Research Projects

Transition metal oxides (TMOs) are characterized by their unique electronic and magnetic properties due to the competing forces of transition metal (TM)-3d electrons. These properties are influenced by the electron-electron Coulombic interaction and TM 3d-O 2p hybridization, which facilitate the delocalization of electrons. TMOs can be classified as Mott-Hubbard insulators or charge transfer insulators based on their charge transfer energy (Δ). However, TMOs with high covalent strength contribute to the ground state, revealing charge fluctuations and potentially causing charge disproportionation (CD) in the system. TMOs with negative charge transfer energy values exhibit various physical properties, such as superconductivity, diamagnetism, and ferromagnetism. The charge transfer energy Δ depends on the TM 3d-O 2p hybridization and decreases with increased hybridization. To adapt to a negative Δ value with a 3dn+1L ground state, TMOs can be tuned via strain in the system. Cobaltate perovskites are relevant materials due to their higher covalent character and low Δ values. This project aims to experimentally examine the electronic structure manifestations of negative charge transfer energy in cobaltates using combined resonant photoemission spectroscopy and X-ray absorption near edge spectra, along with microscopic insights from first-principles calculations. The study aims to probe the interplay between charge transfer energy, charge disproportionation, strain, and spin in transition metal oxide thin films, as tuning Δ is crucial for potential applications where mobile/localized O-2p holes are used instead of strongly localized TM-3d electrons to improve functionalities.