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

Question, March 8, 2023

What day is it?

A: International Working Women's Day

And?

And then.

And then it's still a day at the March meeting of the American physical Society, and don't underestimate this day, a seemingly mundane day that could change the world and the progress of human physics.

The Dias team at the University of Rochester claims to have discovered a near-atmospheric pressure room temperature superconductor, a ternary phase of hydrogen, nitrogen and lutetium. The team believes it can achieve room temperature superconductivity of about 294K (about 21 ℃) at about 10kbar, or 1GPa, or about 10,000 atmospheric pressure.

Superconductor | the picture is from wiki. At this time, someone wants to ask, what is superconductivity, and why is it so excited to find a room temperature superconductor?

01. Superconductivity and its application value superconducting state is a special state of the material. In the superconducting state, the material is in a state of zero resistance. Physics in the second year of junior high school tells us that resistance is a common property of the material. When the current flows through the material, the internal lattice and impurities will hinder the movement of carriers, and the energy carried by carriers will be transferred to the lattice, causing Joule heat macroscopically. The electric potential will drop accordingly.

The superconductor without resistance does not have the above problem at all. If the current flows through the superconductor, there will be neither heat nor voltage drop, so the current can flow through the superconductor without attenuation.

Obviously, the significance of superconductors is obvious. If all our wires use superconductors, there will be no energy decay. The UHV transmission technology we use at this stage is actually to increase the voltage of transmission lines to reduce energy loss as much as possible, but if superconducting wires are used, this problem will not exist at all and the whole industry will be completely rewritten. we can transmit electricity directly at city voltage, without the need for substations at all, and we may be able to use direct current directly.

The superconductivity of our UHV transmission line may be about to change. The picture comes from the network. However, due to the limitation of superconducting Tc (superconducting transition temperature, that is, the temperature at which a superconductor changes from normal state to superconducting state), this idea can not be realized at all. Most of the superconductors we find now are below 77K (- 196 ℃), which is the boiling point of liquid nitrogen. Most of the superconductors below Tc use more expensive liquid helium refrigeration to make them into the superconducting state. Only a small number of copper-based superconductors Tc above 77K can be refrigerated by liquid nitrogen to enter the superconducting state.

Even so, superconductors have been used in our daily life, and superconductors have been used in nuclear magnetic resonance in hospitals, which involves another major application direction of superconductors, that is, the generation of large magnetic fields.

In this thing, there is a superconductor in it. The picture comes from the network. When we need a large magnetic field, what is the first thing we think of? magnet? No, the magnetic field of the permanent magnet is far from meeting our requirements. Recall the physics knowledge of the second year of junior high school. Yes, the electrified solenoid! Using electric current, we can also get a magnetic field. What is more exciting is that the intensity of magnetic induction is proportional to the intensity of the current, that is to say, the greater the current, the stronger the magnetic field.

However, large current will encounter the two problems mentioned above: Joule heat and voltage drop, large current will produce heat, and what is even more desperate is that Joule heat is proportional to the square of the current. Therefore, with each increase in current, the magnetic field will increase by one point. But heat production will increase by the square, and eventually most of the energy will be converted into internal energy.

Currently discovered high temperature superconductors | the source of wiki Joule heat is resistance, as long as there is no resistance, the effect of Joule heat can be completely ignored, so the significance of superconductors here is obvious. If we use superconductor wires to make coils, we can be almost unrestrained (magnetic fields can also suppress superconducting states. It should be noted that the magnetic field generated cannot exceed the critical magnetic field of the superconductor) to increase the current intensity in the coil and obtain a strong magnetic field. This is the source of strong magnetism in nuclear magnetic resonance.

In addition to the above scenario, the Josephson junction made of two different superconductors also has important application value. We can use it to make SQUID. This device is the most accurate magnetic field detection device, and it also has important applications in superconducting quantum computers.

Seeing here, you should have a certain understanding of the significance of room temperature superconductivity. If we can really find room temperature superconductivity at atmospheric pressure, it will bring about major changes in the whole human society, and our existing science and technology may face subversion. The energy problem has been greatly alleviated, which is of great significance for mankind as a whole.

Let's briefly introduce the discovery process and transport properties of superconductors, which is helpful for us to understand the work of Dias.

02. Discovery and mechanism of superconductivity in 1911, Onness improved the refrigeration equipment and took the lead in lowering the temperature below the boiling point of liquid helium. during this period, he found that the resistance of mercury suddenly dropped to zero at 4.2 K. after repeated confirmation, he finally determined that this was not an experimental error or error, but that it was the intrinsic property of mercury. thus, he opened the door to superconductivity, and mercury was also the first superconductor we found, with a Tc of 4.2 K.

In fact, many materials have superconductivity | the figure is derived from the resistance of mercury measured only by wiki Onness, which reveals the electrical transport characteristics of superconductors, that is, zero resistance.

Onness (first from right) | the picture is from Wiki. In 1933, when Meissner measured the magnetic field distribution of tin or lead metal spheres entering the superconducting state, he found that when the material entered the superconducting state, the internal magnetic field would be quickly expelled out of the body. The magnetic field exists only outside the superconductor, and the superconductor shows complete diamagnetism. This is the Meissner effect.

Later studies found that superconductors can be further divided into the first type of superconductors and the second type of superconductors, the first type of superconductors show a complete diamagnetic effect, there is no magnetic field inside. The second type of superconductor allows the magnetic field to generate flux quantum inside the superconductor, that is, to allow the magnetic field to partially enter the superconductor.

The above studies on superconductors still focus on their properties. In fact, we have been looking for the internal mechanism of superconductivity and exploring its essence.

The first attempt was the London equation, but this theory does not reveal the relationship between the depth of penetration and the external magnetic field. Around 1950, the former Soviet scientists Ginzburg and Landau put forward the phenomenological theory to explain superconductivity-Ginzburg-Landau theory (Gmurl theory). The theory is based on Landau's theory of second-order phase transition and uses order parameters to describe superconductors. The theory successfully explains the superconductors. The first type of superconductors and the second kind of superconductors mentioned above are judged by the positive or negative of the interface energy solved by the Gmurl equation.

According to the Gmurl theory, the transition from the normal state to the superconducting state of a superconductor is a second-order phase transition, so we can theoretically find a jump, or a peak, at Tc in the measurement of specific heat. Later, this was also confirmed in experiments.

Electrical and specific heat properties of ideal superconductors | the picture is from wiki. See here, you should also find that superconducting articles are very easy to write. Measure the resistance, measure the magnetic susceptibility, if possible, measure the specific heat, even if the specific heat can not be measured, it is not a big deal.

Finally, I would like to briefly mention that the best theory for us to explain superconductivity at present is the BCS theory. The core of this theory is the possibility that electrons will attract electrons in the coupling between electrons and the lattice, so that the two electrons will form a Cooper pair, and the electrons forming a Cooper pair can be regarded as bosons. At low temperature, "condensation" occurs, and energy can flow in the condensed Cooper pair without dissipation to achieve a superconducting state.

However, the BCS theory can not explain all the superconducting states. According to the BCS theory, we calculate the McMillan limit, that is, the superconductor Tc that accords with the BCS theory will not exceed 40K, but in fact, many superconductors have broken through this limit, such as copper-based superconductors and iron-based superconductors. Such superconductors are called high temperature superconductors, that is to say, compared with the previous superconductors below 20K, the Tc is much higher.

Originally, I also wanted to introduce the acquisition of high pressure in the experiment. Due to the limitation of space, I would like to talk again when I have the opportunity. A previous article also explained the high-voltage technology in the laboratory. If you are interested, you can click here to have a look.

03. With the above preliminary knowledge of the new room temperature superconductivity, we can take a look at this article, which has been published on nature.

Did you see Dias's name? Finally, like most superconducting articles, the Dias team measured the electrical transport, magnetic susceptibility and specific heat of the samples.

The first is the resistance measurement results. The resistance measurement results under 10, 16 and 20kbar (1,1.6, 2.0GPa) are given on the left. The resistance is reduced to 0 at three voltages, which is one of the main characteristics of superconductors. It should be noted that the Tc is the highest at 1GPa, and the lower the voltage is, the higher the Tc is, which is an unexpected result. The illustrations are samples and electrode pictures. On the right, the Vmuri curves of superconducting state and normal state are given.

This picture is the measurement of magnetic susceptibility. An is the 60Oe (Oe is the unit of magnetic field strength in the Gaussian unit system, which can be understood as Gauss, that is, 1T=10000Oe). The magnetic moment of 8kbar (0.8GPa) varies with temperature. It can be seen that the Tc is 277K (4 ℃). Figure b shows the relationship between magnetic moment and external magnetic field, which is also consistent with the characteristics of superconductors, while figure c shows the Mry T curve under different pressures. The Tc here is consistent with the resistance, and the transition temperature range is very small, which is a very good transition. However, it can also be seen in figure a that the research team has done some processing to the original data.

It is mentioned here that the measurement of magnetic susceptibility will be obviously measured by the shape of the sample, background and other factors. Theoretically, the superconductor should show complete diamagnetism (i.e. 4 π χ =-1). However, it is understandable that complete diamagnetism can not be measured in practical measurement. Of course, there is no reduction in Dias's article. The longitudinal axis in figure an is magnetic moment, not magnetic susceptibility.

Dias also measured the heat. The results are shown in the figure above. The measurement results of 10,10.5 and 20kbar are given here. It can be seen that the three specific heat curves can see the transition of superconductivity in the specific heat. The measurement results of Tc and resistance are slightly different, but it is completely understandable. This result is reasonable. However, it should be said that the change in specific heat is not obvious, especially in the curve of 10.5kbar, the peak is not obvious, and the transformation of 10kbar is not as obvious as that of 20kbar. These three specific heat shifts also seem to be somewhat different, especially the data of 10kbar and 10.5kbar, which are only worse than 0.5kbar, but the images are very different. However, considering that it is measured under high pressure, there may be some difficulties that we do not know.

Dias also gives the XRD (X-ray diffraction) results of the sample and draws the cell image, which is of course necessary.

A figure is the XRD result, they use the Mo target, the red line is the result of theoretical calculation, the circle is the result of actual measurement, and the blue line is the error between the two. It can be seen that there is little difference between the measured and calculated results. The sample can be said to be a pure phase. The Dias team calculated that the proportion of the sample is 92.25%, and the impurities are LuN1 − δ H ε and Lu2O3.

The white atom is hydrogen, the green atom is lutetium, and the pink one is nitrogen atom. The chemical formula of the sample given by them is LuH3 − δ N ε, and the space-time group of 61kbar is Fm-3m and Immm, but Dias believes that the superconducting phase space group is the former.

Finally, the superconducting phase diagram of the sample (originally this is the first picture), Tc decreases with the increase of pressure, which is unexpected, and later may also become the focus of the study. B diagram shows that the shape of the sample changes with the pressure, it is blue under normal pressure, and gradually becomes pink with the increase of pressure, and finally shows red, the color of the sample is still very festive.

Space is limited, the supporting materials will not take you to read together, interested students can click on the link to jump to the nature official website to see.

Evidence of near-ambient superconductivity in a N-doped lutetium hydride | Nature

Judging from the article, this work is undoubtedly a breakthrough, and the relevant evidence is also very sufficient. If it can be repeated, we may be able to issue a Nobel Prize in the future. However, after all, the study of physics is not the same opinion, and any scientific research should be able to withstand verification, and this is no exception. This work is bound to be repeated by various research groups in the industry. If it is repeated many times, it will be an epoch-making work to determine the correctness of the results. We will have the strength to predict this year's Nobel Prize.

This work claims to be room temperature superconductivity in the near environment. As you can see above, the highest pressure of Tc is 1Gpa, about 10,000 atmospheres. Although it is still very large, it is much smaller than the previous 2.7 million atmospheres, and it is much less difficult to repeat. I believe many research groups have already begun to repeat experiments.

However, at present, many people hold a wait-and-see attitude towards this result, on the one hand, because the results of repeated experiments have not yet come out, and on the other hand, it may be because of the previous "criminal record" of Dias.

In fact, before that, Dias had already made two breakthroughs. One is metal hydrogen, and the other is superconducting at room temperature.

Dias first claimed to have synthesized metal hydrogen under high pressure, and the article was published on science, but other research groups did not repeat it, but he later claimed that due to improper preservation, the pressure of the device that preserved the metal hydrogen leaked, and eventually the metal hydrogen vaporized away due to lack of pressure. Later, Dias did not synthesize metallic hydrogen either. As a result, metal hydrogen can be said to be a "cold case".

The last hydride room temperature superconductivity was also synthesized by Dias, which achieved a pressure as high as 270GPa, and the relevant results were published on nature, but subsequent research groups failed to repeat the experiment, and because Dias did not disclose the original data, many people thought that it used the wrong method in the data processing of magnetic susceptibility and got the conclusion that it is not correct. So in the face of unanimous protest, the article was eventually withdrawn from nature, and of course, all members of the Dias research team protested against the withdrawal, but it was not undone.

Because of these two incidents, many scientists in the field are actually distrustful of the Dias research team. after all, their data results are always much prettier than others. But this time Dias gives a lot of raw data, which can be said to be comprehensive and rich, and this result only requires the pressure of 1GPa, which is relatively easy to repeat. I think we will soon be able to come to a conclusion on the result. Let's wait and see.

Reference:

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

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). Author: mu Zi

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