Research Suggests We Are Entering the ‘Nickel Age’ of Superconductivity
Last summer, scientists discovered a special class of materials called “nickelates” which were promised a “new age” for high-temperature superconductivity because they can conduct an electrical current without any resistance.
Some materials only superconduct close to absolute zero and thus they are not suitable for technical applications. This has had researchers searching for decades to find a material that can superconduct at high temperatures.
In the 1980s, high-temperature superconductors (copper oxides or ‘cuprates’) were discovered, leading to what became known as the “copper age” of superconductivity. In the context of copper though, these “high temperatures” are actually very cold and for some time now, other possibilities have been explored, including the so-called “iron” age based on iron-containing superconductors.
Last year, the research group of Harold Y. Hwang's research group from Stanford then succeeded in demonstrating high-temperature superconductivity in nickelates-—the “nickel age” of superconductivity.
The ‘Nickel Age’ of Superconductivity
Nickelates share some of the characteristics of copper oxides (‘cuprates’), and this got scientists wondering if they could also be used to superconduct at high temperatures. However, they also have many fundamental differences, the primary one being that nickelate material does not contain a type of magnetism that other superconducting cuprates have.
At the time, it was said that the discovery is an important one which “requires us to rethink the details of the electronic structure and possible mechanisms of superconductivity in these materials,” that would have researchers and scientists scrambling to carry out all sorts of experiments and theoretical works. And that they did.
Soon after this discovery, it became clear that the results from Stanford’s initial research, which was followed up with a further investigation in January, could not be reproduced by other research groups.
An illustration of hydrogen incorporated into the nickelate structure. Image credited to TU Wien
It’s Hydrogen’s Fault
Now, researchers at TU Wien (Vienna) have found out why this is the case: In some nickelates additional hydrogen atoms are incorporated into the material structure during their synthesis. These hydrogen atoms completely change the material’s electronic behavior and so if researchers are to successfully produce new superconductors using nickelates, this effect must be accounted for.
"We analyzed the nickelates with the help of supercomputers and found that they are extremely receptive to hydrogen into the material," reports Liang Si of TU Wien, Vienna.
Incorporating Nickelates and Hydrogen
While nickelates that incorporate hydrogen will completely change the material’s electronic properties, this does not happen with all nickelates. "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,” added Si.
This analysis is supported by recent works from NUS Singapore on April 24th, 2020, that succeeded in producing superconducting nickelates. When synthesizing nickelates, the team here allowed hydrogen that is released in the production process to escape immediately, meaning that the nickelates are unable to incorporate it into their structures.
Predicting the Properties of Nickelates
Although nickelates show promise as superconductors, research on the material is in a very early stage and there is a lot of work ahead. Among other things, researchers are keen to dope the nickelate material in various ways to see how this affects its superconductivity across a range of temperatures.
This not only helps determine whether other nickelates can become superconducting but is also relevant to the potential use cases and applications for the material. At TU Wien, new “extremely complex” computer calculation methods are currently being developed to understand the properties of the new material.