Hong Zhang
M. J. McKenna
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
Measurements of the ultrasonic attenuation and sound velocity were made at frequencies of 32 and 87 MHz in a large single crystal of (La[sub 0.925]Sr[sub 0.075])[sub 2]CuO[sub 4] (~4x4x4 mm[sup 3]), which exhibits a sharp ((Delta)T~1 K) superconducting transition at 37 K. For longitudinal waves propagating along the c axis, the sound velocity shows a sharp softening, (Delta)v/v~100 ppm, concurrent with the sharp superconducting transition in the susceptibility measurements. Following this transition, the susceptibility exhibits a small tail extending to 27 K, where a smaller softening, (Delta)v/v~20 ppm, has been observed in the velocity measurements. Accompanying the ~100-ppm velocity drop, there is a signature in the attenuation; following this feature, the attenuation changes as the temperature is decreased further, with a small kink in the attenuation at 27 K. Additional results of the dependence of the attenuation and sound velocity in external magnetic fields, orientated in the a-b plane, will be presented. [Work supported by ONR.]