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This article comes from the official account of Wechat: ID:fanpu2019, author: Luo Huiqian

There are still several doubtful points about the atmospheric pressure room temperature superconductivity which is popular all over the network, and the authenticity remains to be verified. However, even if other experimental groups can repeat it, there are still many material difficulties to be solved for industrial applications. Although the future has become brighter, the road is still "hindered and long".

Written by Luo Huiqian (researcher, Institute of Physics, Chinese Academy of Sciences)

On March 8, 2023, a piece of "big news" blew up the sensitive nerves of both the tech and financial circles. Diaz (Ranga Dias) from the University of Rochester and others announced at the March meeting of the American physical Society that they had discovered "near-atmospheric pressure room temperature superconductivity", a ternary compound composed of lutetium-nitrogen-hydrogen (Lu-N-H), which can achieve superconductivity at a maximum temperature of 294K (21 °C) at 10,000 atmospheric pressures (1GPa or 10 kbar). On March 9, Nature launched a paper by the Dias team entitled "Evidence of near-ambient superconductivity in a N-doped lutetium hydride (evidence of near-atmospheric superconductivity in nitrogen-doped hydrogen hydrides)" [1]. At the same time, the News & Views article "Hopes raised for room-temperature superconductivity, but doubts remain (room temperature superconductivity gives new hope, but doubts still exist)" [2]. Science also launched a review article "'Revolutionary' blue crystal resurrects hope of room temperature superconductivity (Revolutionary" Blue Crystal "New Hope for rekindling Room temperature Superconductivity)" (figure 1) [3]. For a while, the industry exclaimed that room temperature superconductivity at atmospheric pressure is promising! The future revolution of energy and power technology is coming! However, apart from the surprise, the scientific research colleagues look at the matter very calmly.

Figure 1. Ranga Dias claims to find room temperature superconductivity at near atmospheric pressure (picture from the University of Rochester) Why Nature received the paper again. Indeed, "room temperature superconductivity" is no longer the first time a wolf has come. The last report on "room temperature superconductivity" was on October 14, 2020, when Nature published a paper entitled "Room temperature superconductivity in hydrocarbon sulfides". There were 9 authors, among whom the correspondent was R. P. Dias [4]. Six of the nine authors overlap with the authors of the paper in 2023. At that time, the "room temperature superconductivity" was seriously questioned by many scientists in the field. After two years of controversy, the relevant authors finally failed to produce convincing evidence, and no one in the industry was able to repeat their results. The editorial department of Nature decided to withdraw the manuscript on September 26, 2022, even though the nine authors of the paper did not agree to withdraw the manuscript. In less than half a year, Nature received and published the team's paper again, making people cry, "is the wolf coming again?" And it is reported that the team's previously withdrawn work on hydrocarbon sulphide superconductivity was "repeatedly verified" by themselves not long ago (February 2023), and the relevant papers have been submitted [5]. As to whether this "room temperature superconductivity" is true or not, it is clear that the voice of doubt in the scientific community is far greater than that of belief.

In this paper, the Dias team gave several superconducting evidence of Lu-N-H compounds: 1. Zero resistance effect, that is, the resistance of the material is reduced to absolute zero below a certain temperature; 2. Diamagnetic effect, that is, after the material enters the superconducting state, it can resist the invasion of the external magnetic field and form a negative magnetic susceptibility. Specific heat jump, that is, when a superconducting phase transition occurs, the specific heat of a material has a discontinuous jump, which is a typical characteristic of thermodynamic second-order phase transition (figure 2). Generally speaking, these three pieces of evidence are sufficient to judge the superconductivity of materials. In addition, the data of X-ray diffraction and Raman spectrum of the material are given, and the basic structure of the material is speculated by theoretical calculation. It can be said that the "chain of evidence" is very complete! The paper also gives most of the steps and methods of raw data and data processing on the web page of Nature, which can be downloaded and verified by interested readers. From these angles, it seems to imply that "the wolf is really coming"!

Figure 2. The evidence of Lu-N-H superconductivity given by Ranga Dias et al.: zero resistance, diamagnetism and specific heat jump, but what puzzles the industry is his data conclusion that this material already has superconductivity of more than 200K when the pressure is below 30 kbar, and the lower the pressure is, the higher the superconducting critical temperature Tc is! The highest critical temperature reaches 294K at about 10 kbar, and then the superconducting temperature drops below 100K at lower pressure (figure 3a). What is even more bizarre is that the small crystal, which is blue at atmospheric pressure, turns pink and finally red under pressure, which is completely different from the black samples observed by traditional metal hydride superconductors (figure 3b). Such abnormal temperature-pressure phase diagrams and strange color changes are very suspicious. Moreover, in the molecular formula given in this paper, the ratio of Lu to N is almost 1: 1 and the content of H is only about 3, which is not consistent with the familiar situation that rare earth hydrides contain H in 5, 6 or more. In the case of such a low H content, according to the material structure given by them, the distance between H atoms is so large that superconductivity from the H element itself (similar to metal hydrogen or the previously discovered LaH10) can almost be ruled out. These strange phenomena make people feel that "even if this material is superconducting, it is not very much like a traditional BCS superconductor."

Figure 3. The temperature-pressure phase diagram and color changes of Lu-N-H superconductors given by Ranga Dias et al. Of course, the industry's doubts are not limited to this. Some people have also conducted a simple analysis of their original data and found that their process of processing the data is still too rough. For example, the diamagnetic signal in the magnetic susceptibility data is obtained by processing a very large background signal and a very messy set of measurement signals. Even, such a beautiful zero-resistance transition data is still obtained by the method of "buckling the background and reducing noise". As for how the background is selected and how the noise is "erased", there is no way to know.

Some theoretical revelations in case the experiment comes true, of course, if the research result is true and reliable, it at least brings us some new revelations: 1. Room temperature superconductivity is completely achievable, from the experimental point of view, the critical temperature of superconducting materials, there is no upper limit! two。 Hydrogen compound superconductivity under high pressure is the most promising to find room temperature superconducting materials, they may not need a pressure such as a million atmospheric pressure to achieve, there is hope at lower pressure; 3. If we can further reduce the pressure or with the help of the chemical stress inside and outside the material to achieve atmospheric pressure stable superconducting material, then the so-called "atmospheric pressure room temperature superconducting material" will really be realized! [6]

What does it mean if room temperature superconductors are really found at atmospheric pressure?

It means that the dream that scientists have chased for more than a hundred years has finally come true!

It means that the journey of refreshing critical temperature of superconducting materials has entered a new era of superconducting at room temperature. (figure 4)

It means that the physical door of the new world has been opened since then!

Figure 4. The age and critical temperature of the discovery of all kinds of superconducting materials [6] We can imagine as much as we can. Room temperature superconductors at atmospheric pressure bring us more surprises, and everything seems to be in sight in the future!

If it comes true, will it be enough to ignite the technological revolution? Obviously, the social response to this incident of "room temperature superconductivity at near atmospheric pressure" is much more lively than that in the scientific research circle. As far as I know, many media are particularly interested in the incident, and many media have conducted bombardment interviews with scientists in the study of superconductors, which makes people a little at a loss. Regardless of whether the data in this paper are true or not, will the findings in the paper alone be enough to ignite the future scientific and technological revolution? Obviously "too excited to show". The reason is that the "near atmospheric pressure" (10 kbar) mentioned in the paper is actually far from the familiar atmospheric pressure (1 bar, that is, 1 atmospheric pressure). In fact, 10,000 atmospheres are ten times more intense than the Mariana Trench, the deepest in the world. Under such high pressure, how to facilitate the application of large-scale industry? What's more, the amount of samples prepared under high pressure is very small, most of them are of the order of micrograms or milligrams, and the samples produced by devices with large pressure chambers are only a few grams. In the face of industrial applications reaching tonnage, it is an insurmountable gap.

In addition, scientists have several practical problems that need to be faced with materials.

First of all, does the realization of room temperature superconductivity at atmospheric pressure mean that superconductivity can be used cheaply on a large scale from now on? and be not so! The critical temperature parameter is only one of the bottlenecks that limit whether superconducting materials can be used on a large scale. In fact, superconductors have critical magnetic fields, and most superconductors also have two critical magnetic fields (upper critical field and lower critical field). Once breaking through this magnetic field, the flux line will enter the interior of the superconductor, causing energy dissipation and even completely destroying zero resistance. Not only that, superconductors also have a critical current density, not because the resistance is zero, a little voltage, the current can be infinite. For a specific superconducting material, once the current density exceeds the critical value, it will instantly "quench", that is, the resistance will heat up quickly and the superconductor will disappear completely. These three critical parameters, critical temperature, critical magnetic field and critical current, are like grabbing the throat of superconducting applications and are often determined by the intrinsic characteristics of the material (figure 5). Therefore, it is not enough for light to have a superconductor whose critical temperature reaches room temperature. It also depends on whether it is a superconductor with "three high" critical parameters, which needs to be studied. For example, copper oxide high temperature superconductors have been discovered for nearly 40 years, and their critical temperatures are also very high, easily reaching more than 100 K, or even need only liquid nitrogen (77 K) to cool, but their large-scale high-power applications have not been fully realized. One of the reasons is that its lower critical field is too low [7].

Figure 5. Three critical parameters of superconductor: critical temperature Tc, critical magnetic field Hc, critical current density Jc [6] secondly, if the "three high" room temperature superconductor is found, does it mean it is easy to use? Not exactly! Because it also depends on the specific behavior of the critical parameters, for example, for most superconductors, they belong to the "second kind of superconductors" and there are two critical fields. In particular, some superconductors with high critical temperature often have strong "anisotropy" in the critical field. You can think of a superconductor as a small sheet. When the magnetic field is perpendicular to and parallel to the sheet, the critical temperature pressing effect of the superconductor is tens or even thousands of times different! Then, once the magnetic field reaches the lowest critical field, the complete diamagnetism or even zero resistance of superconductivity is destroyed. The situation is so bad that it is not the upper limit of the parameter, but the lower limit of the parameter, that determines the ceiling of the strong electricity application. Therefore, even if the upper critical field of copper oxide is more than 100 or even more than 200 T, the upper critical field of copper oxide in the other direction can be lower than 1T, and for the lower critical field, it can even be as low as 0.01 T. Just imagine, with a little magnetic field, the flux line penetrates before it is used, and its motion inside is difficult to predict. There are a variety of rich configurations (solid, liquid, plastic, glass, etc.). The reliability of superconducting materials is shaken under a higher magnetic field (figure 6) [8].

Figure 6. Schematic diagram of the flux phase diagram of copper oxide superconductors [8] what if we find room temperature superconductors at atmospheric pressure with good comprehensive critical parameters and low anisotropy? New problems still exist! For example, although the critical temperature of iron-based superconducting materials is not as high as that of copper oxides, their critical magnetic fields are still very high, which can reach tens or even hundreds of T, and are almost isotropic at low temperatures. Obviously, iron-based superconductors seem to be very suitable application materials, and the more advantage is that the current-carrying capacity of iron-based superconductors does not deteriorate in a strong magnetic field, and it can still maintain good performance after several times of rising and cooling. Unfortunately, the current critical current density index of iron-based superconductors is not high, and the production capacity is still in the stage of laboratory application, and further efforts are needed in the future (figure 7) [9]. Even if the constraints of physical factors are overcome, iron-based superconductors will still face the constraints of chemical factors-- most iron-based superconductors are arsenides containing alkali or alkaline earth metals, which are afraid of air, water and poisonous. Once prepared on a large scale, these security risks need to be overcome.

Figure 7. The relationship between the critical current of all kinds of superconductor strips and the external magnetic field [9] finally, what if the above conditions are overcome? This kind of atmospheric pressure room temperature superconductor should be easy to use, right? Not necessarily! We also take copper oxide materials as an example, assuming that similar materials have overcome the above difficulties, and there is another difficulty in copper oxide materials, that is, copper oxides as ceramic materials are very fragile and have poor mechanical properties. Therefore, it is unrealistic to use copper oxide as wire directly, it is difficult to form, and it is basically impossible to maintain good current-carrying performance under all kinds of crimp winding. Of course, scientists have made a lot of efforts over the past few decades. There are two main ways: powder casing method and baseband coating method. In the former, the superconducting powder is sheathed into a metal tube, then drawn into a wire, and heat treated to improve the superconducting performance. With the help of a thin metal baseband, the superconducting material is coated on the middle layer of the baseband through various buffer layers and protective layers. the thickness of the baseband is about 100 μ m and the thickness of the superconducting layer is about 1 μ m (Fig. 8). In the end, it is found that the largest part of the cost of copper oxide superconductor tape is not the superconducting material itself, but the metal casing or metal baseband used, coupled with all kinds of heat treatment processes, the yield is not very good [6]. Therefore, if we have a room temperature superconductor at atmospheric pressure, we would prefer it to be a material similar to metal ductility and toughness.

Figure 8. Schematic diagram of the structure of copper oxide high temperature superconducting tapes the examples above are only about the strong electrical applications of superconductors. In the weak electric application of superconductivity, even if the room temperature superconductor at atmospheric pressure is realized, in addition to the general "three high" critical parameters, we also hope that it has the characteristics of good impedance, large coherence length, insensitive to air, easy to process in micro-and nano-scale and so on. This is also often difficult to take into account.

Therefore, tens of thousands of superconducting materials have been found today, and there are also many superconducting materials with a critical temperature above 20K, but the traditional Nb-Ti alloy is most commonly used in strong superconducting applications, which has excellent strength, toughness and repeatability, but the superconducting temperature is below 9K, which is much lower than room temperature! Secondly, there are Nb3Sn, Nb3Ge, Nb3Al and so on, and the superconducting temperature does not exceed 24K! In the application of superconducting weak electricity, most of the chips of superconducting quantum computers use aluminum or niobium, pure Nb is used in superconducting resonators, and NbN is used in superconducting single-photon detectors. the superconducting temperature of these materials is less than 20K, and only superconducting filters and terahertz detectors use high temperature superconducting thin films. In traditional superconductors, it is found that the critical temperature of MgB2 can reach 39 K, but its critical parameters are so low that it can only be used in applications below 3 T, and the hardness of the material is too high for processing (figure 9) [10].

Figure 9. The relationship between the upper critical field and the critical temperature of various MgB2 materials [10] We can imagine that even with room temperature superconducting materials at atmospheric pressure, the prospect of large-scale application of superconductivity becomes brighter, but it is still "channel resistance and long". For this reason, the exploration of superconducting materials, the study of superconducting mechanism and the basic research of application need to make continuous efforts in order to select the most suitable materials in various application scenarios. For specific materials, its rich physical properties need to be fully excavated. There are no "unusable" materials in the world, only materials that you "can't use"! Material exploration is like fishing in an electronic sea. The fish caught are strangely shaped, but each has its own purpose (figure 10) [6].

Figure 10. Material exploration is like fishing in the electronic ocean [6] in this wave of "room temperature superconductivity" heat, I hope everyone will keep a cool head, adhere to the attitude of rational analysis, and finally judge on the basis of facts. The exploration of superconductivity will still be full of surprises in the future. I hope you can maintain your attention to basic research and learn more about the connotation of the research, instead of reading the news and asking, "what's the use of this research?" . Read more books on superconductivity and you will gain more (figure 11)! (full text)

Figure 11. Superconducting "tiny Times"-- the past Life, present Life and Future of Superconducting (Tsinghua University Press, 2022) [6] references

[1] N. Dasenbrock-Gammon et al., Nature 615244 (2023).

[2] C. Jin and D. Ceperley, Nature 615221 (2023).

[3] Science News, DOI: 10.1126/science.adh4968

[4] E. Snider et al., Nature 586373 (2020).

[5] H. Pasan et al., arXiv: 2302.08622.

Luo Huiqian, tiny Times of Superconductor-- past Life, present Life and Future of Superconductor (Tsinghua University Press 2022).

Wen Haihu, Physics, 2006, 35 (01): 16-26 and 35 (02): 111124.

Zhang Yuheng, Superconducting Physics, University of Science and Technology of China Press, 2009.

[9] H. Hosono et al. Mater.Today 21,278-302 (2018).

[10] C. Buzea, T. Yamashita, Supercond.Sci.Technol. 14115146 (2001).

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