Anomalies in the internal friction and sound velocity in the high-temperature superconductor La/sub 1. 8/Sr/sub 0. 2/CuO/sub 4/

The internal friction Q/sup -1/ and the Young's-modulus sound velocity v/sub E/ have been measured in La/sub 1.8/Sr/sub 0.2/CuO/sub 4/ using the vibrating-reed technique. Measurements were performed at a frequency of 700 Hz at temperatures between 0.2 and 100 K. A drastic change of slope in the internal friction versus temperature curve is observed at T = 44 K just above the superconducting transition temperature T/sub c/ = 40 K. A plateau in Q/sup -1/ is observed between 44 and 5 K. The sound velocity shows a decrease with decreasing temperature between 100 and 20 K and remains almost constant between 2 and 0.2 K.