Polaron formation as origin of unconventional isotope effects in cuprate superconductors

TL;DR

This paper investigates the unconventional isotope effects in high-temperature cuprate superconductors, attributing them to polaron formation and electron-phonon coupling, which influence key superconducting properties.

Contribution

It demonstrates that polaron theory explains the isotope effects on Tc, penetration depth, and the superconducting gap in cuprates, highlighting the role of Jahn-Teller mode coupling.

Findings

Isotope effects on Tc and penetration depth are explained by exponential squeezing of hopping integrals.

Superconducting gap isotope effect is comparable to that on penetration depth.

Electron coupling to Jahn-Teller Q2 mode is identified as the origin of isotope effects.

Abstract

Various unconventional isotope effects have been reported in high-temperature superconducting copper oxides which are beyond the scheme of BCS theory. Their origin is investigated within polaron theory which leads to a renormalization of the single particle energies and introduces a level shift here. It is found that the exponential squeezing of the second nearest neighbour hopping integral carries the correct isotope effect on the superconducting transition temperature Tc, as well as the one on the penetration depth. The average superconducting gap is predicted to have an isotope effect comparable to the one on the penetration depth. The results imply that the coupling of the electronic degrees of freedom to the Jahn-Teller Q2-type mode is the origin of these isotope effects.