Ab initio study of theCoSi2(110) surface

Abstract

We present results of ab initio calculations for the (110) surface of ${\mathrm{CoSi}}_{2}$. The full-potential linearized augmented plane-wave method, designed for free slabs and bulk systems, was applied to calculate energetical quantities as well as electronic-structure properties. For precise calculations of surface energies we present a computationally useful scheme based on a proper bulk reference. The surface energy of 2.19 J/${\mathrm{m}}^{2}$ for the unrelaxed surface at experimental lattice spacing was reduced to 1.92 J/${\mathrm{m}}^{2}$ at the local-density approximation equilibrium lattice parameter. The surface energies were reduced by 4% and 2%, respectively, when the surface atoms were allowed to relax. Relaxations of atomic positions in the surface and subsurface layers were calculated by force minimization, leading to a small rumpling including a small lateral relaxation of Si atoms. A model is suggested for estimating the critical cleavage stress for ideal brittle fracture by relating elastic and cleavage quantities. We found that the energy for adsorbing a complete monolayer equals the bulk cohesive energy, for which we developed a scheme for deriving the free-atom reference energy by stretching a monolayer. We combined projected bulk bands with the slab band structure, which allows the detection of surface states more clearly than the usual schemes for finite slabs. Some typical surface states are analyzed by presenting their charge density. At about 1.2 eV above the Fermi level, a peak of antibonding Co-Si surface states (originated basically from states around X-bar) arises in the density of states. The local density of states at the surface is enhanced at Fermi energy.