Excitations of a diamagnetic hole state in the copper oxide superconductors

Abstract

The strong infrared (ir) absorption features observed in the copper oxide superconductors are interpreted to be internal electronic and vibrational excitations of a spin-zero charge-transfer complex ($P^{+}$) formed between a hole and an essentially single paramagnetic square planar Cu${\mathrm{O}}_4$ unit ($P^0$). The large magnitude and bimodal frequency dependence of the electronic polarizability implied by the ir data for $P^{+}$ favors a combined excitonic and "charged phonon" mechanism for superconductivity. It is possible that the carriers of the superconductivity are the highly correlated electrons belonging to the $P^0\text{'}\mathrm{s}$, while the $P^{+}\text{'}\mathrm{s}$ play the role of highly polarizable impurities. The $P^{+}\text{'}\mathrm{s}$ can delocalize as band polarons.