Strange metal turns out to superconduct after all

At low temperatures, certain materials lose their electrical resistance and conduct electricity without any loss. Known as superconductivity, this phenomenon was first discovered in 1911, but it is still not fully understood. And that is a pity, because finding a material that retained superconducting properties at high temperatures would probably trigger a technological revolution.

A discovery made by a team of solid-state physicists at the Vienna University of Technology (TU Wien) in Austria, reported in a paper in Nature Communications, could represent an important step in this direction. The team studied an unusual material – a so-called 'strange metal' made of ytterbium, rhodium and silicon (YbRh₂Si₂).

Strange metals display an unusual correlation between electrical resistance and temperature. In this particular strange metal, the correlation can be seen over a particularly wide temperature range, and the underlying mechanism is known.

Contrary to previous assumptions, it now turns out that this material is also a superconductor, and that its superconductivity is closely related to its strange metal behavior. This could be the key to understanding high-temperature superconductivity in other classes of materials as well.

In ordinary metals, electrical resistance at low temperatures increases with the square of the temperature. In some high-temperature superconductors, however, the situation is completely different: at low temperatures, below the so-called superconducting transition temperature, they show no electrical resistance at all, and above this temperature the resistance increases linearly instead of quadratically with temperature. This is what defines 'strange metals'.

"It has therefore already been suspected in recent years that this linear relationship between resistance and temperature is of great importance for superconductivity," says Silke Bühler-Paschen, who heads the 'Quantum Materials' research area in the Institute of Solid State Physics at TU Wien. "But unfortunately, until now we didn't know of a suitable material to study this in great depth."

In the case of high-temperature superconductors, the linear relationship between temperature and resistance is usually only detectable over a relatively small temperature range. What is more, various effects that inevitably occur at higher temperatures can influence this relationship in complicated ways.

Many experiments have already been carried out with YbRh₂Si₂. These have revealed that the material displays strange metal behavior over an extremely wide temperature range – but, surprisingly, no superconductivity seemed to emerge from this extreme 'strange metal' state. "Theoretical considerations have already been put forward to justify why superconductivity is simply not possible here," says Bühler-Paschen. "Nevertheless, we decided to take another look at this material."

At TU Wien, a particularly powerful low-temperature laboratory is available. "There we can study materials under more extreme conditions than other research groups have been able to do so far," explains Bühler-Paschen.

First, the team was able to show that in YbRh₂Si₂ the linear relationship between resistance and temperature exists over an even larger temperature range than previously thought. Then they made the key discovery – at extremely low temperatures of just one millikelvin, the strange metal turns into a superconductor.

"This makes our material ideally suited for finding out in what way the strange metal behavior leads to superconductivity," says Bühler-Paschen.

Paradoxically, the fact that the material only becomes superconducting at very low temperatures ensures that it is particularly suited for the study of high-temperature superconductivity. "The mechanisms that lead to superconductivity are visible particularly well at these extremely low temperatures because they are not overlaid by other effects in this regime," explains Bühler-Paschen. "In our material, this is the localization of some of the conduction electrons at a quantum critical point. There are indications that a similar mechanism may also be responsible for the behavior of high-temperature superconductors such as the famous cuprates."

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.