Finally, superconductivity at room temperature!

Imagine a world where diagnostics early medical care for cancers and the Stroke obtained with a IRM are as easy to make and widespread as those made by ultrasound or with x-rays classic. Imagine a world where the transport ofenergy electricity is done without loss – which would help to make a smooth ecological transition – and where one can reach Beijing, for example, from Kiev (in Ukraine) in just one hour thanks to hypersonic trains in magnetic levitation circulating in vacuum tubes. Finally imagine a world where tokamaks like Iter would also be made more efficient thanks to revolutionary magnets.

Such a world, where a new revolution in the field of electronics could also have taken shape with less energy-consuming and more efficient devices, would be possible if we discovered materials that are superconducting at temperature and pression moreover, easy and inexpensive to manufacture while being mechanically and chemically robust.

« We live in a society of semiconductors, and with this kind of technology, you can bring society into a superconducting society where you never need things like batteries again. », Declares on this subject Ashkan Salamat of the University of Nevada in Las Vegas, co-author with Ranga Dias of the University of Rochester and other colleagues of a promisingly resounding article just published by the newspaper Nature.

Superconductivity at room temperature, the hydrogen sulfide trail

The physicists indeed announce there to have obtained a superconducting phase in a simple compound based on Hydrogen sulfide H₂S which has been mixed with methane CH₄. The temperature record is broken since the critical temperature below which we obtain this phase, where electricity can flow without resistance thanks to quantum effects, is only 15 ° C!

But all is not rosy for all that because the pressures necessary to obtain the supraconductivité are of the order of 2.6 millionatmospheres. For the moment, therefore, it is only a laboratory curiosity. We can nevertheless think that it is one more encouragement to go on the track of superconductors. exotic that would allow the technological revolutions discussed previously.

This feat is not completely a surprise and it does indeed give additional hope. To understand this, let us first remember that the supraconductivité was discovered more than 100 years ago, on April 8, 1911. It has fascinated many physicists, such as Vitaly Ginzburg and Pierre-Gilles of Gennes, and has given rise to the attribution of several Nobel prizes such as that of Lev Landau.

It turns out that in 1935 another Nobel Prize winner physique of Hungarian origin, Eugene Wigner, had his famous prediction of the existence at high pressures of a metallic phase ofhydrogen, with his colleague the American physicist Hillard Bell Huntington. From the end of the 1960s, a famous physicist from solid, the British Neil William Ashcroft, comes to the conclusion that, not only metallic hydrogen may be a superconductor, but that it could remain so under conditions of ambient temperatures and pressures because being metastable.

Recall that the diamond is a well-known example of the phenomenon of metastability because it is obtained from the graphite initially brought to high pressures and high temperatures, such as those prevailing more than 150 kilometers inside the Terre. However, it remains as a diamond on the Earth’s surface even billions of years after its formation, unless it is heated to high temperatures again.

In search of a metastable superconductor at room temperature

A quest for this holy grail of superconducting metallic hydrogen at room temperature has been engaged for some time and, at the start of 2020, a significant milestone on this road had been reached as Futura explained it in a previous article on the announcement of the success of a French team of Paul Loubeyre and Florent Occelli, two researchers from the CEA (Commissariat à l’Énergie Atomique et aux Énergies Alternatives) and Paul Dumas, researcher emeritus of the Institute of Chemistry CNRS, seconded to the Synchrotron Soleil.

Their results, also published in the scientific journal Nature, concerned in fact obtaining a metallic phase of hydrogen for the first time and in a way which appears indisputable to the three researchers. In his interview, Florent Occelli explained to us at the time that, on the one hand, he and his colleagues had obtained molecular hydrogen and not yet really metallic hydrogen but that in addition, they were not not yet to show that it was superconducting.

When we asked him to confirm that there were indeed hydrogen-based compounds which are superconductors at high pressures, the physicist replied: ” Yes, like hydrogen sulfide (H₂S) and in particular the hydride of lanthanum (LaH₁₀) but these compounds are not metastable and therefore do not remain superconducting at ambient pressure. We have reason to believe that at least one compound is needed binary with hydrogen, i.e. metal hydrides with at least two types of metal elements associated with atoms of hydrogen. This is a research path currently being explored, especially since the hydrides already obtained are superconductive at pressures lower than those of metallic hydrogen and which are today rather easy to produce. ».

The American physicists proceeded as in the case of their French colleagues to make their discovery, precisely with a binary hydrogen compound that they compressed using a diamond anvil press.

The superconductivity of hydrogen sulfide at 190 K could be explained

Article by Laurent Sacco published on 04/25/2015

At high pressure, hydrogen turns metallic. The addition of atoms of sulfur can then turn it into material superconductor, a property that hydrogen sulfide seemed to retain up to a temperature of 190 kelvins. Which suggested that it perhaps contained the key to the enigma of cuprates. But this is not the case…

Almost 30 years ago, when the first high-critical-temperature superconductors were discovered, it was hoped that there would be rapid progress both theoretically and practically in the direction of creating superconducting materials at room temperature. Unfortunately, the exotic superconductors what are the cuprates always jealously guard their secrets. However, we know that, unlike conventional superconductors, the Cooper pairs formed there do not originate according to a mechanism well understood within the framework of the BCS theory.

Different strategies are being studied to solve the enigma of cuprates, and in the meantime, other types of exotic superconductors have been discovered, such as those at heavy fermions. The high temperature record attested for cuprates is 164 Kelvin (K). However, high pressure must be exerted to obtain this result because, at ambient pressure, it is only 133 K. Recently, a group of researchers obtained an intriguing result by compressing to 150 GPa of the simple Hydrogen sulfide (H₂S).

Hydrogen sulfide would impose anharmonic quantum oscillators

The theorists of course immediately looked into the question. This is a simple physical system and such high critical temperatures are usually only encountered with superconductors not described by BCS theory. It could therefore be a window on the mechanisms of exotic superconductivity. But an international team of English, Canadian, Chinese, Spanish and French researchers has just published on arXiv an article which suggests that it is not.

To reach this conclusion, physicists have, as they should, used the laws of quantum physics. They began by establishing that hydrogen sulfide lost its stability under high pressure and that the H₂S became either an HS material₂ either a crystalline solid based on H₃S forming a cubic network (the two compounds in fact take a metallic form at the high pressures studied). They then showed that this last form was well capable of exhibiting a superconducting phase at a high critical temperature above 200 GPa.

As in the framework of BCS theory, Cooper pairs are formed under the effect of the interactions of electrons with the phonons of crystal lattice, that is to say the analogues of photons for the sound waves in networks of crystallized solids. But, in the present case, the simple model of a harmonic oscillator to describe small vibrations hydrogen atoms around their equilibrium position is no longer sufficient. We must take into account a more complex oscillator model with vibrations which are said to be anharmonic.