Origins of surface alloy formation: Cu(001)c(2×2)-Pd as a case study

Abstract
We present a rigorous theoretical treatment which, in agreement with recent experiments, supports the formation of the ordered c(2×2) phase of the CuPd surface alloy on the Cu(001) substrate, and does not support the formation of an ordered overlayer of only Pd atoms on such a substrate. Based on ab initio electronic structure of random CuPd and Pd-vacancy overlayers, we study their stability against ordering using the generalized perturbation method adapted to strongly inhomogeneous systems. The approach, the first of its kind, is general and can be applied to other metal-on-metal overlayers.