PhD Thesis: Electronic correlations in multiorbital systems

TL;DR

This thesis investigates the impact of electronic correlations, especially Hund's coupling, on multiorbital systems like unconventional superconductors and 2D materials using ab-initio and many-body methods.

Contribution

It provides a detailed analysis of electronic correlations in multiorbital systems, emphasizing the role of Hund's coupling through combined ab-initio and slave-spin approaches.

Findings

Electronic correlations significantly influence material properties.

Hund's coupling $J_H$ plays a crucial role in multiorbital systems.

The study advances understanding of correlation effects in high-$T_c$ superconductors and 2D materials.

Abstract

The role of electronic correlations in Condensed Matter is at the heart of various important systems, like magnetic materials, superconductors, topological materials, optical lattices, etc. Electronic correlations are those which change the motion of individual electrons when considering the interaction with other electrons in the material. Among the available systems to study electronic correlation effects, in this thesis I focus on unconventional superconductors, specifically in high-$T_c$ iron-based superconductors, and on two-dimensional materials, like the recent magic-angle twisted bilayer graphene or the itinerant ferromagnet $Fe_3GeTe_2$. In the introduction, I explained in detail the importance of electronic correlations, and how their strength can be modeled by the quasiparticle weight $Z$. I briefly reviewed the most important aspects of the Fermi Liquid Theory. Chapter 2 is devoted to explain in detail the role of electronic correlations in multiorbital systems, with a special attention to the role played by the Hund's coupling $J_H$. Chapters 3 to 6 describe the calculations done in various real systems. During this thesis, I used a combination of ab-initio and modelling, plus slave-spin many-body approaches to study the effect of the Hubbard interaction $U$ and the Hund's coupling $J_H$ in various real systems. A detailed description of the techniques can be found in the appendixes.