Bimal K. Sarma
Dept. of Phys., Univ. of Wisconsin---Milwaukee, Milwaukee, WI 53201
Ultrasonic attenuation and velocity measurements have proved to be a useful probe for the study of conventional superconductors, where the electron--phonon coupling is the dominant pairing mechanism. Since the discovery of high T[sub c] superconductors, ultrasonic measurements have been performed on these systems with moderate success. In the absence of large single crystals, measurements have been performed on isotropic sintered pellets and partially oriented sintered pellets of these high T[sub c] compounds. Low field ultrasonic measurements in the melt textured YBa[sub 2]Cu[sub 3]O[sub 7] have proved to be interesting. With shear waves one can see a coupling of the sound waves to the flux lattice. As the field is increased, there is an abrupt jump in both the sound velocity ((Delta)V) and the attenuation ((Delta)(alpha)) at a field H[sub c1][sup '], which is close to H[sub c1]. The temperature dependence of H[sub c1][sup '] is the same as that of H[sub c1]. (Delta)V is found to be proportional to the square root of H[sub cl][sup '], that is inversely to the average distance between the flux lines. (Delta)(alpha) goes to zero at T[sub c] and at T=0, going through a maximum at ~55 K. Measurements have been done for various orientations of the magnetic field. As no anisotropy is seen in the value of H[sub c1][sup '], it is surmised that H[sub c1][sup '] is the lower critical field for the intergranular matrix. [Work supported by ONR.]