Electronic, Magnetic and Local structure of Some Selected Strongly Correlated Systems

Abstract

According to independent electron band structure theories, transition metal oxides (TMOs) with partially filled 3d valence band are predicted to be metallic in nature. Though in reality, most of them are insulators. Mott and later Hubbard introduced electron-electron interactions in order to explain such insulating behaviors. These systems are often known as Mott-Hubbard insulators (or Mott insulators). Such systems have attracted a great deal of attention in the last several decades, not only due to the intriguing physics observed in these materials with variations in pressure, temperature, doping, etc., but also due to their diverse application potentials. The discovery of high-temperature superconductivity (high-TC) in the copper-based TMOs regenerated the interest in these systems characterized by strong electron-electron interactions; such systems are generally called strongly correlated systems (SCS) to include materials that are not necessarily insulating, but whose properties are believed to be controlled by strong electron-electron interactions. These SCS, particularly those based on the 3d TMOs, are of great importance and cover various phenomena such as metal-insulator transitions (MIT), high-TC superconductivity, and colossal magnetoresistance. All such exciting properties, shown by 3d TMOs, are fundamentally important due to the presence of a partially filled 3d valence band (VB) and to understand such properties, one needs to investigate the electronic and crystal structures of these materials. In this thesis, we have investigated the electronic, magnetic, and local geometric structures of some selected strongly correlated systems with interesting properties like MIT, and strong magnetoelectric coupling with the help of different high energy spectroscopic techniques along with dielectric and magnetic measurements. The samples reported in this thesis were prepared by various synthetic routes, such as solid-state reaction, sol-gel method, and d.c arc melting. These samples were characterized by x-ray diffraction, magnetic susceptibility, optical absorption, dielectric constant and energy dispersive analysis of x-rays. Various spectroscopic techniques like Hard X-ray Photoemission Spectroscopy (HAXPES) and Extended X-ray Absorption Fine Structure (EXAFS) were used to probe the electronic and local structures of the samples of Nickel Oxide (NiO), La1-xCaxVO3, Ga-doped YMnO3 and La-doped SrTiO3