Research: New View on Unconventional Superconductivity
The mechanism that leads to unconventional superconductivity in copper oxide based and iron based superconductors has been much disputed. Uppsala physicists now show that conventional lattice vibrations may well give rise to unconventional superconductivity, which was not previously thought.
Superconductivity in a material arises when two electrons which normally repel each other form a Cooper pair that moves freely through the material without loss of energy. In most materials it is the lattice vibrations, which also are called phonons, which cause the Cooper pairs to form. The superconducting state is furthermore characterised by a superconducting gap that is formed in the electrons’ energy band. In many superconductors, the superconducting gap is independent of the electrons’ wave vector and thereby isotropic and has the same sign and magnitude in the so called reciprocal space.
But there are also unusual, important superconductors which do not fit in the classical picture though. This foremost applies to high temperature copper oxide and iron based superconductors which have a superconducting gap that is not isotropic but changes sign as a function of the wave vector. These superconductors are therefore called unconventional. It has long been a mystery what mechanism binds the Cooper pairs in an unconventional superconductor. Because it was thought that lattice vibrations could not give rise to an unconventional gap symmetry it has become common to attribute unconventional superconductivity to electronic mechanisms, above all spin fluctuations which may explain the unusual gap symmetry.
Through detailed calculations, the Uppsala physicists Fabian Schrodi, Peter Oppeneer and Alex Aperis have now shown that conventional, isotropic lattice vibrations very well can explain unconventional superconductivity in cuprates, iron based and strongly correlated superconductors. This they have done by developing a theory of superconductivity including the next order Feynman diagrams for electron-phonon interaction. The theory is based on Eliashberg’s theory of superconductivity but goes one step further. The Uppsala researchers’ careful calculations, surprisingly enough, led to the discovery of the unusual gap symmetry which earlier had been observed in experiments, which means that the lattice vibrations actually may be the cause of unconventional and high temperature superconductivity.
“It was astonishing that our calculations for specific materials yielded exactly the type of unconventional superconductivity known from the experiments. This opens up to come a step closer to solving the issue what the mechanism behind unconventional superconductivity really is,” says Fabian Schrodi, who recently defended his PhD theses at the Department of Physics and Astronomy.
F. Schrodi, P. M. Oppeneer, and A. Aperis, Unconventional superconductivity mediated solely by isotropic electron-phonon interaction. Physical Review B – Letter 104, L140506 (2021). Publication date: October 28, 2021
English translation: Johan Wall