Effect of oxygen disorder on superconductivity-induced self-energy effects in impurity-free YBa₂Cu₃O_7-δ

We investigated various areas of impurity-free YBa₂Cu₃O_7-δ single crystals with Raman spectroscopy. A definite correlation between the appearance of the group- theoretically forbidden chain-related Raman lines at 232 and 598 cm^-1 and the superconductivity-induced self-energy effects observed for the 340 cm^-1 B_1g-like mode was established: Disorder in the chains in pure crystals has a similar effect as the introduction of impurity atoms, both leading to an increased gap value for 2Δ/kT_c as deduced from the phonon self-energy effects. The absence of the forbidden chain modes in the Raman spectra may be used as an extremely sensitive quality criterion of impurity-free crystals.