Wen Haihu's team at NTU overturns US room temperature superconductivity

At the March 2023 APS meeting in Las Vegas, Ranga Dias and his team at the University of Rochester in New York described their claimed achievement as the culmination of a more than century-long quest for the condensed "holy grail": a material capable of conducting electrons with zero resistance at ambient temperature. Knowing that such a breakthrough would mark an important step toward a future in which room-temperature superconductors transform electrical grids, computer processors and medical diagnostic tools, the group's discovery could rank among the greatest scientific advances of the 21st century if it is true.

The audience reaction to such disruptive research was muted or hesitant.

Part of the audience was cautious because - they had encountered this before, in fact - in 2020, Dias' team announced in a paper in Nature that they had created a material, carbonaceous sulfur hydride, that was superconducting at 287 K and 267 GPa, and researchers and the news media praised the discovery, but two years later the paper was retracted after outside researchers found oddities in the results.

Another reason is that the findings themselves are highly suspect, with the samples studied in the latest findings ranging from 70 to 100 μm in diameter and 10 to 20 μm in thickness.

Unfortunately, the chemical composition (stoichiometry) of the new material made at very high pressure is unknown (surprisingly the article does not specify the details of the synthesis method): X-ray diffraction points to H3-δNε ( 92.25 %) containing LuN1-δHε (7.29 %) and Lu2O3 (0.46 % ); future studies will have to clarify it.

The problem is that it is impossible to explain the observed superconductivity with this composition (the distance between hydrogens is too large); the current-voltage (IV) curve is perfectly linear (pointing to unconventional superconductivity); the magnetization rate curve (Meissner effect) shows strange (or unexpected) behavior, and it is unclear why the reviewers did not ask for a detailed discussion.

Physicists believe that Dias' paper does not present a detailed approach to replication, which makes replication difficult and makes this research less feasible.

(a) The current-voltage (IV) curve is perfectly linear

(b) These curves are very similar to those criticized by Jorge Hirsch (University of California, San Diego) in his retracted article

(c) This graph shows the original data in the paper (left) and the graph that appears in the article (right). The data show a resistance between 2 and 9 mΩ (milliohms), adjusted by five polynomials that should produce the curve that appears on the right. Subtracting the polynomial (to eliminate the observed "residual resistance") shows almost zero between 125 and 225 K, with a sharp shift between 225 and 275 K.

Due to the sensational nature of the study and the multiple doubts, several groups have already studied the high pressure superconductivity of Lu-H compounds, but none of them have been able to reproduce the results of the room temperature superconductivity experiments of Dias' team. Less than two months after the first draft of the paper was posted to the preprint site arXiv, the work of Nanjing University professor Wen Haihu's team, which overturned the U.S. room-temperature superconductivity study, was published online in the international journal Nature on May 11.

The research results were published in the journal Nature on May 11 under the title "Absence of near-ambient superconductivity in LuH2±xNy".

Haihu Wen Image credit: Nanjing University

On March 15, Wen Haihu's team submitted a 16-page research paper including 14 authors to the preprint site arXiv, which bluntly rejects Dias' findings. The paper concludes, "Our experiments clearly show that superconductivity does not exist in the lutetium nitrogen-hydrogen material LuH2±xNy from ambient pressures down to 6.3 GPa and temperatures down to 10 K (about -263 degrees Celsius)."

Nature originally intended to publish the paper as a Matter Arising Argument, but Wen Haihu's team added so much high-quality data in response to three reviewers' comments that the article became so colorful that Nature's editors decided to publish it as a research paper after discussion.

"The conclusion was definitely overturned, no doubt about it." Wen Haihu, director of the Center for Superconductivity Physics and Materials Research at Nanjing University, sounded resolute enough when he uttered these words to Science China. "It is more difficult to disprove a study than to prove a study because it needs to be proven from multiple perspectives that the sample you are studying is almost identical to the sample of the object being questioned. In addition one has to use fine measurements, a lot of data and analysis to make the argument."

The main point of the analysis, based on the paper submitted by the team, Absence of near-ambient superconductivity in LuH2±x Ny, is as follows: a priori, it is not difficult to synthesize nitrogen-doped lutetium hydride at high pressure (perhaps for this reason, Dias and his team in Nature omitted it).

In fact, the XRD spectra of Ming et al. is almost identical to that of Dias, with the advantage of being cleaner and with fewer impurities (no traces of Lu2O3 were observed, 0.46% in Dias' sample). Unfortunately, LuH 2±x N y does not show any color change between 1 GPa and 5.2 GPa (suggesting that Dias may be looking at something else).

The nitrogen-doped lutetium hydride shows the resistance versus temperature curve properties of the metal, linear between 40 K and 300 K, and quadratic below 40 K. There is no sign of superconductivity (only a small slope change at 315 K, which could be a systematic error of the measurement instrument). As shown above, for pressures between 1.3 and 6.28 GPa, there is also no sign of superconductivity at temperatures between 10 and 350 K.

The magnetic moment curves as a function of temperature at 1 GPa and 2.1 GPa also show no sign of antimagnetism due to the Meissner effect (when the results given by Dias are textbook curves). The results given by the team of Haihu Wen are not superconducting at pressures of the order of LuH 2±x N and 1 GPa (the results are in good agreement with previous studies on lutetium hydrogenation).

During the review process, the reviewers had questioned the sensitivity of the experimental setup of Haihu Wen's team. In response to this question, they chose a copper oxide superconductor [(Cu, C)Ba2Ca3Cu4O12-d] that had been studied for many years (transition temperature of 112 K) and performed measurements under the same conditions. If the experimental LuH2±xNy material is superconducting, then its magnetic measurement signal intensity should be consistent.

However, compared to a real superconductor, the signal of LuH2±xNy material is about 100 times weaker and shows only a very weak signal of positive magnetic moment. From the results, there is no anti-magnetic signal for the relevant superconducting transition at temperatures in the 100-350 K range. The experimental results strongly support the conclusion of Haihu Wen's team and are supported by the reviewers.

Meanwhile, Nature also sent the experimental data back to Dias' team, but they indicated that it was the sample inconsistency that led to the different results. Overall, based on the results available so far, there is plenty of evidence to point to problems with Dias's data unless they can provide convincing new data - which is very difficult.

Removing falsehoods and preserving truth is a very important part of scientific research, and science spirals forward in a process of "discovery-disproof-re-discovery. Although the research of Haihu Wen's team is only a rejection of the room temperature near atmospheric pressure superconductivity results of Dias' team, it is still very important.

It seems that there is still a long way to go for room-temperature superconductivity research, and this study by Haihu Wen's team has stimulated scientists to explore hydrogen-containing compounds as room-temperature superconducting materials at near-ambient pressure.

Reference links:

[1] https://www.chinatimes.com/cn/realtimenews/20230513003467-260409?chdtv

[2]https://www.nature.com/articles/s41586-023-06162-w

[3]https://finance.sina.com.cn/tech/roll/2023-05-14/doc-imyttsvu8492943.shtml