Long-range order near the Cu3Au(001) surface by evanescent x-ray scattering.

Abstract

We present an experimental study of the surface effects on the order-disorder phase transition in the binary alloy ${\mathrm{Cu}}_{3}$Au. By x-ray scattering under the condition of total external reflection, we obtained depth profiles of the order parameter at the (001) surface when the transition temperature ${\mathit{T}}_{0}$ is approached. The scattered intensity is analyzed in the framework of the distorted-wave Born approximation, which is briefly outlined. The temperature and depth dependence of the evanescent (100) superlattice intensity is consistent with a wetting transition, which is driven by the first-order bulk transition: Close to ${\mathit{T}}_{0}$ the disordered phase appears at the (001) surface and grows according to L(t)=(13.1 \AA{})ln\ensuremath{\Vert}1/t\ensuremath{\Vert} with reduced temperature t. During these measurements the near-surface Debye-Waller factor was monitored via the (200) evanescent Bragg intensity, which, however, showed no significant surface effects at ${\mathit{T}}_{0}$. In several temperature quench experiments, where the system was rapidly cooled from T>${\mathit{T}}_{0}$ to ${\mathit{T}}_{0}$-\ensuremath{\Delta}T, we measured time- and depth-resolved evanescent superlattice intensities. We present depth-resolved near-surface relaxation times that exhibit a very distinct depth dependence. A first quantitative analysis of the relaxation processes is indicated and gives results consistent with the assumption of an ordinary ``surface-induced disorder.''