M. J. McKenna
Hong Zhang
Bimal K. Sarma
Moises Levy
Dept. of Phys., Univ. of Wisconsin---Milwaukee, Milwaukee WI 53201
T. Kimura
K. Kishio
K. Kitazawa
Univ. of Tokyo, Tokyo, Japan
The temperature dependence of the ultrasonic attenuation and the relative sound velocity were measured in a single crystal of (La[sub 1-x]Sr[sub x])[sub 2]CuO[sub 4] (x=0.075, nominal) with dimensions of about 4x4x4 mm[sup 3], in a temperature range of 4.2 to 50 K. The superconducting transition of this sample measured by ac susceptibility is at 36.6 K. The frequency of longitudinal sound waves propagating along the c axis was extended up to 305 MHz. The attenuation shows two peaks, one at about 27 K and the other at around 36.7 K, while the sound velocity exhibits softenings of about 20 and 80 ppm at the corresponding temperatures, respectively. Magnetic fields of 11 KOe were applied in the a-b plane to study the effects of the field on both the temperatures and the heights of these features in attenuation and velocity. The results, combined with the frequency dependences of these features, suggest that the transition at 36.7 K may possibly be produced by both a structural transition and the superconducting transition. In addition, results on the temperature dependence of the attenuation upon cooling in magnetic fields will be presented. [Work supported by the Office of Naval Research.]