Interplay of spin-orbital correlations and structural distortions in Ru- and Cr- based perovskite systems

Abstract

The transition metal oxides are emerging as the natural playground where the intriguing effects induced by electron correlations can be addressed. Since the s electron of the transition metals are transferred to the oxygen ions, the remaining electrons near the Fermi level have strongly correlated d character and are responsible for the physical properties of the transition metal oxides. These electron correlations, together with dimensionality and relativistic effect, play a crucial role in the formation and the competition of different electronic, magnetic and structural phases, giving rise to a rich phase diagram: Mott insulators, charge, spin and orbital orderings, metal-insulator transitions, multiferroics and superconductivity. The investigation of correlated electron physics usually refers to 3d transition metal oxides, mainly because of high-temperature superconductivity in the cuprates and in the iron-pnictides, and colossal magneto-resistance in manganites, but also because the highly extended 4d-shells would a priori suggest a weaker ratio between the intra-atomic Coulomb interaction and the electron bandwidth. Nevertheless, the extension of the 4d-shells also points towards a strong coupling between the 4d-orbitals and the neighbouring oxygen orbitals, implying that these transition metal oxides have the tendency to form distorted structure with respect to the ideal one. As a consequence, the change in the Metal-Oxygen-Metal bond angle often leads to a narrowing of the d-bandwidth, bringing the system on the verge of a metal-insulator transition or into an insulating state. Hence, 4d materials share common features with 3d systems having additionally a significant sensitivity of the electronic states to the lattice structure, effective dimensionality and, most importantly, to relativistic effects due to stronger spin-orbit coupling. The main purpose of this thesis is a study of the mechanisms and the fundamental interactions that control the formation and the competition of different magnetic and structural phase driven by the electronic correlations, dimensionality and relativistic effects in Ru-, Cr- and Mn- based perovskite systems, also considering what happens in hybrid or eutectic structures. The mean field theory of itinerant uniform ferro/ metamagnetism and its consequences are introduced. We present two analytically solvable models: the M6 Landau theory and the full analytical solution of one-dimensional tight binding density of state. We compute the analytical thermodynamic functional, the phase diagram, the quantum critical endpoint and the critical magnetic field. Necessary and sufficient conditions to have itinerant metamagnetism are examined. We analyse the interface Sr2RuO4-Sr3Ru2O7. We study the modification of the electronic structure induced by nanometric inclusions of Sr2RuO4 embedded as c-axis stacking fault in Sr3Ru2O7 and viceversa. The change of the density of states near the Fermi level is investigated as a function of the electron density, the strength of the charge transfer at the interfaces between the inclusion and the host, and of the distance from the inclusion. Then, we examine how the tendency towards long range orders is affected by the presence of the nanometric inclusions. This is done by looking at the basic criteria for broken symmetry states such as superconductivity, ferromagnetism and metamagnetism. We show that, according to the strength of the charge transfer coupling, the ordered phases may be enhanced or hindered, as a consequence of the interplay between the host and the inclusion, and we clarify the role played by the orbital degree of freedom showing an orbital selective behaviour within the t2g bands. A discussion on the connections between the theoretical outcome and the experimental observations is also presented. We study the effect of electronic correlation at interface Sr2RuO4-Sr3Ru2O7. We study in detail the role of the electronic correlation in systems based on nanometric inclusions of Sr2RuO4 embedded as c-axis stacking fault in Sr3Ru2O7 and viceversa. The metamagnetic properties in mean field theory approach using the realistic density of state are analyzed. We study the analysis of the electronic reconstruction at the interface Sr2RuO4-Sr3Ru2O7. We study the fermiology of Sr2RuO4 and Sr3Ru2O7 from first principles: comparison, main features and calculation of effective hopping Ru-Ru are performed. Effect of the octahedral rotation and dimensionality are analyzed studying ab-initio the interface Sr2RuO4-Sr3Ru2O7. We show that the rotations strongly reduce the main hopping parameter of the dxy band, making near the Van Hove singularity to the Fermi level. We study the tetragonal-monoclinic transition in the compound KCrF3. We present the electronic structure and the volume relaxation study for the KCrF3 in the two different crystalline phases. Following the usual definition of the eg orbital | _ >= cos _ 2 |3z2 −1⟩+sin _ 2 |x2 −y2⟩, the calculation of the orbital gives _ = 110:5◦ for the tetragonal structure, that is similar to LaMnO3. For the monoclinic phase, we find _ = 120:9◦ and 102:2◦ for the two types of octahedron. We discuss similarities with KCuF3 and LaMnO3 in the orbital order. We deepen the study of KCrF3 studying the low-energy physics and the non-collinear properties of its antiferromagnetic ground state. We present and compare the hopping parameters for the cubic, tetragonal and monoclinic structures of KCrF3 using the eg basis and the Maximally localised Wannier functions. Moreover, we analyse the strength of electronic correlation using the Cococcioni method based on linear response approach. Although, the atomic number of chromium is relatively small, it is observed experimentally that the spin-orbit effect can play a non trivial role at low temperature. We go beyond the spin collinear approximation, the spin-orbit coupling and the weak ferromagnetism are also examined. Finally, we study from first principles the magnetic, electronic, orbital and structural properties of the LaMnO3 doped with gallium atoms. The gallium atoms reduce the Jahn-Teller effect, and accordingly reduce the charge gap. Surprisingly, the system does not go towards a metallic phase. The doping tends to reduce the orbital order by weakening the antiferromagnetic phase and by favoring an unusual insulating ferromagnetic phase due to the effect of the correlated disorder. It is also presented a general discussion on the results obtained and some comments on prospective and open questions. [edited by author]