With use of low-energy ${\mathrm{Ne}}^{+}$ scattering and low-energy electron diffraction both long-range order and Au segregation have been found at the ${\mathrm{Cu}}_3$Au(001) surface. The Au concentrations in the first and second layers are essentially constant at 0.5 and 0, respectively, for $T<~400°$C, beyond which they approach each other. Calculations of shadowing for the ordered surface agree with experiment, e.g., for shadowing of Cu atoms in the second and third layers by Au atoms in the first layer.

• Received 7 December 1981

DOI:https://doi.org/10.1103/PhysRevLett.51.43