Pairing interactions and pairing mechanism in high-temperature copper oxide superconductors

Abstract

The polaron binding energy ${E}_{p}$ in undoped parent cuprates has been determined to be about $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the unconventional oxygen-isotope effect on the antiferromagnetic ordering temperature. The deduced value of ${E}_{p}$ is in quantitative agreement with that estimated from independent optical data and that estimated theoretically from the measured dielectric constants. The substantial oxygen-isotope effect on the in-plane supercarrier mass observed in optimally doped cuprates suggests that polarons are bound into the Cooper pairs. We also identify the phonon modes that are strongly coupled to conduction electrons from the angle-resolved photoemission spectroscopy, tunneling spectra, and optical data. We consistently show that there is a very strong electron-phonon coupling feature at a phonon energy of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ along the antinodal direction and that this coupling becomes weaker towards the diagonal direction. We further show that high-temperature superconductivity in cuprates is caused by strong electron-phonon coupling, polaronic effect, and significant coupling with $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ charge transfer fluctuation.