ULTRASONIC INVESTIGATIONS ON THE LATTICE INSTABILITY OF La 2-x Sr x CuO 4-y AND Nd 2-x Ce x CuO 4-y SUPERCONDUCTORS

Ultrasonic measurements were carried out on a series samples of La 2-x Sr x CuO 4-y and Nd 2-x Ce x CuO 4-y by 10 MHz longitudinal and transverse waves in the temperature range between 10 K to 300 K. A striking velocity softening was observed and this softening temperature T 0 decreases almost linearly with the increasing of x. A restiffening of velocity takes place at even lower temperature, implying another possible structural phase transition occurred. The anomalous changes in both attenuation and velocity around 170 and 200 K were found for the superconducting samples of Nd 2-x Ce x CuO 4-y (x=0.15) during cooling, whereas no anomalies can be found in this region for non-superconducting samples. The implication of these phenomenon is discussed.