The world of superconductivity is in an extraordinary state. In 2015, a team in Germany discovered that hydrogen sulphide can superconduct at 203 kelvin (-70 degrees Centrigrade). That’s the highest temperature ever recorded for a superconductor. But nobody is quite sure how it does it, although there are one or two theories.
Today, the field is set to be turned on its head once again by the discovery that a simple organic molecule, more usually found in sunscreen, can be made to superconduct at 123 K at ambient pressure. That’s almost 100 degrees higher than the current record for an organic superconductor and a similar temperature to the very best ceramic superconductors.
If confirmed, the discovery raises the prospect of a new focus for materials scientists hoping to achieve ever higher superconducting temperatures. It should also set theorists scrambling for an explanation—for the moment nobody is quite sure why this molecule should superconduct at all at such temperatures.
First some background. Ordinary metallic superconductors operate at relatively low temperatures of up to 30 K (-243 degrees Centigrade). When cooled, electrons within the metallic lattice join up to form Cooper pairs that interact with coherent vibrations in the lattice. When the temperature is low enough, these vibrations conspire to ease the passage of the Cooper pairs through the lattice with zero resistance.
This phenomenon of zero resistance is a delicate state. Raise the temperature just slightly and it breaks down and the resistance increases dramatically. This phase transition in conductivity at a critical temperature is one of the hallmarks of superconductivity.
Another is the so-called Meissner effect, in which a superconductor expels the magnetic field within it. Physicists demand evidence the Meissner effect in any claim of superconductivity.
A final test of superconductivity is the isotope effect. Because superconductivity depends on lattice vibrations, it is tremendously sensitive to the mass of the atoms in the lattice. Change their mass by replacing them with lighter or heavier isotopes and the critical temperature changes too.
This change in critical temperature is yet another crucial sign of superconductivity. Physicists usually demand all three of these signatures before they accept any claim of superconductivity.
So what of the new claim? The molecule in question is an aromatic hydrocarbon called para-terphenyl or sometimes diphenylbenzene. As a material used in laser dyes and sunscreen, it is unremarkable.
That looks set to change now that Ren-Shu Wang and pals at Hubei University in China say they have made it superconduct at 143 K by doping it with potassium.
Their method is relatively straightforward. In a high vacuum, they mixed pure para-terphenyl with pure potassium cut into small pieces at a ratio of three to one. They then packed the mixture into quartz tubes and heated it to 260 degrees Centigrade for up to seven days.
Finally, they put the mixture into non-magnetic capsules and measured the magnetic and conducting properties of the material over a temperature range of 1 to 300 K.
The results make for interesting reading. Ren-Shu and co say the magnetic properties of the material change dramatically at a temperature of 123 K. “This shape of the magnetization susceptibility curve is consistent with the well-defined Meissner effect,” they say. “The superconducting transition at temperatures higher than 120 K in this molecule was unambiguously confirmed from these measurements.”
That’s an interesting result but it is far from a slam dunk. It’s not hard to imagine physicists asking whether they observed a phase transition in conductivity at the same temperature and a change in the critical temperature when ordinary atoms were replaced with isotopes.
Unfortunately, Ren-Shu and co have nothing to say on these issues. That makes their announcement tentative at best.
Physicists well know that the field of superconductivity is littered with claims of high temperature phenomenon that have turned out to be impossible to reproduce. So more work is clearly needed here.
Nevertheless, it raises some interesting ideas. If para-terphenyl is superconducting at these temperatures, how does it do it?
Ren-Shu and co have a tentative answer and some evidence for it. Polymers that conduct at all are relatively new to physics, having been first developed at the turn of the century. To explain how they work, physicists think that electrons move across a molecule by interacting with the way it vibrates.
This combination of a vibration and an electron is called a polaron and the way polarons move through an organic molecule explains their conductivity. In some molecules polarons can pair up to form bipolarons.
This is what Ren-Shu and co think is happening in para-terphenyl when it is doped with potassium. The doping allows bipolarons to form, and when the structure is cooled, the bipolarons travel resistance-free in exactly the same way as Cooper pairs in conventional superconductors.
They say they have gathered evidence of this by studying the vibrations at work in the superconducting molecules using a technique known as Raman scattering.
That’s an interesting suggestion. If bipolarons can cause organic materials to superconduct, para-terphenyl is unlikely to be the only example. And maybe this same process will work at higher temperatures in other materials.
That’s a lot of “ifs,” and all this should be taken with a pinch of salt until the exotic behavior of doped para-terphenyl is confirmed. One thing is for certain though—para-terphenyl is about to become one of the most closely studied molecules on the planet. We’ll be watching to see what it reveals.
Ref: arxiv.org/abs/1703.06641: Superconductivity Above 120 Kelvin in a Chain Link Molecule
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