A New Technique Predicts High-Temperature Superconductors
King's College London
The breakthrough could bring the search for room-temperature superconductors one step closer.
King's College London physicists and partners have developed a novel approach that explains why a specific class of superconductors can function at high temperatures. By focusing on cerium superhydride (CeH9), the team identified the missing ingredient behind its superconductivity, revealing that it can operate at a temperature twice as high as previously predicted.
The researchers believe that by incorporating this discovery into a more comprehensive theory, they have laid the foundation for a computational search for room-temperature superconductors. They selected cerium superhydride (CeH9) as it was proven superconductive in a 2019 experiment under lower pressures than any other superhydride, yet state-of-the-art theory struggled to explain it.
Superconductors, which operate at zero resistance, can power electric technologies with minimal energy loss from heat caused by resistance, making them ideal for addressing the world's growing energy demands. While they are currently used in technologies like MRI scanners, the vast majority require extremely low temperatures (-196C) to function, making them expensive and impractical for widespread use.
Physicists have been searching for decades for room-temperature superconductors. Hydrogen-rich compounds, particularly LaH10 (the decahydride of Lanthanum, a rare-earth metal), have been experimentally proven to function at the highest temperature on record, approaching 250 Kelvin (-23C). However, LaH10 only functions under extreme pressures, comparable to those found at the Earth's core, making it highly impractical.
The King's team, in collaboration with researchers from the University of Cambridge, Vienna University of Technology, and the Université catholique de Louvain, aimed to find a new, better theory. Dr. Yao Wei, a former PhD researcher at King's, stated, 'We picked one of the most challenging compounds in the hydride class - cerium superhydride (CeH9). Its superconductivity was proven in a 2019 experiment for lower pressures than in any other superhydride, but state-of-the-art theory failed miserably to describe it.'
The team discovered that, alongside phonon-electron interactions, electron-electron interactions or electron scattering actually held the key to the better-than-expected superconductivity. Understanding CeH9 is challenging due to its substantial electronic complexity, with over sixty electrons per formula unit, including very heavy electrons.
Dr. Jan Tomczak, a Senior Lecturer in Physics, explains, 'Contrary to electrons in copper, Ce-borne electrons hesitate to move through the solid because the repulsion between them is especially large. Imagine electrons in copper acting like water; the electrons in Ce are more like viscous honey, with scattering or friction slowing them down.'
Dr. Siyu Chen, a former PhD student at the University of Cambridge, added, 'While superconductivity mediated by phonons has been understood since 1957, what is special here is that we finally understand how beneficial electron scattering is to superconductivity.'
In essence, this scattering reduces the electrons' energy, and the more negatively charged, low-energy electrons, the more the nuclei's positive charges are shielded, causing nuclei to repel each other less. Samuel Poncé, a Professor at the Université catholique de Louvain, said, 'The atomic lattice of the crystal can be compared to an array of masses connected with springs; these springs have now become softer, facilitating vibrations. Low-energy electrons and soft phonons are the key ingredients to superconductivity, and we just got more of both!'
When the team considered the effect of electronic scattering on both electrons and phonons, the 50% discrepancy between experimental findings and previous theoretical modeling was essentially eliminated. The new theory reproduces the transition temperature within 1%.
Dr. Yao Wei concluded, 'Our work establishes a versatile and predictive computational tool that could speed up the exploration and discovery of promising phonon-mediated superconductors functioning at high temperatures and lower pressures.'
The team believes this framework is transferable to many other systems and could predict phonon-based superconductors at even higher temperatures. It could also be used to explore different crystal structures, potentially reducing the high pressures required for these superconductors. Additionally, they believe their approach could advance the role of machine learning in finding superconducting materials, as Dr. Tomczak suggested.
Dr. Yao Wei added, 'While experimental observation remains the definitive test of superconductivity, there is too much chemical and structural freedom to synthesize all possible materials and check them for superconductivity in the lab. Our work establishes a versatile and predictive computational tool that could speed up the exploration and discovery of promising phonon-mediated superconductors functioning at high temperatures and lower pressures.'
