Direct evidence of anomalous Cu-O vibrational modes near ${\mathrm{T}}_{\mathrm{c}}$ in ${\mathrm{ErBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Ion channeling along the c axis in ${\mathrm{ErBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ reveals a normal (Debye-type) temperature dependence for thermal vibrations of the Er and Ba atoms between 40 and 300 K. In contrast, a substantially stronger than normal temperature dependence is found for the Cu-O atom vibrations, and a large, anomalous change (∼30%) is seen at the superconducting transition. The large magnitude of the vibrational changes found for the Cu-O rows across ${\mathrm{T}}_{\mathrm{c}}$ suggests that the atoms in these rows become strongly coupled in the superconducting state.