Sound velocity and attenuation in single-crystal YBa2Cu3O7-

We have used a modified vibrating reed technique to measure the temperature dependence of the sound velocity and attenuation for modes corresponding to the Young's modulus and c44 shear modulus of single crystals of YBa2Cu3O7-. The Young's sound velocity shows a tremendous softening during the superconducting transitiona smeared discontinuity of as high as 190 ppm. Below the transition, the temperature dependences of both sound velocities harden considerably. The discontinuities of the sound velocities and their temperature derivatives at Tc are related to the specific-heat jump at the superconducting transition.