Calculation of electronic and magnetic properties of transition-metal surfaces: Comparison of LMTO and tight-binding methods.

Abstract

Low-dimensional systems have attracted a great deal of attention during the past decade. The lowered symmetry and coordination number give rise to new and interesting electronic and magnetic phenomena. Several ab initio numerical methods have been developed to calculate the electronic and magnetic structure of materials, but these calculations require a great deal of computational effort. In order to study complex systems it is of interest to be able to make use of simpler approximative methods. For transition metals such an alternative approach is provided by the tight-binding approximation. Within the conventional parametrized tight-binding approach the lowered dimensionality of surfaces gives rise to an unphysical filling of the surface d orbitals (sp-d charge transfer), which in turn gives rise to lowered surface magnetizations. Results obtained by applying the linear muffin-tin orbital (LMTO) method to a repeated sequence of slabs and empty spheres show the existence of a spillover coming from the s and essentially p surface orbitals, with the d-band occupation remaining nearly the same as in the bulk materials. We suggest in this work a simple way of parametrizing the tight-binding Hamiltonian in such a way that the characteristics observed in LMTO calculations are preserved and the simplicity of a tight-binding approach remains valid. This is obtained by only adding a new layer of orbitals on the surface in order to simulate the spillover. We compare in this contribution results for Rh, Fe, and Cu (001) monolayers and five-layer slabs obtained using LMTO and an unrestricted Hubbard Hartree-Fock Hamiltonian with the surface parametrization.