Tuning the electronic properties of transition-metal compounds: an ab initio study

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

Effects of applied pressure in ZnV2 O4 and evidences for a dimerized structure

The series of V spinels [A2+] V2 O4 (A = Cd, Mn, Zn, Mg) provides an opportunity to tune the V-V distance continuously, in the frustrated pyrochlore lattice of the spinel. This system has been shown to approach the metallic state when V-V distance is reduced. The proximity to the transition leads to a dimerized structure in ZnV2 O4 caused by lattice instabilities. A different manner to tune the V − V distance of this structure is to fix the A2+ cation (in our case, Zn) and apply pressure. We have analyzed the evolution of the electronic structure of the system in the dimerized state. Such structure prevents the system to present a metallic phase at moderate pressures. We have also calculated the transport properties in a semiclassical approach based on Boltzmann transport theory. Our results support the validity of this structural distortion by providing a nice fit with experimental measurementsThe authors thank the CESGA (Centro de Supercomputacion de Galicia) for the computing facilities and the Ministerio de Educación y Ciencia (MEC) for the financial support through the project MAT2009-08165. A.S.B. thanks MEC for an FPU grant. J.B. and M.P. acknowledge Deputación da Coruña and the Isabel Barreto program respectively for financial support. We are also thankful to the Xunta de Galicia for financial support through the project INCITE08PXIB236052PRS

Electronic structure analysis of the quasi-one-dimensional oxide Sr6Co5O15 within the LDA+U method

The quasi-one-dimensional cobalt oxide Sr6Co5O15 is studied using first-principles electronic-structure calculations and Boltzmann transport theory. We have been able to describe the electronic structure, characterized by the structural one-dimensionality and a particular type of charge ordering, with unexpected electronic structure of the different Co atoms. The origin of the large unquenched misaligned orbital angular momenta comes out naturally from a correct description of the different crystal-field environments. The evolution with the on-site Coulomb repulsion (U) of the electronic structure and the transport properties is discussed, with a best agreement with experiment found for the smallest value of U that allows to converge the correct in-chain ferrimagnetic ground stateThe authors thank the CESGA for the computing facilities, the Ministerio de Educación y Ciencia (MEC) for the financial support through the project MAT2009-08165, the Ministerio de Ciencia e Innovación (MICINN) for the project MAT2007-60536 and the Xunta de Galicia for the project INCITE08PXIB236052PR. A.S.B. thanks MEC for a FPU grant. M.P. and J.B. thank Isabel Barreto program and Deputación da Coruña, respectively, for financial supportS