RANDOM-FIELD STRUCTURAL TRANSITION IN YBA2CU3O6.5

Synchrotron x-ray diffraction measurements on a high purity single crystal reveal that the absence of long-range ortho-II order in YBa2Cu3 O6+x is most likely due to random-field effects. The ortho-II superstructure peaks show three qualitative features consistent with random-field behavior: First, instead of the development of a sharp Bragg peak at the transition TOII=125(5)°C (for x=0.5), the initial diffuse anisotropic Lorentzian (critical scattering) above TOII transforms to a Lorentzian-squared form below TOII. Second, the equilibration time increases dramatically at the onset of ordering. And third, the domain growth is logarithmic in time below TOII. © 1995 The American Physical Society.