Shear-modulus change below T/sub c/ in YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/

The ultrasonic transverse-wave velocity in the high-T/sub c/ metal--oxide superconductor YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ between 295 and 4 K was measured. Above T/sub c/ the velocity shows normal behavior: during cooling, it displays a monotonic increase with decreasing slope. During cooling below T/sub c/ the shear modulus departs from normal behavior, increasing as ..delta..G/G = b(1-(T/T/sub c/)/sup n/), where n = 3. These results depart dramatically from those expected for a simple second-order normal-superconducting phase transition.