Describing surfaces: Semi-infinite versus thin film approaches

In theoretical studies of the electronic structure and magnetic properties of solids with a surface thin film concepts as well as approaches based on semi-infinite geometries are used leading very often to quite similar results, but also sometimes substantial disagreement can be found in the literature. Furthermore, since usually different computational schemes are applied a direct comparison between the two basic concepts seems to be out of reach. By discussing the boundary conditions inherent to these two concepts and by making use of a model that combines on equal footing both aspects the main similarities but also differences can be pointed out. This model is applied to free and magnetically coated surfaces of fcc Cu100 and fcc Pt111 as well as to a free surface of bcc Fe100. It is shown that local quantities such as surface spin and orbital moments can be determined equally well using either of the two concepts while the very details of the corresponding Friedel oscillations generally are much less compatible. In particular, for the magnetic anisotropy energy of magnetic overlayers on highly polarizable nonmagnetic substrates or of free surfaces of magnetic solids the conceptual differences become apparent.