Anomalous behaviour of sound velocity in the high T<SUB>c</SUB> superconductor YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7</SUB>

The longitudinal acoustic wave velocity in YBa2Cu3O7 has been measured in the range 300 to 85 K using the ultrasonic pulse superposition technique. The sound velocity shows a steep drop in its value at 260 K and decreases continuously till the superconducting transition temperature. Below Tc the sound velocity increases steeply. A hysteresis is seen when the experiments are performed during a cooling-warming cycle. The results are compared with recent measurements by Ewertet al on the samples with the same chemical composition