Crystallographers Use Computers To Find New Superconductor

196° Celsius below zero counts as a high temperature for people who work with superconductors, because this is the boiling point of nitrogen at 77 Kelvin, the units of measurement used by physicists. Thus a material can be cooled down to this temperature with liquid nitrogen, which is cheap and easy to manufacture. This is why materials that are superconducting at this temperature are attractive for technical applications. Such materials exist and are called “unconventional superconductors” – unconventional because the mechanism of their superconductivity is not understood.

The highest transition temperature found up to now – the temperature below which a material becomes superconducting – was for a cuprate, a copper compound, and lies at 166 K (-107°C). The problem is that cuprates have a similar consistency to graphite, which we know from pencil leads. Cuprates are difficult to work mechanically; for example, attempts to produce long wires from them have hitherto been unsuccessful. Cuprates are also difficult to manufacture and are often toxic.

Conventional superconductors have none of these problems, but those currently known do not become superconducting until far below the boiling point of nitrogen, somewhere between absolute zero and 39 Kelvin (–234°C).

13 degrees still missing

Scientists are now searching for a conventional superconductor whose transition temperature is above 77 Kelvin. A team of scientists led by Artem Oganov at the Laboratory of Crystallography of ETH Zurich has moved a big step closer to this goal. As they reported recently in Physical Review Letters, computer calculations have shown that germanium hydride (GeH4) is a conventional superconductor with a transition temperature of 64 Kelvin. Thus they are now only 13 degrees short of the nitrogen limit.

It might be possible to bridge these 13 degrees by doping the material with tin or silicon. However, germanium hydride must also be under high pressure to become superconducting: about two megabars are needed, i.e. about a million times more than the pressure in the tyres of a motor car. Such pressure cannot be achieved industrially, although it can in laboratories. A laboratory in Germany already plans to expose germanium hydride to this pressure this month.

Artem Oganov has no doubt that the results of the laboratory experiment will agree with his calculations. His algorithms have already proved correct for other materials. For example, the calculations for SiH4 agreed with the measurements, whereas earlier calculations by other groups were wide off the mark. When Oganov first published his method three years ago, it caused great excitement throughout the world. Oganov says that, when he advertised for a post-doc position, one of the applicants was a professor, “one of China’s cleverest theoretical physicists.” Yanming Ma obtained the post and was involved in numerous discoveries during his two years in Oganov’s group at ETH Zurich. He has since returned to China. The current publication on germanium hydride was the result of collaboration between the groups led by Ma and Oganov. The first author is Ma’s doctoral student Guoying Gao.

On 1 December Oganov himself took up a professorship at the State University of New York where he will lead his own laboratory in which, among other things, he will continue the search for superconductors. One aim is to find all the possible stable structures composed of defined elements such as Ge and H – perhaps one of them will be an interesting superconductor.

Superconductivity

When a material becomes a perfect conductor below a certain temperature, this is called superconductivity, which means that an electric current can flow through it without any resistance. The phenomenon was discovered in 1911 by the Dutchman Kamerlingh Onnes. Nowadays, superconductors are used only in a few areas, for example in magnetic resonance tomography, in the magnets for the LHC particle accelerator at CERN and in a few magnetic levitation railways.