Finding New Superconductors: Where To Look

The successful prediction of superconductivity in a material that is not related to known superconductors remains a significant theoretical challenge, to say the least, though some theorists are trying. As a consequence, the past decade or two of superconductor discovery can best be summarized by the phrase “expect the unexpected”. Breakthrough discoveries of new classes of superconductors have been made, but not where even experts in the field would have imagined to look, magnetic clays (sodium cobalt oxyhydrate) and poisonized  lodestone (lanthanum iron oxyarsenide) being the most recent examples. Once the windows are opened onto a new superconducting family, rational materials design takes over and optimizing the superconducting transition temperature proceeds in an understandable, predictable fashion.  But where to look to find the breakthrough classes of superconductors? Although I come to the conclusion from a different scientific pole, chemistry rather than physics, like the previous speaker I believe that the key is to look for superconducting compounds that lie in an intermediate regime of chemical and physical behavior not so easily found. In the language of chemistry, the best superconductors should be materials where the bonding is neither fully ionic nor fully covalent, not sodium chloride and not Cu₃Au, but something in between; where the charge counting we learn in freshman chemistry seems to work but also makes us nervous.  Our compound walking the chemical middle road should also be bordering magnetic, or close to disproportionating into different charge states, or almost structurally unstable to give it the best chance for hosting superconductivity. I’ll make the case for these views in my talk.