In 1986, the discovery of superconductivity in copper-based ceramic materials — cuprates — overturned physicists' ideas. The transition temperature to the superconducting state suddenly rose several times higher than the previous limit, and the familiar BCS theory, which explained the phenomenon by lattice vibrations, stopped working.
An international team led by Professor Nigel Hussey of Radboud University and HFML-FELIX, together with colleagues from China, the United Kingdom and the United States, has finally found the key mechanism. The researchers showed that in electron-doped cuprates such as lanthanum-cerium-copper oxide, electrons form pairs precisely through interaction with spin fluctuations — rapid oscillations of the tiny magnetic moments of atoms.
To prove this, the scientists created a special thin film with a smooth gradient of critical temperature along its length. Lithographic contacts made it possible to measure resistance at different values of the transition temperature while maintaining the same level of disorder. Analysis of the data in strong magnetic fields revealed that the main source of resistance is the scattering of electrons on fluctuating spins, and the strength of this interaction correlates directly with the superconducting temperature.
The analogy is simple: imagine a stream of electrons as a crowd of people in the subway. Spin fluctuations are sudden "nudges" from neighbours, which in an ordinary metal hinder movement, but in cuprates help electrons "pair up" and move without losses. It is this connection, and not ion vibrations, that turned out to be decisive.
However, the work also revealed a second, as yet unknown "ingredient". Even by strengthening the interaction with spins, it is impossible to reach the record temperatures observed in other families of cuprates. The search for the missing factor now becomes the main task for theorists.
The results are published in Nature Physics and bring closer the moment when high-temperature superconductors will be able to operate at temperatures accessible without expensive cooling. This will open the way to ultra-efficient power lines, powerful magnets for medicine and the quantum technologies of the future.
