Evidence for Large Static and Dynamic Distortions in High $T_c$ Superconducting ${\mathrm{YBa}}_2{{\mathrm{Cu}}_{3}O}_{7-\mathit{\delta }}$ Crystals over a Wide Temperature Range

Large deviations from the expected atomic thermal vibrational amplitude in Cu-O rows in a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ $(T_c=92.5\mathrm{}\mathrm{K})$ crystal have been observed in a wide temperature range (300–30 K) by ion channeling. In contrast, Y-Ba as well as Cu-O rows in nonsuperconducting deoxygenated YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_{6.2}$ specimens show a normal decrease in the atomic vibrational amplitude as the sample temperature is lowered. These anomalous results in the YBCO superconductor provide a clear indication of uncorrelated static and dynamic distortions associated with Cu-O rows. Possible implications of these structural changes are discussed.