Optical measurements on oriented thinYBa2Cu3O7−δfilms: Lack of evidence for excitonic superconductivity

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 \ensuremath{\simeq}0.37 eV reported earlier, are not observed. It therefore seems unlikely that the high-${T}_{c}$ superconductivity in cuprates could arise from exciton-mediated electron pairing.