AB-INITIO THEORY OF SURFACE SEGREGATION - SELF-CONSISTENT DETERMINATION OF THE CONCENTRATION PROFILE

The parameters of the effective Ising Hamiltonian governing segregatio n and ordering phenomena in the surface region of an alloy are determi ned from first principles, Employing the force theorem. the total ener gy of semi-infinite disordered alloys is mapped onto an Ising Hamilton ian. The band term is treated within the generalized perturbation meth od, and, in addition, the contributions to the on-site terms of the Is ing 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 Hamilt onian parameters are then recalculated for the profile, and the whole procedure is repeated until self-consistency between the electronic an d atomic structures of the alloy surface is achieved. As an illustrati on, the results for an fcc(001) surface of the Cu-Ni alloys as calcula ted within the all-electron fully relativistic tight-binding linear-mu ffin-tin-orbital coherent potential approximation method are discussed .