Electron spectroscopic study of 3d transition metal oxides and metal-insulator transitions.

Abstract

This thesis investigates the electronic structure of 3d transition metal (TM) oxides. Many different electron spectroscopy and x-ray absorption spectroscopy (XAS) measurements have been performed on a sequence of transition metal oxides NiO, CoO, FeO, Fe$\sb2$O$\sb3$, Fe$\sb3$O$\sb4$, MnO, Cr$\sb2$O$\sb3$, (V$\sb{1-x}$M$\sb{x}$)$\sb2$O$\sb3$ (M = Cr,Ti), and Ti$\sb2$O$\sb3$. XAS and resonance photoemission spectroscopy (RESPES) studies were made at the TM 2p and oxygen 1s edges, and the photoemission spectroscopy (PES) work also includes high resolution, variation of temperature studies of the metal-insulator (MI) 'Verwey' transition in Fe$\sb3$O$\sb4$ and the famous MI transition in the V$\sb2$O$\sb3$ system. In this thesis, I present various new spectroscopic data, suggest a new physical picture for the early TM oxides in framework of the Anderson model Hamiltonian and discuss new interpretations of the two unsolved metal-insulator transition problems. The systematic study for the sequence leads to a reclassification of early TM oxides from a traditional Mott-Hubbard picture to one using the Anderson Hamiltonian with very strong metal-oxygen hybridization. The lowest lying PES states are not of the $d\sp{n-l}$ character, but are bound states of mixed configuration character pushed out of the oxygen band, toward the lowest one electron addition state. The occurrence of a gap is then controlled by the strong hybridization, which also pushes bound state satellites out of the oxygen 2p band at high binding energies. (V$\sb{1-x}$M$\sb{x}$)$\sb2$O$\sb3$ (M = Ti, Cr) system shows a complex phase diagram which contains antiferromagnetic insulating, paramagnetic metallic, and paramagnetic insulating phases. Varying the temperature through the MI transitions, I observed not only gap openings in the high resolution spectra but also spectral changes in all the measured spectra. The spectral changes through the MI transitions are well explained by the variation of the V $3d$-O2p hybridization T and the charge transfer energy $\Delta$ as proposed in the new picture. The new data also enable us to estimate the size of band gap in both the low T and the high T insulating phase. The spectroscopic study on the valence of the dopants in the (V$\sb{1-x}$M$\sb{x}$)$\sb2$O$\sb3$ system shows interesting results. The valence of Ti is not 3+, as in Ti$\sb2$O$\sb3$, but is 4+, as in SrTiO$\sb3$, while that of Cr is as expected 3+, as in Cr$\sb2$O$\sb3$. In Fe$\sb3$O$\sb4$, XAS and RESPES at the Fe 2p edge enable resolution and observation of separate Fe valence states even above the transition temperature T$\sb{V}$. On heating through T$\sb{V}$ the electrical gap is not collapsed, but merely reduced by $\sim$50meV, showing that a metal-insulator transition does not occur. This change in the gap is perfectly consistent with the two orders of magnitude conductivity jump at T$\sb{V}$. These new spectroscopic data allow us to construct an energy diagram in connection with the Verwey transition.