Order-Disorder Transformation at the {100} Surface ofCu3Au

The long-range order parameter of the {100} surface of ${\mathrm{Cu}}_3$Au has been determined as a function of temperature by low-energy electron diffraction. The order parameter of the surface, unlike the bulk order parameters, appears to be a continuous function of temperature, and the disordering process at the surface begins about 60°K below a common critical temperature. A Monte Carlo calculation of the order parameter in terms of a model incorporating nearest- and next-nearest-neighbor interactions reproduces the experimental results.