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DF: Dielektrische Festkörper

DF 3: Poster Session

DF 3.21: Poster

Montag, 27. März 2006, 09:30–17:00, P1

Thermal motion induced resonant *forbidden* reflections in Ge and ZnO — •Alexey Oreshko¹, Vladimir Dmitrienko², and Elena Ovchinnikova¹ — ¹M.V.Lomonosov Moscow State University, Physical Department, Moscow, Russia, 119992 — ²V.A.Shubnikov Institute of Crystallography RAS, Moscow, Russia, 119333

In this report we discuss thermal-motion-induced (TMI) *forbidden* reflections. Atomic displacements can change the symmetry of an atomic site, and thus induce an additional anisotropy of the atomic scattering factor near an absorption edge and therefore give rise to Bragg reflections, otherwise forbidden. Since TMI scattering arises from the relative displacement of adjacent atoms, its strength depends crucially on the thermal population of optical phonons modes. The search for suitable materials in which to study this effect is therefore governed by the requirements for: (1) a suitable space group; (2) site symmetry; (3) an absorption edge resonance at a wave-length suitable for crystal diffraction; (4) a low-lying optical phonon mode; (5) a high quality single crystal sample. Ge, which crystallizes with the diamond structure, and w-ZnO, which crystallizes with the Wurtzite structure, satisfy the above conditions. Strongly temperature dependent TMI reflections were recently observed in Ge and w-ZnO. Owing to interference with the temperature-independent contribution, their intensities can increase and decrease with temperature. In this report we present ab initio simulations of the temperature dependence and diffraction spectra for Ge and w-ZnO.