by the (microscopic) jiggling motion of the crystal, and the investigators intuited that the right amount of nitrogen could induce the right amount of jiggling — to produce superconductivity at room temperature but without destabilising the crystal.
The nitrogen-doped lutetium hydride that Dias et al. produced was stable in ambient conditions (with a blue colour) but not yet superconducting. When they applied a thousand atmospheres of pressure to this material, it turned red, indicating a change in the nature of the electrons in the material. When the scientists measured the material’s electrical resistance, magnetic properties, and thermal properties in these conditions, they concluded that the material had become a superconductor.
The data they have reported in their paper shows a sharp drop in the electrical resistance around room temperature, the expulsion of magnetic fields, and a hump in the heat capacity (the sample expels heat from itself when cooled, as the electrons organise into the more-ordered superconducting state). These are all telltale signs of superconductivity, which they found in about 35% of the samples they tested.
Questioning the study
However, this story of blue to red is not so black and white. On purely scientific grounds, the claims made by the authors depend strongly on the correctness of the way they processed their data.
For one, the group inferred that the material’s electrical resistance had dropped to zero by collecting resistance data and then subtracting the contributions from sources other than the material. The validity of this procedure has to be carefully ascertained; some experts have already expressed an inclination to outright reject the technique as being completely unfounded.
The measurement of the material’s diamagnetism (i.e. when it expels magnetic fields) also suffers the same criticism. To measure the heat capacity, the authors have developed a new method that they claim to have validated using a known superconductor, magnesium diboride (MgB2). This also has to be carefully vetted.
While there are clearly stated scientific criticisms, they are neither alone nor, among physicists themselves, the most urgent. The main point of contention lies with the principal investigator of this work, Dr. Dias, who has had a controversial past. His recent claim of room-temperature superconductivity in a carbonaceous sulphur hydride published in the same journal, Nature, was retracted by the editors (as opposed to the authors requesting for it) after several experts pointed to serious problems with the data presented. Other physicists have also published (non-peer-reviewed) papers online with a lot of supporting data analysis that anyone with the inclination can inspect and study. They directly accuse Dr. Dias of scientific misconduct, including fabricating data.
While Dr. Dias has steadfastly denied any wrongdoing, the community’s rather bemused reaction to his new paper is understandable.
Curiosity will prevail
On the whole, when all is said and done, the scientific curiosity of researchers — and the obvious significance of the discovery that has been claimed — is likely to prevail, and more research groups are likely to explore ways to reproduce the results. The data analysis and subtraction methods used by the authors will be the first to be scrutinised, in microscopic detail. Further, the fact that the claimed phenomenon is occurring at a thousand-times atmospheric pressure will allow more research groups to enter the fray. The previous systems required pressures of more than a million atmospheres, facilities for which are available only with a handful of groups around the world.
It remains to be seen if the physics community will heed Dr. Dias’s call this time around.
Vijay B. Shenoy is Professor, Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bengaluru.
