We studied several different materials systems with the focus on the appearance of defects. Thereby, alloys, oxides and surfaces were considered from the theoretical point of view using ab-initio methods. The main results were achieved with the Korringa-Kohn-Rostoker Green’s function method (KKR), which is based on the multiple scattering theory and the density functional theory. This combination offers advantages compared to other ab-initio techniques when dealing with disordered systems or magnetic properties. For example, the coherent potential approximation describes an effective medium, which approximates randomly distributed atoms within the material and was recently extended to include also nonlocal effects like short-range ordering. On the other hand, the magnetic force theorem offers a fast and effective method for the calculation of the magnetic exchange interactions.

With these tools, we studied for example disorder in metallic alloys ranging from periodic long-range order over short-range order and totally disorder to segregation of the alloy constituents. We showed for AgPd alloys that the short-range order alters strongly the electronic structure [1]. The latter might be then an indicator for short-range order in experimental investigations. Defects play also a crucial role in oxide systems. The influence of oxygen vacancies and antisite disorder on the electronic and magnetic properties was studied for SrCoO₃[2] and Sr₂FeMoO₆[3]. We obtained good agreement with experimental observations. Besides, we calculated the crystal field parameters for single holmium atoms on a Pt surface from ab-initio and used exact diagonalization in order to resolve the splitting of the energy levels. The aim was to understand possible excitations and the high magnetic stability of the holmium atoms observed in surface tunneling experiments[4].
References:
[1] Hoffmann et al J. Phys.: Condens. Matter 28, 305501 (2016)
[2] Hoffmann et al Phys. Rev. B 92, 094427 (2015)
[3] Saloaro et al ACS Appl. Mater. Interfaces 8, 20440 (2016)
[4] Miyamachi et al Nature 503, 242 (2013)