Ultrasonic study of the elastic softening in superconducting Bi2Sr2CaCu2O8 single crystals above room temperature

The elastic moduli associated with the a‐b plane are obtained by the pulse echo overlap technique from room temperature to 200 °C for Bi2Sr2CaCu2O8 single crystals. The temperature dependence of elastic moduli, especially C′, exhibits an anomalous variation. The change of C′ with decreasing temperature is an overall trend of softening over a wide temperature range, and the maximum amplitude of softening reaches about 24%. This behavior is strikingly different from that of other moduli measured in the experiment. Three obvious softening minima of C′ around 65, 90, and 150 °C reflect the property of lattice instability and a shear‐type phase‐like transition. The relation between the lattice instability and the softening of the elastic mode is discussed.