Ab initio theory of surface segregation: Self-consistent determination of the concentration profile

The parameters of the effective Ising Hamiltonian governing segregation and ordering phenomena in the surface region of an alloy are determined from first principles, Employing the force theorem. the total energy of semi-infinite disordered alloys is mapped onto an Ising Hamiltonian. The band term is treated within the generalized perturbation method, and, in addition, the contributions to the on-site terms of the Ising Hamiltonian from core stales, and the double-counting and Madelung terms are included. The concentration profile is determined by using Monte Carlo simulations. The electronic structure and the Ising Hamiltonian parameters are then recalculated for the profile, and the whole procedure is repeated until self-consistency between the electronic and atomic structures of the alloy surface is achieved. As an illustration, the results for an fcc(001) surface of the Cu-Ni alloys as calculated within the all-electron fully relativistic tight-binding linear-muffin-tin-orbital coherent potential approximation method are discussed.