Photoemission Studies in Transition Metal Oxides and Chalcogenides

Abstract

Photoemission spectroscopy (PES) has been traditionally used to yield information on the electronic structure on a large energy scale (typically a few eV). For example, in the UV range, the band dispersion in solids can be investigated. Although the photoemission results have often been compared with calculations of the ground state band structure, a photoemission spectrum directly gives, in principle, the density of elementary excitations at all energies: from the band width range to the meV range. The latter is particularly interesting because the low energy excitations (in the kT range) are responsible for the thermodynamical properties of solids. Recent technical progress has led to a significant improvement of the energy resolution of PES allowing a determination of these low energy excitations. In the last few years, high resolution photoemission has been exploited to study electronic systems characterized by the existence of a low energy scale. For example, high resolution photoemission spectra in Ce- or Yb-based compounds have revealed the Kondo energy (kT_K≈1-10 meV), which governs the physical properties of heavy fermion [1–2]. In high-Tc superconductors, very useful information have been obtained from spectroscopic studies, namely the magnitude of the superconducting gap (Δ≈20 meV), its anisotropy, the nature of the normal state [3–4].