Optical measurements on oriented thin YBa2Cu3O7δ films: Lack of evidence for excitonic superconductivity

Abstract
Optical transmission and reflection spectra (mid IR through uv) and Raman spectra of superior-quality 90-, 180-, 400-, and 1000-nm-thick superconducting Y-Ba-Cu-O films are reported. Characteristic excitonic bands, and in particular the absorption band at ≃0.37 eV reported earlier, are not observed. It therefore seems unlikely that the high-Tc superconductivity in cuprates could arise from exciton-mediated electron pairing.