Resonant and angle resolved photoelectron spectroscopic studies of strontium hexaboride, europium hexaboride and samarium hexaboride.

Abstract

This dissertation investigates the electronic structure of three metal hexaborides, SrB₆, EuB₆ and SmB₆ and probes the issues and controversies concerning these systems using the experimental techniques and methods of photoelectron spectroscopy (PES). The body of research problems connected to these materials provides an opening to the investigation into the electronic properties of other metal hexaborides. The general class of metal hexaborides, as exhibited by these compounds, display a wide variety of interesting and unusual ground state phenomenon. This includes mixed valence, heavy fermion properties and exotic narrow gaped semi-metallic behavior and many characteristics associated with systems containing localized f-electron states that exhibit strong Coulombic interactions with valence and conduction band states. The electronic spectrum is measured by (PES) and is the spectrum to add or remove electrons from the system using photons. SrB₆ and EuB₆ are both divalent hexaboride compounds with EuB₆ divalence being demonstrated by performing 4f resonance photoemission. SrB₆ is the simplest of the compounds studied in this thesis in terms of its electronic structure. EuB₆ is a ferromagnet. Using (PES) we directly observe X-point Fermi surface pockets for both SrB₆ and EuB₆ and obtain numerical values for the electron number densities. SmB₆ is a unique material with very unusual and complicated characteristics. SmB₆ is mixed valent with valence intermediate between 2<super>+</super> and 3<super>+</super>. With (PES) we observe a dispersing conduction band and new 4f-electron multiplets. We also obtain an approximate Fermi surface from the k-dependent Fermi energy crossings of the conduction band. The electronic structure of these hexaboride systems are studied in detail using improved sample quality and better state of the art experimental instrumentation. From this, questions are raised that lay the basis for continued studies.