Acoustic Response of YBa2Cu3Ox Films

Abstract

Since the discovery of high temperature oxide superconductors many different experiments have been conducted to find some clues to the understanding of the basic mechanism. Acoustic attenuation measurement was one of the techniques applied to the conventional metal superconductors to verify the existence of a superconducting gap (Morse and Bohm, 1957). It would be natural to apply the standard acoustic echo technique to the oxide superconductors. However, unlike the conventional metal superconductors, large single crystals of the oxide superconductors suitable for the bulk acoustic measurement is still not available. Therefore most reported acoustic measurements were done using ceramic samples (Horie et al., 1987; Bhattacharya et al., 1988; Xu et al., 1989). Unfortunately, the reported data on ceramic samples showed subtantial variations from samples to samples. It is not surprising because the porosity and the process-dependent intergrain materials of the ceramic sample can cause a large scattering background as well as sample-to-sample variations. Nevertheless several broad peaks of the acoutic attenuation as a function of temperature have been observed more than once by different groups. There have been two reported acoustic measurements on single-crystal YBa2Cu3O7 (Saint-Paul et al., 1988; Shi et al., 1989). Saint Paul et al. used a conventional echo technique with a quartz rod buffer between the transducer and a small single crystal. They reported a gradual reduction of attenuation with decreasing temperature without any broad peaks often seen in ceramic samples. On the other hand, Shi et.al. reported such peaks of a single crystal sample using a vibrating reed technique. None of the experiments on either ceramics or single crystals have indicated a super­conducting gap below the superconducting transition temperature.