Relaxation behavior of ultrasonic attenuation in YBa2Cu3O7

It is shown that temperature-dependent ultrasonic attenuation data of YBa2Cu3O7 at various frequencies exhibit anomalies at temperatures close to Tc. These attenuation maxima are found to be the result of a relaxation mechanism added on top of an unusual attenuation background. It is proposed that the grain boundary motions induced by the structural distortion and the propagation of sound waves enhances the energy dissipation around Tc. Whether this structural distortion is the consequence of the onset of a superconducting state remains undetermined. It is also possible that either a tunneling effect or the acoustoelectric effect contributes to sound energy dissipation. The temperature dependence of ultrasonic velocity shows a softening around Tc which may be an intrinsic property of high-temperature superconductors.