Inelastic X-ray scattering studies of transition metal compounds

Inelastic X-ray scattering spectroscopies form a versatile family of experimental techniques that are capable of probing the ground state properties as well as the single-particle and collective excitations of condensed-matter systems. Electronic excitations are of fundamental importance in determining the optical and transport properties of solids and also take part in the screening of the Coulomb interaction, thus contributing to ground-state properties. Transition-metal and rare-earth compounds exhibit a large variety of physical phenomena such as metal-insulator transitions, colossal magnetoresistance, unconventional magnetic ground states etc. These compounds are of fundamental and applied interest due to challenges in their theoretical description and their potential use in various devices ranging from transistors to infrared detectors. This thesis presents four applications of inelastic X-ray scattering methods in transition-metal and rare-earth oxides as well as transition-metal dichalcogenides. First, we demonstrate that valence excitations in 55-nm-thick LaAlO3/SrTiO3 heterostructures can be measured. The data is analyzed utilizing free-ion multiplet and first-principles calculations. Second, it is demonstrated that the electron momentum-density difference across the metal-insulator phase transition in VO2 is observable using the Compton-scattering spectroscopy, which probes the electronic ground state. The experimental results are compared against firstprinciples calculations. Third, a new experimental resonant X-ray emission method that utilizes a X-ray standing wave to excite the resonant scattering process is demonstrated using a Gd3Ga5O13 single crystal. The method is shown to be very sensitive to quadrupole excitations. Its potential in achieving atomic-site sensitivity and electronic-state symmetry selectivity are discussed. Last, a joint experimental and computational study on the highenergy plasmon excitations in the transition-metal dichalcogenides Cu0.2NbS2 and NbSe2 is presented. The primary significance of the first three studies lies in in demonstrations of new types of experiments, with several interesting possible applications in solid-state physics and materials science. The last study contributes to the discussion on the optical properties of transition-metal dichalcogenides. Classification (IUPAP 2010): 71, 71.45.GM, 78.70.-g