Elastic properties of polycrystalline YBa2Cu3O7−δ: Evidence for granularity induced martensitic behavior

In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa2Cu3O7−δ prepared by the sol-gel method. The quality of all samples was checked by X-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (T < 200 K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensitic-like phase above the superconducting critical temperature Tc. We argue that this peculiar behavior can be attributed to the granularity present in the samples.