Tuning the electronic properties of transition-metal compoundsAn ab initio study

Resumo

This PhD Thesis deals with the analysis using first-principles calculations of transition-metal compounds whose electronic properties reside in the vicinity of a metal-to-insulator transition, the ultimate goal being the computational design of materials with new functionalities based on our calculations and the full understanding of these systems in order to make new predictions. A large portion of the technological progress over the past decades has been related to Materials Science and Condensed Matter Physics. Particularly, the latter half of the 20th century witnessed an explosion of new materials and new ways to tune their properties. A powerful driving force behind the technology revolution is the ability to control the electronic properties of a material on demand, giving rise to new functionalities. An important class of these novel materials is represented by transition- metal compounds where outstanding properties such as ferroelectricity, colossal magnetoresistance, high temperature superconductivity and metal-insulator transitions can be observed. In addition to this huge heterogeneity of physical phenomena, there is potential for exciting new discoveries in these materials: the enhancement of thermoelectric properties through nanostructuring or the new phenomena on surfaces and interfaces between complex oxides are clear examples of that and have been explored by a lively scientific community in the last years. Because of the agreement between experimental and computational data, Density Functional Theory (DFT) proves to be a reliable tool to acquire new knowledge in Materials Science, even in the complicated case of strongly correlated electron systems such as transition-metal compounds. For this reason, the methodology to perform the ab initio calculations throughout this Thesis will be DFT as it is implemented in the WIEN2k code. The transport properties calculations will be performed using the BoltzTraP code, a semiclassical approach based on the Boltzmann transport theory within the constant scattering time approximation. First, we will study the changes in electronic structure between the low and high temperature phases of V4O7 across its temperature-induced metalinsulator transition. We will analyze charge and magnetic order, the nature of the observed structural distortions, the degree of charge disproportionation in both phases and the nature and strength of the magnetic couplings. We will also analyze the electronic structure, special magnetic properties and thermopower of a quasi-one-dimensional oxide, Sr6Co5O15. The study of this material serves as a confirmation of our ability to make predictions of the thermoelectric properties of a system. After that we will describe the electronic structure and transport properties of bulk CrN using a diverse set of exchange-correlation potentials. The electronic properties of this material, close to a metal-insulator transition, have been a matter of discussion due to the controversial results found experimentally and we propose how to reconcile the di↵erent pictures. After understanding the properties of these bulk materials, we will deepen into several mechanisms of tuning their electronic properties. To elucidate the origin of the experimentally found metallic behavior of the low-temperature phase of CrN, and to analyze what the e↵ects of a material so close to a metal-insulator transition are when it is nanostructured, we have studied free-standing thin films of CrN. This will allow us to predict an unexpected electronic reconstruction at the surface that leads to emergent conducting states confined to a thickness of less than 1 nm. We will also analyze the e↵ects of quantum confining CrN, studying the electronic structure of CrN/MgO multilayers. This will be the best scenario to study the e↵ect of a drastic dimensionality reduction without surface states appearing, that otherwise complicate the study. The enhancement of the thermoelectric response of La2NiO4+