Just at the grand event of physics, the March meeting of the American Physical Society, there was big news. Ranga Dias from the University of Rochester announced that her team achieved room-temperature superconductivity at Near Ambient Pressure.
The report is titled: Observation of Room Temperature Superconductivity in Hydride at Near Ambient Pressure
Translate: Observed room-temperature superconductivity of metal hydrides at near-ambient pressure
Website: https://meetings.aps.org/Meeting/MAR23/Session/K20.2
Ranga Dias' group achieved superconductivity at 1 GPa (or 1000 MPa), 20°C, in ternary hydrides (Lu-N-H, known as "halogen-egg hydrogen"). The so-called "ternary" means that there are three elements, and hydrides are compounds formed by hydrogen and other elements. N and H are both familiar elements, Lu (Lutetium, lutetium) is slightly unfamiliar, its atomic number is 71, and its electronic structure is: 4f14 5d1 6s2
Here's a summary of the Dias report:
Superconductivity is one of the most esoteric phenomena in nature. However, due to the extremely low temperatures required, this elusive quantum state has not yet caused a revolution in the scientific community. Thus, since Kamerlingh Onnes first observed superconductivity in the element mercury at 4.2 Kelvin more than a century ago, near-normal pressure superconductivity has become one of the most pursued goals in the scientific community. Over the past decade, high-pressure technology has dominated the search for high-temperature superconductivity. The leading route is achieved by hydrogen alloys under "chemical precompression", and the superconducting critical transition temperature (Tc) demonstrated by rare earth hydrides LaH10 and YH9 is close to the freezing point of water. We found room temperature superconductivity in hydrocarbon (C-S-H) (NOTE: THIS WORK HAS BEEN RETRACTED BY NATURE) AND HIGHLIGHT THAT TERNARY OR HIGHER SYSTEMS MAY BE THE KEY TO HIGHER TC AND NEAR-NORMAL PRESSURE SUPERCONDUCTIVITY. Here, we report recent advances in the performance of a new material that exhibits superconductivity under near-normal pressure conditions. These compounds were synthesized under high temperature and pressure conditions, and their fully recoverable material and superconductivity properties were investigated in the compression path. With these materials, the dawn of near-atmospheric superconductivity and applied technologies has arrived, now opening a direct avenue for customizing hydrides under extreme conditions through "material design." (Acknowledgements: ChatGPT)
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【1】
Superconductivity is very good, and it has important applications in a series of important fields such as maglev trains and nuclear fusion. It is marked by the presence of a critical temperature Tc, below which there are: 1. zero resistance phenomenon; 2. Meissner effect (complete diamagnetism).
The superconducting transition temperature of 20°C means that this ternary hydride will have zero resistance phenomenon and Meissner effect at room temperature. In the report, Ranga Dias gives three pieces of evidence for achieving room-temperature superconductivity, in addition to zero resistance, the Meissner effect (susceptibility measurement), and data for specific heat measurements (superconducting transitions are characterized by the presence of specific thermal jumps). It can be said that it is quite sincere.
At the same time, the importance of this work is also that the high pressure required to achieve room temperature superconductivity is not very high, 1000MPa (equivalent to about 10,000 atmospheres) is close to the capacity of ordinary isostatic presses, 500MPa is the standard specification of isostatic presses, if you make a demand, 1000MPa is completely fine. This is a sign that commercial room-temperature superconductivity is already beckoning to us.
The picture on the left sells a 500MPa isostatic press, and the picture on the right says that it is a 1000MPa isostatic press that can be customized.
Of course, we need other study groups to repeat this work independently, and in fact the work in this area is quite controversial. Ranga Dias, a physicist from Sri Lanka, graduated from Colombo University in Sri Lanka in 2006 and received his PhD from Washington State University in 2013. Dias' research area is quantum phenomena of hydrides under extreme conditions, such as high pressure.
Dias Class
In 2020, the Dias group published a similar work in Nature, claiming that ternary hydrides (C-S-H) achieved superconductivity of 15°C at ultra-high pressure of 267GPa. As soon as this work came out, it was praised as a Nobel Prize-level work. Unfortunately, because other physicists could not repeat this result, Nature retracted it in September 2022. It is worth mentioning that complete diamagnetic measurements are experimentally difficult, often requiring subtraction of the background signal, and this part of the "unconventional" operation is often very controversial. It is also easily influenced by people's wishful thinking.
In September 2022, the C-S-H superconductivity article was tragically retracted by Nature
Less than half a year later, Dias made a comeback with a new ternary hydride (N-Lu-H), achieving a higher superconducting transition temperature at a less extreme high pressure (1GPa). The pressure is lower, Tc is higher. Surprised or not? Strange or not?
Originally, 267GPa, we also needed diamond to achieve ultra-high pressure on the top anvil, and now it has been immediately reduced to 1GPa that poor people can also try (Note: Dias still used diamond to do experiments on the top anvil this time), presumably there will be more groups to repeat this time, right? Whether it can be repeated or not is a perfect bet for physicists.
Diamond against top anvil
【2】
The higher the critical temperature, the closer the superconductivity is to practical applications, such as the copper oxide discovered in the 80s of the last century, which is called a high-temperature superconductor because it can achieve superconductivity in the liquid nitrogen temperature region (77K or -195.8 °C). This is already a remarkable step forward because liquid nitrogen is cheap. Perverted physicists also hope to achieve room-temperature superconductivity (around 20°C), which has the advantage of getting rid of refrigeration equipment and advancing our understanding of superconductivity.
Not surprisingly, room temperature superconductivity is known as the holy grail of physicists, of course, the pressure cannot be too high, by default we think of 1 atmosphere, or under normal pressure.
The classical theory of superconductivity holds that the "electron-phonon" interaction is the mechanism that causes electron pairing, in other words, below the critical temperature Tc, the electron will enter a "new" quantum state, that is, a state in which spin and momentum opposing electrons pair in pairs. According to this "electron-phonon" interaction image, there will be the so-called isotopic effect, which simply is:
Here M is the mass of the ion, the smaller the M, the higher the superconducting transition temperature Tc, obviously hydrogen (H) is the smallest. This means that under the BCS framework, if we can achieve metallic hydrogen, it will have the highest superconducting transition temperature. But unfortunately, the interaction between hydrogen and hydrogen is too weak, usually hydrogen comes in the form of gas, or we need to use super super huge pressure to press H into metallic hydrogen.
Dias' work can be said to be a continuation of this simple line of thought.
Metal H is too difficult, what to do? A natural idea is to introduce other elements into it, just as we were going to confine hydrogen to solids, which is too difficult, we introduce other elements to make a cage (or house) for hydrogen, and constrain H in this cage, which is why people look for Tc higher superconductors in hydrides.
LaH10 structure schematic: La, red ball; H, Green Ball.
The first attempts were naturally binary hydrides, and under extreme high-pressure conditions, physicists achieved 155GPa, 203K (-70°C) superconductors in SH3, and then 190GPa, 260K (-13°C) superconductors in LaH10 (note: these two works have been confirmed by many physicists). Naturally, the next step is ternary hydrides.
Green represents superconductors that can be described by the BCS mechanism, with hydrides at high pressure having the highest superconducting transition temperature.
In the figure above, green represents a superconductor that can be described by the BCS mechanism, hydrides at high pressure have the highest superconducting transition temperature and are infinitely close to room temperature (red dashed line), and the highest green dot in the figure (HxSxCx, 267GPa) is the work retracted by Nature.
However, here we encounter a "complexity" problem, more than 100 elements in the periodic table, fixed H elements, completely randomly, for ternary hydrides, we need to test 10,000 possibilities, and this does not take into account the proportions of various elements. Obviously, we can't try randomly, it's better to have a roughly aligned direction, try the most likely several. This is called material design.
In the penultimate row, the last element is Lu
In the framework of BCS, Tc is determined by the "electro-phonon" coupling intensity, which is calculated by the Eliashberg spectral function α^2F(ω). Therefore, we need to calculate the Eliashberg spectral function of many ternary hydrides. It's not very fast, but now we have machine learning/artificial intelligence, and physicists are actually using ML/AI to calculate the Eliashberg spectral function of many, many kinds of ternary hydrides.
Generate Eliashberg spectral functions with machine learning/artificial intelligence
Now we know that today's big news, the hero behind the scenes is still ML/AI. The approximate step is for physicists to train neural networks with Eliashberg spectral functions that are relatively easy to calculate, and after training, the neural network is used to generate more difficult-to-calculate Eliashberg spectral functions for ternary hydrides. Then we can calculate the Tc of the various ternary hydrides, and then we only need to try a few ternary hydrides with the highest Tc.
【3】
To summarize, we have three concerns:
1. Whether this work will be repeated by other groups, and thus whether true room-temperature superconductors at atmospheric pressure will be screened out.
2. If (1) holds, then this will be another victory for the BCS theory, and perhaps superconductivity will really be unified by a BCS-framed theory of superconductivity.
3. Don't say anything more, learn ML/AI, which will be the universal toolkit for all the innovative jobs of the future.
Author: @ianwest