Ultrasonic Velocity Measurement for Superconducting Material Versus Cryogenic Temperatures

Abstract

Ultrasonic pulse propagation in polycrystalline YBa2Cu3O6+x samples was examined during thermocycling from room temperature to well below the critical temperature Tc. An accurate measurement of ultrasonic velocity 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. In this case quasi-longitudinal and quasi-shear modes are obtained with different velocity values and consequently different echo pulses are generated which can appear overlapped. In the time domain this overlap appears as a perturbation of the signal shape which is difficult to quantify. In this paper the signal spectral analysis performed on the ultrasonic echoes, at various temperatures, is demonstrated to be a powerful tool to clarify and to detect this occurrence, which would have been impossible to explain by a simple analysis in the time domain. For some samples, spectral analysis has highlighted the presence of two partially overlapped echo pulses. This overlap begins at about 170 K and is observed up to Tc. The presence of these two pulses leads to the interesting hypothesis that non-pure modes can be excited in particular conditions, giving rise to peculiar effects. Phase transitions and changes in the lattice anharmonicity can be ascribed to possible causes which affect the propagation mode as temperature varies. If the sample structural organization changes versus temperature an out-of-axis propagation can occur inducing non-pure modes. Following this approach, derived from the acousto-elasticity theory, the velocity results obtained in previous works are also revised1,2. The velocity curves versus temperature confirm the presence of two hysteresis loops during thermocycling for all the investigated samples. This result is interesting and new with respect to data generally reported by other groups. Moreover different hysteresis loops are obtained not only for samples with different structural characteristics, but also for nominally equal samples. This discrepancy could be attributed to different orientation between the crystallographic axes and the geometrical sample axes induced by the manufacturing process.KeywordsUltrasonic VelocityLattice AnharmonicityUltrasonic Velocity MeasurementUltrasonic EchoOxide High Temperature SuperconductorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.