Anisotropic elastic and nonlinear acoustic properties of very dense textured Bi2Sr2CaCu2O8+y

The velocities of longitudinal and shear ultrasonic waves propagated in very dense (95% of theoretical), highly-textured, ceramic Bi2Sr2CaCu2O8+y (Bi2212) have been measured as functions of temperature (between 10 and 290 K) and hydrostatic pressure at 290 K. The main finding is a marked anisotropy of the elastic stiffness constants and their dependences on pressure. The authors have observed an elastic stiffness constant in the a-b plane that is substantially greater than that in the c direction. This observation is consistent with the crystallographic structure of Bi2212. The anomalous elastic effects and thermal hysteresis, observed previously by several groups in various Bi-based superconductors in the range of 19-240 K, have not been found in the present material. These observations suggest that these anomalies are not an inherent property of Bi-based cuprates, but instead may depend on the oxygen content and microstructure of the ceramic material. An analysis of sound velocity in a polycrystalline array of elastically anisotropic crystallites is presented. (author)