Excitonic absorption and superconductivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$

Optical and neutron-diffraction studies of superconducting and nonsuperconducting samples of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{y}}$ are described. Superconducting samples, with y≊0.1, showed two strong electronic transitions, at 3000 ${\mathrm{cm}}^{\mathrm{-}1}$/0.37 eV and at 20 000 ${\mathrm{cm}}^{\mathrm{-}1}$/2.5 eV, which are interpreted as charge-transfer bands (i.e., excitons). Nonsuperconducting samples, with y≊0.8 (prepared by heating of superconducting samples in vacuum to deplete the oxygen content), did not show these features. Our measurements support an excitonic mechanism for the superconductivity in high-$T_c$ copper oxide compounds.