Theoretical modelling of nano-scaled systems with heavy ions

Abstract

Systems on the nanoscale are a hot topic in science and technology as they may be applied in future concepts for microprocessors, memory storage and sensors. This PhD thesis covers two nano-scaled systems which promise particularly advantageous properties: rare-earth silicide nanowires on silicon surfaces, a quasi-one-dimensional electronic system, and thin antimony layers on bismuth selenide, a two-dimensional system with topologically protected surface states. These systems have in common that their fascinating properties are due to heavy ions incorporated in the structures. This thesis theoretically investigates them by means of density functional theory (DFT). However, DFT encounters problems when describing such materials since the high atomic numbers of the involved elements give rise to exotic phenomena, e.g. strongly correlated electronic subshells (the incomplete 4f shell of the lanthanoids), strong relativistic effects (topologically non-trivial insulators) and high contributions to the electronic long-range correlation (“van der Waals interactions”). These problems are solved by approaches beyond DFT, including LDA+U, spin-orbit coupling and dispersion corrections. Both systems investigated in this work have higher-dimensional prototype structures, which are explored at first and then scaled down to the final nanostructures. Different structure models are set up and optimised regarding the ionic positions. Their stability is evaluated by means of ab initio thermodynamics and phase diagrams are derived. For the most stable structure models, the electronic properties are calculated, including band structures, Fermi surfaces and simulated scanning tunnelling microscopy. All theoretical findings on the structural and electronic properties are carefully compared with experimental reference. In this way, a conclusive ab initio framework is established for these systems, which permits a deep understanding of the underlying physics. Furthermore, novel and fascinating phenomena are identified. The rare-earth silicide nanowires on Si(557) show a unique dimensional crossover, which gives rise to quasi-one-dimensional, metallic edge states. The thin antimony layers are proven to underlie a complex interplay with the topologically protected surfaces states of the bismuth selenide surface, which involves an intricate series of topological phase transitions.