On March 15th, Haihu Wen’s team submitted a 16-page research paper authored by nine different experts on the preprinted website arXiv, which denied Dias’s research conclusion. The paper concludes: “Our experiments clearly show that there is no superconductivity in lutetium nitrogen hydrogen material LuH2 xNy from ambient pressure to 6.3 GPa and temperature as low as 10K (about-263 degrees Celsius).”
This is only eight days before Dias’s research was released. “This conclusion must be overturned, there is no doubt about it,” said Haihu Wen, director of the Research Center for Superconducting Physics and Materials of Nanjing University.
“This conclusion” refers to the room temperature superconductivity research by the Ranga Dias team of the University of Rochester in the USA. They claim that a lutetium-nitrogen-hydrogen material in R&D has achieved room temperature superconductivity at nearly 10,000 atmospheres (1GPa).
On March 7th, after seeing the results of Dias’s report from the meeting at the US Physics Society, Haihu Wen quickly arranged to repeat the experiment. “Our preliminary samples came out soon, and then some adjustments were made.” Wen questioned why the efficiency was so high before saying that their team had worked overtime on the issue. In fact, this replica experiment is “not very difficult”, but “it is still difficult to measure”, because it needs precise signals and data analysis. Fortunately, they “usually have a good accumulation”.
But the experiment was not completely reproduced. Haihu Wen found that the sample preparation scheme given by Dias was almost infeasible, so they combined their own conditions and synthesized it in a new way and obtained lutetium, nitrogen and hydrogen materials. The X-ray diffractometer examination showed that the structure of the material was almost the same as that of the Dias sample, and nitrogen was also found by energy dispersive X-ray spectrometer analysis.
Haihu Wen’s team immediately measured the resistance of the material at different pressures below 60,000 atmospheres, and found that no superconductivity occurred as low as 10K. At the same time, they also made careful magnetization measurements and found that there was no diamagnetic signal needed for superconductivity. Haihu Wen said that these findings are enough to refute Dias’s superconductivity conclusion at normal temperatures and low pressures.
Since the Dias team did not explain the nitrogen content in the materials they studied, it can only be discussed by the material structure at present. Haihu Wen figured out that although the nitrogen content in the samples may be different, the material structure is the same, and three elements have both. In this case, superconductivity should be produced. “It cannot be said that the change of composition will determine superconductivity or non-superconductivity.”
Why is Dias’ sample preparation scheme not feasible? Dias’s scheme used two small diamonds to pressurize lutetium, nitrogen and hydrogen in a microchamber at 65 degrees Celsius to 10,000 atmospheres. Wen Haihu analyzed that Dias’s material preparation method appears unreasonable. Sixty-five degrees Celsius is too low, and the reaction between metal and nitrogen and hydrogen can be produced at this temperature, which is incredible.
Wen Haihu said that Dias may have given a wrong condition, perhaps because the temperature is one “0” less, “it is difficult to make it unless heated by laser”, but Dias did not mention any use of a laser. Haihu Wen’s team used a high-temperature and high-pressure furnace to heat the materials and soon got lutetium, nitrogen and hydrogen materials. Haihu Wen considered it more rigorously. He said that it is inconclusive whether this material will have high temperature superconductivity at hundreds of thousands of atmospheres, “we are doing it too”.
From 1968 until today, physicists have been studying the superconducting properties related to hydrogen. Hydrogen sulfide, rare earth hydrides, and alkaline earth hydrides, can be transformed into superconducting states at temperatures exceeding 200K. The Dias team changed part of hydrogen in lutetium hydride into nitrogen this time, and claimed to have measured superconductivity at the highest transition temperature of 1GPa and 20 degrees Celsius. If confirmed, it would be an unprecedented step forward.
Earlier, C. Q. Jin, a researcher at the Chinese Academy of Sciences Institute of Physics, mentioned several doubts with regars to Dias’s research in an interview with the Chinese Journal of Science, including an unclear structure of synthetic samples and a low hydrogen content (quite different from hydrogen-rich superconductors discovered before).
Why is hydrogen content so important? This is related to an inherent understanding of superconductivity in academic circles. Generally speaking, the higher the hydrogen content in superconducting materials, the higher the superconducting transition temperature.
Maosheng Miao, a computational chemist and associate professor at California State University Northridge, USA, told the Chinese Journal of Science that hydrogen-rich superconductors and low-hydrogen superconductors are “completely different systems”, and Dias’s conclusion subverts the existing understanding. For example, the superconducting transition temperature of lanthanum decahydride is MINUS 13 degrees Celsius, which is already very high. In Dias’ lutetium-nitrogen-hydrogen material, the molar ratio of lutetium to hydrogen is less than 3, which is far lower than lanthanum decahydride, but its superconducting transition temperature is higher than lanthanum decahydride.
Maosheng Miao said that it is hard to imagine that Dias’ lutetium-nitrogen-hydrogen material will become an electron-phonon coupling superconductor. Based on the electron-phonon coupling theory, the superconducting transition temperature of this material should be more than ten Kelvin. He suggested that the high-voltage experiment is very difficult to do, the samples are very small, the synthesis conditions are difficult to achieve very uniform, and the noise of signal measurement is very large, all of which means that the experiment is prone to misjudgment.
It is worth noting that Dias’s two “criminal records” make people unable to be assured of his research. Testability, repeatability and transparency are important factors for the credibility of scientific research. Dias’s two previous important studies, including the study of metal hydrogen published in Science, and the study of hydrocarbon and sulfur superconductivity published in Nature, did not have these factors.
Nature also notes Dias’s past. C. Q. Jin and David Ceperley of the University of Illinois at Urbana-Champaign, published a commentary in this journal saying that, because Dias’s paper on carbon-sulfide hydride high-temperature superconductivity was withdrawn, independent measurement data of its latest research will help dispel doubts.
Maosheng Miao also warned that since Nature accepted Dias’s work, it is necessary to discuss it seriously. If we want to deny his work, we need more evidence and repetitive verification, and several verification papers should be published soon.