a Study of the Copper Atom Vibration Anomaly in a High Temperature Superconductor.

Previous ion channeling measurements on superconducting rm YBa_2Cu_3O_{7 -delta} (YBCO) bulk single crystals and neutron resonance absorption spectroscopy (NRAS) measurements on superconducting rm Bi_2Sr_2CaCu _2O_8 bulk single crystals have shown anomalies in the motion of copper atoms in the a-b plane near the critical temperature, T_{ rm c}. Such anomalous phonon behavior may be an indication of strong electron-phonon coupling which could create superconductivity in these materials. However, the ion channeling results were not conclusive: two reports found opposite changes in the copper vibrational amplitude. In this work, a detailed study of vibrations in the a-b plane of YBCO is presented. We used NRAS to determine the vibrational momentum of copper atoms in the a-b plane of large, oriented YBCO crystals. We studied the Doppler broadening of the 230eV resonance in the neutron cross section of ^ {65}Cu atoms in the YBCO sample from 10K to 300K. No abrupt changes for copper vibrations in the a-b plane were observed in either the superconducting rm YBa_2Cu_3O_7 or nonsuperconducting rm YBa_2Cu_3O _{6.2} compounds. This result seems to be in contradiction to the ion channeling measurements. Therefore, we measured the temperature dependence of the ion channeling minimum yield (chi _{rm min}) and channeling angular width (psi_{1/2} ) to monitor the atomic vibrational amplitudes in a 2000A crystalline film of YBCO. The contributions to the backscattering yield from Y, Ba, and Cu are separated in energy so that their vibrations could be studied independently. An ^{16}O(alpha,alpha) ^{16}O resonance was used to monitor oxygen motions. An in situ measurement of the sample resistance was employed to quantify the effect of ion beam-induced crystal damage on the superconducting phase transition. psi_{1/2} was determined in a region 10K above and 10K below rm T _{c}. chi_{min } was also measured from 77K to 106K in 1-2K steps. Again, in direct contradiction to the previous ion channeling measurements, we observed no discontinuities. Based on these two separate measurements, we conclude that no irregularities occur for copper vibrations in the vicinity of T_{rm c}.