The ${\mathrm{Cu}}_3$Au(100) surface has been studied close to the bulk order-disorder transition temperature $T_0$ by x-ray diffraction under total external reflection. The order-parameter profile which has been probed in various depths is analyzed in terms of a surface-induced disorder transition. It is demonstrated that the temperature dependence of the superlattice intensity is consistent with the assumption of a disordered surface layer which exists between the ordered bulk and the vacuum at temperatures below $T_0$ and grows logarithmically as $T_0$ is approached.

• Received 23 December 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.2382