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.
Followers of this blog will have read about this work last December, when it was first posted to the arXiv. At the time, physicists were cautious about the work. The history of superconductivity is littered with dubious claims of high-temperature activity that later turn out to be impossible to reproduce.
But in the months since then, 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.
First, some background. Superconductivity is the phenomenon of zero electrical resistance that occurs in some materials when they are cooled below a critical temperature.
This phenomenon is well understood in conventional superconductors, which are essentially rigid lattices of positive ions bathed in a sea of electrons. Electrical resistance occurs because electrons bump into this lattice and lose energy as they move through it.
However, at low temperatures electrons can bond to each other to form Cooper pairs. At the same time, the lattice becomes rigid enough to allow the coherent movement of waves called phonons.
Superconductivity occurs when the Cooper pairs and the phonons travel together through the material, the waves essentially clearing the way for the electron pairs. And it breaks down when the vibrations in the lattice—its temperature—become strong enough to break apart the Cooper pairs. That’s the critical temperature.
Until recently, the highest critical temperature of this kind was about 40 kelvin or -230 centigrade.
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.
While all this experimental work has been ongoing, theorists have scratching their heads over how to explain it. Many physicists had believed there was some theoretical reason why conventional superconductors cannot work above about 40 kelvin. But actually there is nothing in the theory that prevents superconductivity at higher temperatures.
Indeed, 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.
Ref: arxiv.org/abs/1510.05264 : Superconductivity Above ythe Lowest Earth Temperature In Pressurized Sulfur Hydride