Condensed Matter and Biological Physics Seminars

Multiple Bragg Scattering, a situation in which two or more Bragg reflections are excited at the same time, is a source of phase information. Applications to quasicrystals will be presented. The general problem of centrosymmetry, or lack of it, will be discussed, along with experimental results. Resonant scattering is a new technique used to get information about "orbital ordering". The crystal potential gives rise to preferred orientations of aspherical degenerate orbitals, responsible for chemical bonding. This is commonly referred to as "orbital ordering". One of the effects of orbital ordering is to excite forbidden reflections when the energy of the x-rays corresponds to an absorption edge. The phases of these forbidden reflections may be very useful for a complete and accurate description of orbital ordering. Applications will be shown for Ge and LaMnO3. In the latter case, a recent 3-beam experiment has resolved an ambiguity concerning the spatial distribution of the Mn d-electrons.