At high temperatures, superconductors behave very similar to other conducting materials. But when they are cooled below a certain (transition) temperature, their electrical resistance disappears and suddenly they conduct electricity without any loss. The limit at which a material switches between a superconducting and a normally conducting state is called the “critical temperature”.
Many materials become superconducting only just above absolute zero (−273.15°C), while there are some that become so at much higher temperatures. Copper-based (cuprate) superconductors, which were discovered in the 1980s, have long held the record for the highest critical temperature at ambient pressure. In 2019, researchers reported a nickel-based (nickelate) analogue to cuprate superconductors. The arrival of a “nickel age of superconductivity” was even heralded. (Cuprates are layered materials comprising superconducting layers of copper oxide separated by layers containing ions of elements such as lanthanum, barium, and strontium. In many respects, nickelates are similar to cuprates.) Since then, researchers have been trying to identify factors that control superconductivity in such systems.
Motoharu Kitatani of the University of Hyogo, Japan, and associates from the Vienna University of Technology (TU Wien) have now identified some of them. The study, which was published recently in Physical Review Letters, suggests that replacing nickel with palladium could result in compounds superconducting at even higher temperatures than cuprates. Palladates could soon become the new hunting arena for novel high-temperature superconducting materials.
The search for the best possible superconducting materials is difficult. You can put many different elements together in different structures or you can add tiny traces of other elements to optimise superconductivity. Using computer simulations, the team studied how electrons interacted with each other in different materials and showed that there was an optimum interaction strength of the electrons, strong but not too strong. That is, there is a “golden zone” where the highest transition temperatures can be achieved. The study found that this golden zone could not be attained with cuprates or nickelates but with palladates. “Palladium is directly one line below nickel in the periodic table. The properties are similar, but the electrons there are on average somewhat further away from the atomic nucleus and each other, so the electronic interaction is weaker,” said Karsten Held of TU Wien.