The world of superconductivity is in uproar. Last year, Mikhail Eremets and a couple of pals from the Max Planck Institute for Chemistry in Mainz, Germany, made the extraordinary claim that they had seen hydrogen sulphide superconducting at -70 °C. That’s some 20 degrees hotter than any other material—a huge increase over the current record.
Eremets and co have worked hard to conjure up the final pieces of conclusive evidence. A few weeks ago, their paper was finally published in the peer reviewed journal Nature, giving it the rubber stamp of respectability that mainstream physics requires. Suddenly, superconductivity is back in the headlines.
Today, Antonio Bianconi and Thomas Jarlborg at the Rome International Center for Materials Science Superstripes in Italy provide a review of this exciting field. These guys give an overview of Eremet and co’s discovery and a treatment of the theoretical work that attempts to explain it.
A recent experiment has shown a macroscopic quantum coherent condensate at 203 K, about 19 degrees above the coldest temperature recorded on the Earth, 184 K, in pressurized sulfur hydride. This discovery is relevant not only in material science and condensed matter but also in other fields ranging from quantum computing to quantum physics of living matter. It has given the start to a gold rush looking for other macroscopic quantum coherent condensates in hydrides at the temperature range of living matter with critical superconducting temperatures over 200K and less than 400K. We present here a review of the experimental results and the theoretical works and we discuss the Fermiology of H3S focusing on Lifshitz transitions as a function of pressure. We discuss the possible role of the shape resonance near a neck disrupting Lifshitz transition, in the Bianconi-Perali Valletta (BPV) theory, for rising the critical temperature in a multigap superconductor, as the Feshbach resonance rises the critical temperature in Fermionic ultracold gases.
There are essentially three characteristics that physicists look for as proof that a material superconducts. The first is a sudden drop in electrical resistance when the material is cooled below this critical temperature. The second is the expulsion of magnetic fields from inside the material, a phenomenon known as the Meissner effect.
The third is a change in the critical temperature when atoms in the material are replaced with isotopes. That’s because the difference in isotope mass causes the lattice to vibrate differently, which changes the critical temperature.
But there is another kind of superconductivity that is much less well understood. This involves certain ceramic substances discovered in the 1980s that superconduct up to temperatures of about -110 centigrade. Nobody really understands how this works but much of the research in the superconductivity community has focused on these exotic materials.
Eremet and co’s work is likely to change that. Perhaps the biggest surprise about their breakthrough is that it does not involve a “high temperature” superconductor. Instead, hydrogen sulphide is an ordinary superconductor of the kind that had never been seen working at temperatures greater than about 40 kelvin.
Eremet and co achieved their trick by squeezing the material to the kind of pressures that exist only at the center of the earth. At the same time, they have managed to find evidence of all the important characteristics of superconductivity.
in the 1960s, the British physicist Neil Ashcroft predicted that hydrogen ought to be able to superconduct at high temperatures and pressures, perhaps even at room temperature. His idea was that hydrogen is so light that it should form a lattice capable of vibrating at very high frequencies and therefore of superconducting at high temperatures and pressures.
Eremet and co’s discovery seems to be a vindication of this idea. Or at least, something like it. There are numerous theoretical creases that need to be ironed out before physicists can say they have a proper understanding of what’s going on. This theoretical work is ongoing.
Now the race is on to find other superconductors that work at even higher temperatures. One promising candidate is H3S (as opposed to H2S that Eremet initially worked on).
And of course, physicists are beginning to think about applications. There are numerous challenges in exploiting this material, not least because it exists in superconducting form only in tiny samples inside high pressure anvils.
But that hasn’t stopped people speculating. “This discovery is relevant not only in material science and condensed matter but also in other fields ranging from quantum computing to quantum physics of living matter,” say Bianconi and Jarlborg. They also make the thought-provoking point that this superconductor works at a temperature that is 19 degrees higher than the coldest temperature ever recorded on Earth.
That makes this an exciting field to be in and one we’re likely to hear a lot more about in the coming months and years.
SOURCES – Technology Review, Arxiv