Photoemission spectra of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ in the normal and superconducting states provide direct evidence for dimerization of oxygen below $T_c$. ${\mathrm{Cu}}^{2+}$ is found to reduce to ${\mathrm{Cu}}^{1+}$ concomitantly. These changes may be of vital importance to the mechanism of high-temperature superconductivity.

• Received 1 May 1987

DOI:https://doi.org/10.1103/PhysRevB.36.2371