Behind room-temp superconductivity
A paper in Nature this week by scientists from the University of Rochester, who claim that they have created a material that achieves superconductivity at room temperature, has been met with a degree of scepticism. That is because the team’s earlier work has been controversial.
It’s a scientific Holy Grail. Superconductivity, the mysterious property that allows some elements to conduct electricity with zero resistance, remains largely confined to the lab because it usually happens at extremely low temperatures. For more than a century, scientists have been trying to find a material that superconducts at room temperature. That would have revolutionary practical applications, including drastically reducing transmission of electrical energy.
A paper in Nature this week by scientists from the University of Rochester, who claim that they have created a material that achieves this very goal, has been met with a degree of scepticism. That is because the team’s earlier work has been controversial. In 2020, it described another material that it said had superconducting properties at room temperature, but Nature retracted the paper last year after other scientists questioned the findings.
How superconductivity works
It’s a property that was discovered in 1911 by the Dutch physicist Heike Kammerlingh Onnes, who would win the Nobel Prize two years later for the production of liquid helium. Investigating the properties of matter at such low temperatures, Onnes found that mercury has zero electrical resistance at 4.12 degrees Kelvin (°K) (about –269°C).
Over the years, as lower and lower temperatures were generated in the lab, more metals, such as lead (at 7.22°K) and tin (at 3.73°K), were found to be superconductive. Such temperatures were restricted to the lab, so the quest since then has been to harness these properties at manageable temperatures.
The most obvious benefit would be transmission of electricity at zero loss, theoretically. “If room temperature superconductivity is indeed achieved, it would revolutionise many things. For example, if one can make cables of superconductors, electric energy transport would become enormously more efficient, because transmission loss would come down to a very low range,” said Pratap Raychaudhuri, senior professor of physics at the Tata Institute of Fundamental Research.
Among other applications, Raychaudhuri said the use of superconductors in chips could bring greater efficiency to computers used by data firms, which currently generate large amounts of heat.
At the low temperature levels at which superconductivity is possible, practical applications exploit a magnetic property called the Meissner effect. Superconductors can be made to expel magnetic flux, something that is used in MRI devices and the MagLev (short for magnetic levitation) trains in China.
Claim and retraction
In the paper that would eventually be retracted, the researchers used a polyhydride compound they called carbonaceous sulphur hydride, or CSH, made of these three elements.
“When squeezed at enough pressure, using a diamond anvil, using methodologies that do not require super-cool temperatures, this material achieves superconductivity. CSH is itself derived from biomatter,” physicist and mechanical engineer Ranga P Dias, who led both research teams, said in an email response. Objections to those findings came in the months that followed. “What led to the retraction I think are the issues we raised with Dirk van der Marel about data manipulation and fabrication for magnetic susceptibility,” said Jorge E Hirsch of University of California San Diego. The two scientists had raised their issues in the International Journal of Modern Physics B last year.
Hirsch said he has not analysed the data in the new paper. “Based on my previous experience with these authors and my understanding of superconductivity, I do not believe these recent claims are true,” he said in an email.
Dias, for his part, says he still disagrees with the retraction. “The University of Rochester conducted its own inquiry and affirmed the quality of our work,” he said.
The new claims
In their latest paper in Nature, the researchers describe a compound called nitrogen-doped lutetium hydride (NDLH). It exhibits superconductivity at about 21°C and 10 kilobars of pressure, they said.
A kilobar is roughly 986 atmospheres or atms.
While that pressure may appear high, they note that it can be achieved. “Strain engineering techniques routinely used in chip manufacturing, for example, incorporate materials held together by internal chemical pressures that are even higher,” the University of Rochester said in a statement.
In his email response, Dias said the team is calling the material “reddmatter”. “Reddmatter is a compound of ionic elements – and again, using our methodologies, we are able to squeeze it with enough pressure so that it can achieve superconductivity. Reddmatter involves the use of a compound that nature itself would not make.”
“In both cases, we are able to detect superconductivity using methodologies that the physics community accepts as a baseline test,” he added.
Is it for real?
Amid the concerns given the previous experience, there has also been some cautious optimism within the scientific community.
David Ceperley, a physicist at the University of Illinois Urbana-Champaign, co-authored an accompanying article in the same issue of Nature, acknowledging the hopes raised but also the doubts that remain.
“I assume that the latest paper was thoroughly reviewed by experts because of the earlier controversy… I also assume that the authors were educated in how to present their results because of their earlier problems and did not make the same mistakes again,” Ceperley said in an email response.
“But in the end the community has to give them some slack and trust what they are saying at least to the extent that we consider the compound as interesting to investigate. The discovery would be so consequential that we need to take the risk. However, it is urgent that another lab reproduce their findings,” he said.
Raychaudhuri of TIFR, too, stressed the importance of other labs reproducing the findings. “They have a very controversial record, so it is very difficult to make up one’s mind one way or the other. Prima facie, if you look at the data, the way they present it, they seem to have a rather strong case,” he said.
He noted that the new paper shows data for magnetic properties, which is the accepted yardstick for establishing superconductivity, given that zero resistance is difficult measure with instruments. The earlier paper too had such data, he noted, but it later transpired that “they had used data processing methods that were not scientifically acceptable”.