When hydrogen becomes incorporated into the nickelate structure, the nickelate stops being a superconductor. Image: TU Wien.
When hydrogen becomes incorporated into the nickelate structure, the nickelate stops being a superconductor. Image: TU Wien.

Last year, a new age for high-temperature superconductivity was proclaimed – the nickel age. This followed the discovery that there are promising superconductors in a special class of materials known as nickelates, which can conduct electric current without any resistance even at high temperatures.

It soon became apparent, however, that these initially spectacular results from Stanford University could not be reproduced by other research groups. Researchers at the Vienna University of Technology (TU Wien) in Austria have now found the reason why: in some nickelates, additional hydrogen atoms are incorporated into the material structure, completely changing the electrical behavior of the material. In the production of the new superconductors, this effect must be taken into account. The researchers report their findings in a paper in Physical Review Letters.

Some materials are only superconducting near a temperature of absolute zero, making them unsuitable for technical applications. For decades, therefore, scientists have been looking for materials that remain superconducting at higher temperatures.

In the 1980s, so-called ‘high-temperature superconductors’ were discovered. But in this context, ‘high temperature’ is still very cold: even high-temperature superconductors must be cooled to fairly low temperatures in order to reveal their superconducting properties. Thus, the search for new superconductors that work at even higher temperatures continues.

"For a long time, special attention was paid to so-called cuprates, i.e. compounds containing copper. This is why we also speak of the copper age," explains Karsten Held from the Institute of Solid State Physics at TU Wien. "With these cuprates, some important progress was made, even though there are still many open questions in the theory of high-temperature superconductivity today".

But for some time now, other possibilities have also been under consideration. There was already a so-called ‘iron age’, based on iron-containing superconductors. Then, in summer 2019, Harold Hwang's research group at Stanford University demonstrated high-temperature superconductivity in nickelates (see ‘Jenga’ chemistry produces first superconducting nickel oxide material).

"Based on our calculations, we already proposed nickelates as superconductors 10 years ago, but they were somewhat different from the ones that have now been discovered. They are related to cuprates, but contain nickel instead of copper atoms," says Held.

After some initial enthusiasm, however, it has become apparent in recent months that nickelate superconductors are more difficult to produce than initially thought. Other research groups reported that their nickelates do not possess superconducting properties. This apparent contradiction has now been clarified by the team at TU Wien.

"We analyzed the nickelates with the help of supercomputers and found that they are extremely receptive to hydrogen into the material," reports team member Liang Si. During the synthesis of certain nickelates, hydrogen atoms can become incorporated, completely changing the electronic properties of the material.

"However, this does not happen with all nickelates," adds Si. "Our calculations show that for most of them it is energetically more favorable to incorporate hydrogen, but not for the nickelates from Stanford. Even small changes in the synthesis conditions can make a difference." Recently, Ariando Ariando and his team at the National University of Singapore (NUS) reported that they had also succeeded in producing superconducting nickelates, by letting the hydrogen generated in the production process escape immediately.

At TU Wien, new computer calculation methods are being developed and used to understand and predict the properties of nickelates. "Since a large number of quantum-physical particles always play a role here at the same time, the calculations are extremely complex," says Si. "But by combining different methods, we are now even able to estimate the critical temperature up to which the various materials are superconducting. Such reliable calculations have not been possible before."

In particular, the team at TU Wien was able to calculate the allowed range of strontium concentration for which the nickelates are superconducting – and this prediction has now been confirmed by experiments.

"High-temperature superconductivity is an extremely complex and difficult field of research," says Held. "The new nickelate superconductors, together with our theoretical understanding and the predictive power of computer calculations, open up a whole new perspective on the great dream of solid state physics: a superconductor at ambient temperature that hence works without any cooling."

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.