ULTRASONIC-ATTENUATION AND SOUND-VELOCITY CHANGES IN A SUPERCONDUCTING LA2-XSRXCUO4 SINGLE-CRYSTAL

The ultrasonic attenuation and relative sound velocity were measured a t frequencies up to 305 MHz in the superconducting state of a La1-xSrx CuO4 single crystal. For longitudinal sound waves propagating along th e c-axis, the attenuation increases with increasing temperature with a small peak at 27 K and a large peak at about 37 K. These two peaks, e specially the small one, become more obvious at higher frequencies. Si multaneously, the velocity exhibits a jump of about 100 ppm around the superconducting transition. At about 27 K, there is another discontin uity in the sound velocity of about 20 ppm, which corresponds to the s mall peak in the attenuation measurement.