REAL-SPACE DETERMINATION OF ANISOTROPIC ELECTRONIC-STRUCTURE BY ELECTRON-ENERGY LOSS SPECTROSCOPY

Electron energy loss spectroscopy (EELS) measures the dielectric function of a material, and as such provides information on its electronic structure. In the case of non-cubic anisotropic materials, like graphite and the oxide superconductors, the dielectric function is a tensor. For uniaxial or tetrahedral anisotropy it has two orthogonal components, epsilon-perpendicular-to and epsilon-parallel-to. The convergence angle and collection geometry typically used in scanning transmission electron microscopy (STEM) are such that both components are included in any spectrum. The exact contribution from each component will be dependent on specimen orientation and the collection apertures used. A model is proposed whereby EELS spectra obtained from the core-loss region are decomposed into the two components resulting from excitations parallel and perpendicular to the c-axis. The effect of varying aperture sizes on the relative intensity of the two components can be calculated. Agreement with experimental results obtained for a series of spectra of the carbon K-edge of graphite is satisfactory, leading to momentum-transfer-resolved spectra being obtained. The small probe size of the STEM, coupled with this method of spectral decomposition, enables orientation dependence to be studied when the real-space resolution is on the nanometre scale.