Building better superconductors with palladium

It is one of the most fascinating contests in modern physics: How can we produce the best superconductors that remain superconducting even at the highest possible temperatures and ambient pressures? In recent years, a new era of superconductivity has begun with the discovery of nickelates. These superconductors are based on nickel, which is why many scientists talk about the “nickel age of superconductivity research”. In many respects, nickelates are similar to cuprates, which are based on copper and were invented in the 1980s.

But now a new class of materials is coming into play: in cooperation between the TU Wien and universities in Japan, it became possible to simulate the behavior of various materials more precisely than ever before on a computer.

There is a “Goldilocks zone” in which superconductivity works particularly well. And this zone is reached not with nickel or copper, but with palladium. This may usher in a new “Age of Palladates” in superconductivity research. The results have now been published in the journal Physical Review Letters.

A search for higher transition temperatures

At high temperatures, superconductors behave like any other conducting material. But when they cool below a certain critical temperature, they change dramatically: their electrical resistance disappears completely and suddenly they can conduct electricity without any loss. This limit, at which the material changes between the superconducting and normally conducting states, is called the critical temperature.

“We have now been able to calculate this critical temperature for a complete range of materials. With our modeling on high-performance computers, we have been able to predict the phase diagram of nickelate superconductivity with a high degree of accuracy, as experiments later show. Later,” TU Prof. from the Institute of Solid State Physics at Wien. says Carsten Held.

Many materials become superconducting only above absolute zero (-273.15 °C), while others retain their superconducting properties even at much higher temperatures. A superconductor that is still superconducting at normal room temperature and normal atmospheric pressure would fundamentally revolutionize the way we generate, transport and use electricity.

However, such material has not yet been found. Nevertheless, high-temperature superconductors, including the cuprate class, play an important role in technology – for example, in the transmission of large currents or the generation of extremely strong magnetic fields.

Copper, nickel or palladium?

The search for the best possible superconducting material is difficult: there are many different chemical elements that come into question. You can put them together in different structures, you can add small traces of other elements to optimize superconductivity. “To find suitable candidates, you have to understand at the quantum-physical level how electrons interact with each other in the material,” Prof. says Carsten Held.

This shows that the interaction energy of the electron is optimal. The interaction should be strong, but not too strong. In between is a “golden zone” that makes it possible to achieve the highest transition temperature.

Palladates as the best solution

This golden zone of moderate interaction can be reached neither with cuprates nor with nickelates—but one can hit the bull’s eye with a new type of material: so-called paladates. “Palladium is a line directly below nickel in the periodic table. The properties are the same, but the electrons there are on average a little further from the atomic nucleus and from each other, so the electronic interaction is weak,” says Carsten Held.

Model calculations show how to obtain optimal transition temperatures for palladium data. “The computational results are very promising,” says Carsten Held. “We hope that now we can use it to start experimental research. If we have a whole new, additional class of materials available with palladates to better understand superconductivity and make better superconductors, then this whole research field can bring forth.”