CHANNELING OF 7.6 MEV HE IN YBCO

Channeling dips for 7.6 MeV He ions in YBa2CU3O7-x have been measured for all constituents to investigate possible anomalies in thermal vibr ations near the transition to superconductivity. Quantitative agreemen t with simulations is obtained for all the dips with a single set of s tructural parameters, Angular scans for crystal temperatures in the ra nge 30-220 K confirm the presence of an anomaly at T-c for the width o f CuYBa backscattering dips, which can be interpreted as a freeze-out of Cu vibrations at the transition to superconductivity. A detailed te mperature scan reveals for the first time that the change in width tak es place over a temperature interval of similar to 15 K below the tran sition temperature, in agreement with recent EXAFS measurements of rel ative Y-Cu vibrations. Our data for oxygen are not conclusive but may indicate a similar small anomaly. (C) 1998 Elsevier Science B.V.