Understanding the science: ‘Room-temperature superconductivity’ claim fails tests
Contrary to claims by Korean scientists, the material LK-99 does not show signatures of superconductivity at room temperature, CSIR-NPL study finds
Not for the first time, a claim on superconductivity at room temperature, for long a Holy Grail in science, has failed verification tests. Scientists from CSIR-National Physical Laboratory (NPL) are among those who have reported results contrary to the claim.
Superconductivity is a property that allows certain materials to conduct electric current without resistance, but its full potential remains largely untapped because it happens at extremely low temperatures. Zero resistance is one of the two defining signatures of superconductivity, the other one being the ability to cause a magnet to levitate. Levitation is caused when the material repels a magnetic field completely, a property called perfect diamagnetism.
Last month, Korean researchers claimed to have demonstrated superconductivity in a material not only at room temperature (like many others before them) but also at ambient pressure (which is a first, because previous claims have involved high pressure).
Subsequently, scientists from NPL synthesised the material as described and found that it does not show any signature of superconductivity at room temperature. They found that the material is indeed diamagnetic at room temperature, but not strong enough to cause levitation on a magnet.
Why superconductivity matters
Superconductivity was discovered in 1911 by the Dutch physicist Heike Kammerlingh Onnes, who found that mercury has zero electrical resistance at 4.12 degrees Kelvin (°K) (about –269°C). In subsequent years, more and more metals were found to be superconductive at extremely low temperatures, such as lead at 7.22°K and tin at 3.73°K.
At zero resistance, superconductivity holds the promise of revolutionising power transmission; cables made of superconductors would bring transmission losses down drastically. The promise, however, remains unfulfilled because of the impracticably low temperatures at which materials become superconductive. Magnetic levitation, however, has been exploited in MRI devices and MagLev trains.
The new claim
The Korean physicists reported their results in two separate papers on the online repository arXiv. synthesised a material, LK-99, and reported that it superconducts at a critical temperature of 400°K (126.85°C), which means it remains a superconductor below that level. It also exhibits magnetic levitation at room temperature and pressure, they said. The material is a lead apatite, a kind of crystalline structure, and contains a phosphate of copper besides lead.
“Once the material is made, it becomes a superconductor at room temperature,” Hyun Tak Kim, a co-author of one of the two papers, said over email. Kim is a research physicist at the College of William & Mary, Virginia, while the other authors are associated with the Seoul National University and other South Korean institutions.
Why two papers? The question becomes intriguing when Kim distanced himself from one of them. Kim co-authored his paper with five other researchers, and two of these co-authored the other paper with one more researcher, Young Wan-Kwon of Korea University, who was not involved in the first.
“The author (Kwon) who uploaded the paper to arXiv left us last winter,” Kim said. Kwon’s paper has grabbed attention on account of the way it makes its claims. It begins with the phrase, “For the first time in the world,” and concludes emphatically with: “We believe that our new development will be a brand-new historical event that opens a new era for humankind.”
HT has sent an email to Kwon, who was yet to respond at the time of writing this.
What other scientists think
To establish any material as superconductive, scientists need to demonstrate its two primary properties: zero resistance and perfect diamagnetism. When a claim like this is made, other laboratories try to verify it by replicating the results.
Scientists at CSIR-NPL synthesised LK-99 as described in the Korean papers. “The LK-99 synthesised is as good as reported by the Korean team. In fact, the purity of the material is even better than that of any reported before in terms of powder X-ray diffraction (a technique for identification of crystalline material) results,” said Dr VPS Awana, chief scientist at CSIR-NPL. Awana and colleagues have published these findings on arXiv
“The material is weakly diamagnetic, but not superconducting. It is diamagnetic right from room temperature down to 5K. Levitation or repulsion of the magnet from the sample or vice versa is not seen at room temperature,” said Awana, who, along with colleagues, has published these findings on arXiv.
Independently, Peking University researchers have published a preprint, also on arXiv, that infer that LK-99 is not superconductive. Their results, however, are different from those of the NPL group. After synthesising the material, they found partial levitation, but said it is not due to diamagnetism but caused by ferromagnetic content (iron, steel etc are ferromagnetic materials; this property attracts them to magnets, but can also cause levitation under certain circumstances).
“The half-levitation is caused by the magnet’s repulsion of the specimen’s far end, while attracting its near end. The net force is attractive rather than repelling,” Peking University physicist Yuan Li said over email.
Independent of these results, other scientists were already questioning the Korean team’s findings. “The paper has some drawbacks. It does not show zero resistance. It only shows that there is sharp drop in the resistance above room temperature. The other drawback is there is no data showing perfect diamagnetism. There are lots of materials which are diamagnetic, for example tomato. So, the authors have to do more measurements to ratify their claim,” said Dilip Pal, professor of physics at IIT Guwahati.
Jorge E Hirsch, a physicist at the University of California, San Diego, described the Korean team’s findings as “not convincing at all”. “This is not superconductivity but experimental artifacts and wishful thinking,” he told HT.
The previous big claim
Hirsch has previously been previously critical of another claim about superconductivity at room temperature, made by University of Rochester researchers and published in Nature in 2020. Following issues raised by Hirsch and another physicist, Nature retracted the paper last year.
That was not the end of it, though. In March this year, the same team published another paper, describing another material as being superconductive. That paper still stands, but the team leader, Ranga P Dias, has since been caught in a new controversy.
In 2021, Dias had published a paper in Physical Review Letters. This was not about superconductivity; it described the electrical properties of manganese disulphide. PRL is set to retract that paper amid concerns over data fabrication, according to a report by Nature’s news team, which works independently of the journal.
In April this year, Nature reported allegations that Dias had plagiarised portions of his PhD thesis, submitted in 2013, from a 2007 thesis by a Washington University researcher.