Measurement of ultrasonic parameters for high Tc oxide superconductors: evidence of non-pure modes excitation

Ultrasonic pulse propagation in polycrystalline YBa₂Cu ₃O_6+x samples was examined during thermo-cycling from room temperature to well below the critical temperature T_c . Ultrasonic velocity and attenuation were measured with high accuracy using a particular reference signal. An appropriate evaluation of these ultrasonic parameters is strictly correlated to the knowledge of the propagation modes of the ultrasonic waves which are induced in the material. If the ultrasonic propagation direction is not perfectly aligned with the sample symmetry axis, non-pure propagation modes can be excited, and in this case quasi-longitudinal and quasi-shear modes are obtained. These non-pure modes are characterized by different velocity and attenuation values. For this purpose the signal analysis in the frequency domain was demonstrated to be a powerful tool to identify the existence of these non-pure modes. In light of these results, a clearer analysis of velocity and attenuation versus temperature behavior is possible