Magnetic and Structural Properties of f-electron Systems from First Principles Theory

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: A series of studies of f-electron systems based on density functional theory methods have been performed.  The focus of the studies has been on magnetic and structural properties, as well as investigating ways to handle strong electron correlation in these systems.A version of the self-interaction correction (SIC) method has been developed for a full-potential linear muffin-tin orbital method. The method is demonstrated to have the strong capabilities of previous SIC implementations, to study energetics and phase stabilities of d- and f-electron systems with localisation-delocalisation transitions, but with no geometrical constraints from the underlying band structure method. The method is applied to the high-TC superconductor CeOFeAs, in which the f-shell of the Ce atoms is argued to undergo a Mott transition to a delocalised state under pressure.The non-collinear magnetic structures of two rare earth compounds, TbNi5 and CeRhIn5 have been studied, and in both cases the complex magnetic ordering can be attributed to the effects of Fermi surface nesting.The magnetic properties of the FeMnP0.75Si0.25 system has been studied and found to have an extreme sensitivity to the amount of disorder of the Fe-Mn sublattice.Elements with valence f electrons typically exhibit very complex phase diagrams, with the frequently occurring phenomenon that they melt from a bcc phase that is unstable in calculations based solely on the electronic structure. The high temperature bcc phase of the elements La and Th were studied by means of the self-consistent ab initio lattice dynamics method that accounts for phonon-phonon interaction.Delicate magnetic and structural properties are often sensible to details of how the Brillouin zone (BZ) integration is performed. An improved scheme is proposed that adapts to the BZ mesh and allows for better energy convergence of small energy differences in the smearing type methods.Correlation effects in the 5f-states of plutonium has in recent years been the focus of attention for many theoretical studies employing extensions to DFT schemes. These different schemes have often produced large variations in 5f occupation numbers, and therefore a survey was made of experimental occupation numbers and 4f core level shifts to establish a value for the 5f occupation without any computational bias.