Jan Kune. To the right: Nickel, oxygen atoms and moving electrons between. Credit: Vienna University of Technology
High-temperature superconductivity remains a mystery even after over 30 years of research. It is not yet clear what causes certain materials to conduct electricity at high temperatures without resistance.
A new class of superconductors with promising potential was discovered two years ago: the so-called "layered nickelates". A research team from TU Wien has been able to determine the important parameters of these new superconductors for the first time by comparing theory with experiment. This is the first time that a theoretical model can be used to study the electronic mechanisms of superconductivity at high temperatures in these materials.
Looking for superconductors at high temperatures
Although there are many superconductors known today, most are only superconducting at very low temperatures. Materials that are superconducting at higher temperatures can be called "high-temperature SUPERCONDUCTORS". These temperatures, often in the order of magnitude of -200C or less, are still very cold according to human standards.
It would be revolutionary to find a material that is still superconducting at higher temperatures. This would open up the doors for many new technologies. The so-called cuprates, a class of materials that contain copper atoms, were for a long time considered to be a promising candidate. However, a new class of materials is emerging that could prove to be even more exciting: Nickelates. These have a similar structure as cuprates but contain nickel instead of copper.
"There have been many studies on cuprates and it has been possible dramatically increase the critical temperature at which the material remains superconducting." It would be a significant step forward if similar progress is made with newly discovered nickelates," Prof. Jan Kune, Institute of Solid State Physics at TU Wien, says.
Parameters that are difficult to access
Existing theoretical models that describe the behavior of superconductors are available. However, these models are only useful if you have a good understanding of the material parameters. Jan Kune explains that the charge transfer energy is a crucial part of this model. This value indicates how much energy is required to convert an electron from a nickel-atom to an oxygenatom.
This value cannot be measured directly and the theoretical calculations are very complicated and imprecise. Atsushi Hariki is a Jan Kune researcher and has developed a method to indirectly determine this parameter. When the material is examined using X-rays the results also depend upon the charge transfer energy. Jan Kune explains, "We calculated details about the Xray spectrum that are especially sensitive to this parameter and compared these results with measurements using different Xray spectroscopy techniques." This allows us to determine the correct value and can be used in the computational models that describe the superconductivity.
It is important to have a good understanding of the requirements for searching for better nickelates
This allows us to understand the electronic structure of the material and create a parametric theoretical model to describe superconductivity. Jan Kune says, "With this we can now get down to the bottom of what the mechanics behind the effect can be explained at an electronic level." Which orbitals play a crucial role? What parameters are important? This is what you should know in order to learn how to improve the material so that you can one day produce new nickelates with superconductivity that persists at even higher temperatures.
Further information: Keisuke higashi et., Core-Level Xray Spectroscopy Of Infinite Layer Nickelate: LDA+DMFT Stud, Physical Review X (2021). Information from the Journal: Physical Review X Keisuke higashi et. al. Core-Level X-Ray Spectroscopy Of Infinite-Layer Nitrate: LDA+DMFT Stud, (2021). DOI: 10.1103/PhysRevX.11.041009
