Electronic transitions and correlation effects : From pure elements to complex materials

Sammanfattning: Macroscopic properties of real materials, such as conductivity, magnetic properties, crystal structure parameters, etc. are closely related to or even determined by the configuration of their electrons, characterized by the electronic structure. By changing the conditions, e.g, pressure, temperature, magnetic/ electric field, chemical doping, etc. one can modify the electronic structure of solids and therefore induce a phase transition(s) between different electronic and magnetic states. One famous example is the Mott metal-to-insulator phase transition, in which a material undergoes a significant, often many orders of magnitude, change of conductivity caused by the interplay between itinerancy and localization of the carriers.Electronic topological transitions (ETT) involve changes in the topology of a metal’s Fermi surface. This thesis investigates the effect of such electronic transitions in various materials, ranging from pure elements to complex compounds.To describe the interplay between electronic transitions and properties of real materials, different state-of-the-art computational methods are used. The density functional theory (DFT), as well as the DFT + U method, are used to calculate structural properties. The validity of recently introduced exchange-correlation functionals, such as the strongly constrained and appropriately normed (SCAN) functional, is also assessed for magnetic elements. In order to include dynamical effects of electron interactions we use the DFT + dynamical mean field theory (DFT + DMFT) method.Experiments in hcp-Os have reported peculiarities in the ratio between lattice parameters at high pressure. Previous calculations have suggested these transitions may be related to ETTs and even crossings of core levels at ultra high pressure. In this thesis it is shown that the crossing of core levels is a general feature of heavy transition metals. Experiments have therefore been performed to look for indications of this transition in Ir using X-ray absorption spectroscopy. In NiO, strong repulsion between electrons leads to a Mott insulating state at ambient conditions. It has long been predicted that high pressure will lead to an insulator-to-metal transition. This has been suggested to be accompanied by a loss of magnetic order, and a structural phase transition. In collaboration with experimentalists we look for this transition by investigating the X-ray absorption spectra as well as the magnetic hyperfine field. We find no evidence of a Mott transition up to 280 GPa. In the Mott insulator TiPO4, application of external pressure has been suggested to lead to a spin-Peierls transition at room temperature. We investigate the dimerisation and the magnetic structure of TiPO4 at high pressure. As pressure is increased further, TiPO4 goes through a metal to insulator transition before an eventual crystallographic phase transition. Remarkably, the new high pressure phases are found to be insulators; the Mott insulating state is restored.MAX phases are layered materials that combine metallic and ceramic properties and feature layers of M-metal and X-C or N atoms interconnected by A-group atoms. Magnetic MAXphases with their low dimensional magnetism are promising candidates for applications in e.g., spintronics. The validity of various theoretical approaches are discussed in connection to the magnetic MAX-phase Mn2GaC. Using DFT and DFT + DMFT we consider the high temperature paramagnetic state, and whether the magnetic moments are formed by localized or itinerant electrons.