Session M17 - Semiconductors IV: Static and Dynamic Structural Properties.
MIXED session, Wednesday morning, March 22
205D, MCC
The 600 is a forbidden reflection, like the 222 (h+k+l=4n+2). Unlike the 222, it cannot be produced by an anisotropic charge density. F_600 is always zero, even when interstitial charges are placed along the bonds between atoms. Resonantly excited forbidden reflections in Ge had already been observed by D. and L. Templeton.(D. H. Templeton and L. K. Templeton, Phys. Rev. B 49), 14850 (1994) The new ingredient in the ESRF experiment is the fact that multiple reflections were excited, with the purpose of determining phases. The method of phase determination consists in measuring the 600 reflection vs. the azimuthal angle \psi, near an ``Umweganregung" reflection, typically in a 3-beam situation, and utilize the asymmetry observed at the base of the ``Umweg" reflection as a source of phase information. Two ``Umweg" reflections have been obtained at the resonant energy of 11,111 eV, the (3 7 \bar5) and the (3\bar3\bar5). The second reflection has also been measured out of resonance, at 10,800 eV. By fitting experimental azimuthal plots with theory we could determine phases and amplitudes on an absolute basis. Both were found to be critically dependent on azimuthal angle. In one case a phase value of -90^\circ was found, corresponding to a non-centrosymmetric charge density. The energy dependence of the (600) two-beam value was found highly asymmetric.