Ultrasonic properties of oriented ceramic high T_c superconductors

The results of both attenuation and velocity measurements on sinter-forged polycrystalline YBa₂Cu₃O_7-δ samples are discussed. Optical micrograph and X-ray diffraction measurements indicate that these sinter-forged samples have more than 80% of the crystallites aligned with the c axis parallel to the forging axis. Attenuation measurements of longitudinal waves perpendicular to the forging axis exhibit three relative maxima as a function of temperature at 70 K, 180 K, and 250 K, whereas longitudinal waves parallel to the forging axis only exhibit one relative maximum at 180 K. These measurements imply that the high- and low-temperature maxima are associated with disturbances in Cu-O planes of these high T_c superconductors, and therefore analysis of these maxima may yield information about interaction in these planes. The velocity data show lattice softening at temperatures much higher than T_c as the sample is cooled. This softening is arrested below T_c. A strong hysteresis is observed in the temperature dependence of these velocity curves.