Shulou(Shulou.com)11/24 Report--

Recently, South Korea's room temperature superconductor "LK-99" became famous for a time.

All major media platforms pay more attention to it.

The superconducting academic community has stepped up its work of reproduction.

However, whether it is experimental or theoretical calculation,

The results of different papers are quite different.

Some papers give several pieces of evidence to support superconductivity [1] [2].

Some say it is only ordinary magnetic material [3]

Even false signals of impurities [4].

The prospect of LK-99 is constantly reversed with the update of the paper.

The fog that hangs over LK-99 seems to be getting thicker.

In more than a hundred years of exploring superconductivity, LK-99 is not the first material to be claimed to achieve room temperature superconductivity, nor is it the first material to be controversial in subsequent verification. In fact, many kinds of so-called "room temperature superconductors" are inconclusive in the end, and physicists mimic the UFO UFO and jokingly call them "Unidentified Superconducting Object,USO". So, how many suspected room temperature superconductors have there been in the world? What about their follow-up?

Now, how many kinds of room temperature superconductors are there? In the past three weeks, the South Korean team's arxiv article has once again pushed the concept of room temperature superconductivity to the top of the wave. In recent years, related big news one after another, stirring up people's curiosity and expectations, but there is not a satisfactory result.

In fact, the pursuit of room temperature superconductivity has not just begun in recent years. In the past decade, teams have repeatedly claimed to have found superconductors at or near room temperature.

Back in October 2020, the editor felt such widespread concern about room temperature superconductivity for the first time. At that time, an "achievement" of room temperature superconductivity by Diaz's team in the United States was published in Nature [5], claiming that the superconducting transition temperature of hydrocarbon sulfide (C-S-H) compounds synthesized by green laser was as high as 288K under 267GPa pressure. From then on, when it comes to superconductivity, everyone has to sigh: although the pressure is ridiculously high, room temperature superconductivity is finally coming.

But what a pity! After a period of waiting, Diaz ate a lot of melons before the experiment was repeated. The experimental data of the work are suspected of being modified and manipulated by peers. For example, after careful analysis, Jorge Hirsch, a theoretical physicist at the University of California, published two papers questioning and criticizing the results. After a long period of sawing, the article was finally withdrawn in September 2022.

A big RETRACTED was added in front of the paper. Three years later, room temperature superconductivity has come again! Diaz announced the discovery of room temperature superconductors at the American physical Society meeting in early March 2023, claiming that the hydrogen nitrogen (Lu-N-H) compounds synthesized under high temperature and high pressure can achieve room temperature superconductivity at 294K under 1GPa pressure, and published in Nature [6]. People who have been "shaken" by the last result are more likely to take the attitude of "Let the bullet fly for a while". Previously, Hirsch, who was "angry" at Diaz, also came to the meeting "alignment":

The scene picture of the meeting of the American physical Society in early March 2023 shows Hirsch with injury on the front and Diaz on the back. Sure enough, this result has not been widely repeated, but there are many confirmatory experiments negative. for example, the experiments of the domestic Nanjing University team and the Physics Institute team [8] [9]. People's attention to room temperature superconductivity has also partly shifted to Diaz, the scientist who "discovered" room temperature superconductivity twice, and began to understand his origin and past. Results colleagues found that his doctoral thesis was similar to many papers, and a PRL (Physics Review Letter, an authoritative journal in the field of physics) was also strikingly similar to charts and other literatures, pointing out that there might be academic misconduct in Diaz. As a result, C-S-H-related papers were investigated and the PRL was withdrawn.

Diaz's widespread attention is inseparable from people's growing awareness of the importance of superconductors and the development of modern science and technology. But in fact, there have been many examples of room temperature superconductors claimed to have been found before. For example, in 2018, two Indian researchers claimed that adding silver nanoparticles to gold nanoarrays could obtain superconductivity of 236K [10]. Their data were questioned by Brian Skinner of the Massachusetts Institute of Technology because the noise patterns of the experimental data were the same, which was impossible in real experiments. Later, Indian scholars came to refute the rumor as the "quantum noise effect".

Ag-Au nanostructure schematic diagram, (a) TEM image of nanostructure (b) HRTEM image of single structure (c) HAADF-STEM image of single structure (d) figure (c) element distribution along red line | figure from [10] or, for example, in 2016 Kostadinov (Ivan Zahariev Kostadinov) claimed to have found a superconductor with a transition temperature of 373K [11], but did not disclose the composition and preparation process of the superconductor. There is no follow-up in a confidential manner.

Arxiv articles related to 373k, mysterious abstracts | there are many earlier screenshots from the web page. In 2012, a team announced that graphite powder specially treated with pure water had superconductivity at 300K [12]. In 2003, a team claimed that n-type diamond could have a superconducting phase at room temperature and pressure after coupling with electrodes and vacuum.

We have to admit that before the emergence of real room temperature superconductors (or proved impossible by reliable theories), such similar news may be one after another, and may cause a burst of attention and provoke a stock volatility. Let people down. It is undeniable that the real arrival of room temperature superconductivity will bring great changes to the world, but the road of exploration may be long and difficult, we might as well pay attention to it with peace of mind and look forward to it.

Here, we summarize some of these historical "room temperature superconductors" into the following table, which will also be updated with the increase of relevant news to help us have a more comprehensive understanding of the relevant events.

Table 1 "room temperature superconductors" in history

According to incomplete statistics, it is claimed that room temperature superconductivity (close to or higher than 300K) is not less than 7 times in history. In order to feel the pressure here intuitively, two examples are given for reference: the atmospheric pressure is 101kPa, that is, 0.0001GPa, and the geocentric pressure is 370GPa.

After watching so many "room temperature superconductors" events, we should not lose heart. From elemental superconductors to copper-based superconductors to iron-based superconductors, our understanding of superconductivity is deepening step by step. It is worth noting that at present, the superconductor Hg-Tl-Ba-Ca-Cu-O has the highest transition temperature of 138K [15] at atmospheric pressure. Under high pressure, the transition temperature of LaH ceramics reaches 252K [16], which has been widely verified by experiments.

Why after so long, we are not sure whether it is superconducting or not, people may be puzzled by the question of whether a material is a superconductor or not, is it not a "black-and-white" question that can be widely confirmed or denied quickly? In this way, there will not be so many ambiguous questions, and it will not be like LK-99 that everyone has been eating melons for three weeks. Actually, the problem is not that simple.

In order to be recognized, new superconducting materials need not only convincing data from the author, but also the same effect from other colleagues-superconducting materials in Beijing should also be superconducting in New York, which is the persistent belief of physical people. To determine whether a new material is superconductive, it is always necessary to use an instrument to do something about a sample. Therefore, the verification of suspected superconductors can be divided into at least two parts: obtaining a high-quality sample and completing the test of the sample.

It is not an easy task to prepare samples. Although people often joke that the preparation of samples is like alchemy, after all, not all elixir can make people immortal (it seems that all elixir cannot live forever). For superconducting materials, a "high quality sample" often represents a clean single crystal of the right size. The crystal defects used for testing should be as few as possible, and the impurities should be almost completely eliminated. As a result, polycrystals with disorganized grain boundaries and a large number of impurities are easy to be sintered, but the test results are difficult to convince rigorous reviewers and colleagues. The preparation of usable samples requires expensive high-purity raw materials, complex sintering conditions, unspeakable experience and some luck.

The small LK-99 samples obtained by the experimental group of Huazhong University of Science and Technology, such as the small black spots in the picture, are difficult to support the measurement of resistance. Since figure [2], even if a useful sample is obtained, how to use it to measure persuasive data is also a technical task. The measurement of superconducting samples at atmospheric pressure is easier, but there are many steps. First of all, the sample needs to be cleaned and polished with fine sandpaper-if the grinding force is light and the impurities on the sample surface are not stripped off, it will bring a false signal; if the grinding is heavy, the sample may directly fall apart. After a few millimeter-long samples are ground, four conductive electrodes are glued side by side to test the resistance in a way similar to that of a middle school voltmeter. The electrodes should be parallel and equal in length and leave enough distance from each other. From polishing to sticking the electrode, the fine work under the microscope must be completed quickly, otherwise the sample will be oxidized and deteriorated in the air, and all previous work will be wasted.

The verification of high-voltage superconductivity will be more difficult. Not to mention that the experimental conditions of millions of times atmospheric pressure have discouraged most laboratories from participating in the verification efforts, considering the testing technology itself is surprisingly complex. It is necessary to find a way to evenly apply and conduct pressure on the sample without damage; to cool and add a magnetic field to the sample together with the pressurizing device; to draw four wires from the pressurizing mechanism to connect the voltmeter and ammeter of the testing equipment; and to suppress noise signals generated by complex devices and extreme conditions. Therefore, the fate of high-pressure superconductivity seems to be somewhat ill-fated: the sample of metal hydrogen superconductor volatilized and went nowhere; although the carbon and hydrogen sulfide with a critical temperature of more than 200 K was famous for a while, it was eventually withdrawn last year; and the nitrogen-hydrogen superconductor in March this year is now nearing a hasty end.

Schematic diagram of diamond anvil needed for extremely high pressure | from [17] superhydride: a miracle? Having said so much, what kind of material will room temperature superconductors appear on?

Of all the "room temperature superconducting materials", superhydride is the most likely to be realized in theory and the most widely studied at present. According to the BCS theory that can explain conventional superconductivity (named after three discoverers, Bardeen, Cooper and Shriver), the critical temperature Tc of a superconductor is inversely proportional to the square root of the atomic mass M that makes up the superconductor. In this way, scientists naturally think that if you want to increase the critical temperature of superconductivity, then the best way is to use the lightest element-hydrogen.

Millions of atmospheric pressure must be applied to turn hydrogen at atmospheric pressure with a boiling point of-253 ℃ into a solid conductive material, metallic hydrogen. As it happens, the increase of pressure is also beneficial to the increase of superconducting critical temperature. As a result, hydrogen is injected into diamond anvil in high pressure superconducting laboratories around the world. However, it is not easy to press gaseous hydrogen into a solid, keep it stable and then complete the measurement. Over the past few decades, to this day, few people have successfully produced metallic hydrogen. The only claim to success came from Diaz, the American who claimed to achieve room temperature superconductivity in March. But after being questioned by his peers, he claimed that the sample was improperly preserved and gasified and disappeared. The cold case came to an end in such a funny and sad way.

The temperature and pressure conditions of superconductivity of superhydride, the experimental results of green inverted triangle labeling, the theoretical calculation results of blue triangle labeling, and the calculation results of red pentagram labeling given in a literature. Figure from [19] because metal hydrogen is too difficult to prepare, scientists turned to rare earth hydrides. The lanthanide elements at the bottom of the periodic table can combine with multiple hydrogen atoms to form molecules and exist relatively stably. This compound, which contains multiple hydrogen atoms, is called a hydrogen-rich compound, and if there is more hydrogen in the molecule, it is called a superhydride. Among them, the most studied material is La-H system. LaH hydrogen can be obtained by irradiating lanthanum and hydrogen at high pressure, which is the highest critical temperature superhydride verified by experiments. It can achieve about 252K (- 13 ℃) superconductivity at 1.65 million atmospheric pressure.

Zero resistance signal of LaH metal oxide [16] at present, the research on rare earth-rich hydride is mainly focused on binary systems. With the deepening of the research, the superhydride of ternary system has attracted more and more attention. maybe it can refresh the critical temperature record of high pressure superconductivity in the future. It should be pointed out that hydride-rich superconductors are a kind of conventional superconductors, which can be explained and predicted by the BCS theory proposed in the 1950s. Although the success of hydride-rich is another proof of BCS theory, its significance is almost only in science and extreme condition experimental technology. The pressure of millions of atmospheric pressure makes it almost impossible to become practical.

In conclusion, perhaps more "room temperature superconductors" will appear and be falsified in the future; perhaps atmospheric room temperature superconductors do not exist at all. But mankind's exploration of superconductivity under mild conditions will not stop. This is the expectation of engineering and the pursuit of science. The editor believes that with the progress of experimental technology and the breakthrough of basic theory, more "room temperature superconductors" will appear in the future.

In the future, we will continue to pay attention to it with you. At the initiative of the Institute of Physics to Academician Tao, we will recently launch a web page to collect previously claimed "room temperature superconducting materials" to track the future development of room temperature superconductivity. Maybe one day, the real room temperature superconducting materials at atmospheric pressure will appear on this website; maybe on that day, it will be our Chinese scientists or even those working in the Institute of Physics who will be on the Nobel Prize podium at the room temperature superconductor station.

This article examination expert: Luo Huiqian researcher, Institute of Physics, Chinese Academy of Sciences, Associate researcher Liu Miao

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This article comes from the official account of Wechat: Institute of Physics of the Chinese Academy of Sciences (ID:cas-iop), by Xiao Fan and Tibetan idiots.

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