Realistic calculations for correlated materials

Abstract

Strongly correlated fermionic systems nowadays stand in the forefront of condensed matter physics. A plethora of phenomena, ranging from unconventional superconductivity, gigantic and colossal magneto-resistance and metal-to-insulator transitions, are attributed to the effects of electron correlation. Given the spectacular progress on the experimental side, today - more than ever - the understanding of the underlying microscopic mechanisms, and the explanation or even prediction of experimental observations becomes a necessity. The advancements of theoretical and computational methodologies together with a concurrent increase of computational power, allows for both the ab initio study of realistic materials and the investigation of low-energy effective Hamiltonians inspired and designed to resemble whole classes of compounds. This work is conceptually divided into two major parts. In Chapter 3 and Chapter 4, we present our results, obtained by the state-of-the-art merger of density functional and dynamical mean-field theory, for two realistic systems: the layered LaNiO2/LaGaO3 superstructure, where we focus on the orbital resolved single-particle spectral functions and study the effect of electron and hole doping; and the ruthenate system Ca2RuO4, for which we provide a clear understanding and theoretical support of the experimentally observed semi-metallic state under the application of DC current. The second conceptual part of this work deals with the study of low-energy effective Hamiltonians. In Chapter 4, we investigate a generic t2g model Hamiltonian in the presence of non-spherical crystal-field potentials and/or spin-orbit coupling in order to shed more light on the distinct features that arise on the single-particle level and, most importantly, on the two-particle observables, such as the uniform and static magnetic susceptibilities. In Chapter 5, we investigate the multi-orbital extension of the periodic Anderson model, as inspired by the family of cerium-based heavy-fermion compounds, with a clear focus on the evolution of the dynamic behavior of the systems' moments.