Bulk modulus anomalies at the superconducting transition of single phase YBa sub 2 Cu sub 3 O sub 7.

We report sound velocity measurements on polycrystalline single phase YBa sub 2 Cu sub 3 O sub 7 with a T sub c of 90K. The sound velocity is found to increase as the temperature is reduced from room temperature in contrast to the decrease observed in La sub (1.85) Sr sub (.15) Cu O sub 4. At T sub c there is no discontinuity in the sound velocity within experimental resolution, as expected from thermodynamic arguments and the small value of dT sub c/dP. As the temperature is lowered below T sub c the sound velocity increases at a greater rate. A similar increase below T sub c is observed in La sub (1.85) Sr sub (.15) Cu O sub 4. The magnitude of the sound velocity increase below T sub c is much larger than observed in conventional superconductors.