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Electronic structure and magnetic properties of strongly correlated transition metal materials

Final Activity Report Summary - MAGNET (Electronic structure and magnetic properties of strongly correlated transition metal materials)

Materials with strong electron correlations form a remarkable class of systems, with distinctive electronic properties, such as metal-insulator transitions or high-temperature superconductivity. Numerous examples of current interest include transition-metal oxides and rare-earth compounds. Conventional electronic structure methods usually fail to work properly for these materials. The present project aimed at the development and application of a powerful method, called the 'LDA+DMFT', which combined ab-initio band structure calculations with the dynamical mean-field theory, a many-body methodology to handle strong electron correlations. The target materials included several transition metal oxides, in particular vanadium dioxide and vanadium sesquioxide. The key interplay of structural aspects and electron correlations was revealed in both materials and a consistent description of their electronic structure was achieved during this project.

The recently developed LDA+DMFT approach, which was a combination of the dynamical mean field theory and the local density approximation was a powerful and promising tool in order to study from first principles the magnetic and electronic properties of strongly correlated materials. DMFT included the local aspects of electronic correlations in a fully dynamical manner. It was based on a mapping onto a single-site quantum impurity model supplemented by a self-consistency condition. The continuous time quantum Monte-Carlo was used for a solution of the quantum impurity model. This allowed describing the physics of the realistic compounds at experimental temperatures. The LDA+DMFT method was successfully applied to the investigation of the electronic and magnetic properties of the titanium based perovskites, LaTiO3 and YTiO3, and vanadium oxides, V2O3 and VO2.

We found that for V2O3 the transition was driven by a correlation-induced enhancement of the crystal-field splitting within the t2g manifold, which resulted in a suppression of the hybridisation between the a1g and eg bands. The results were in good agreement with experimental findings and had predictive power. We also investigated a quarter-filled two-band Hubbard model involving a crystal-field splitting. The nature of the Mott metal-insulator transition was found to depend on the magnitude of the crystal-field splitting. At large values, a transition from a two-band to a one-band metal was first observed as the on-site repulsion was increased, followed by a Mott transition for the remaining band. At small values of the crystal-field splitting, a direct transition from a two-band metal to a Mott insulator with partial orbital polarisation was found, taking place simultaneously for both orbitals.