Anisotropy of the electron momentum density of graphite studied by (gamma,e gamma) and (e,2e) spectroscopy - art. no. 155204

The electron momentum density (EMD) of two different modifications of graph ite has been measured and the results of the measurements have been compare d with theoretical calculations from three different theories: a full poten tial linear muffin-tin orbital, a modified augmented plane wave, and a pseu dopotential calculation. Experimental results have been obtained by two dif ferent methods. The complete three-dimensional EMD is determined by inelast ic photon-electron scattering, i.e., by the so-called (gamma ,e gamma) expe riment, and by electron-electron scattering, the (e,2e) experiment, cuts in the spectral electron momentum density are studied. For the (gamma ,e gamm a) experiment 180 keV synchrotron radiation from the PETRA storage ring at the Deutsches Elektronen-Synchrotron has been used with coincident detectio n of the recoil electrons. The (e,2e) experiments were carried out at the n ew (e,2e) spectrometer at the Australian National University using 40 keV p rimary electron energy and simultaneous detection of the outgoing electrons in an equal energy sharing mode. As samples we have prepared approximately 20 nm thin self-supporting graphite foils either by thermal evaporation (T E) or by laser plasma ablation (LPA). They are thin enough to suppress in e ssence electron multiple scattering. Electron diffraction analysis revealed that the LPA foil contains graphitic basal planes with a random distributi on of c axes, whereas the TE foil was strongly c-axis oriented in the sense that the basal planes were parallel to the foil surface. In the analysis o f the results special attention was devoted to anisotropies in the EMD reve aled by comparison of TE and LPA foils. The (e,2e) measurements showed furt hermore a strong orientation dependence of the intensity of pi and sigma st ates (here we have for comparison additionally measured highly oriented pyr olytic graphite). The EMD's obtained by both techniques show anisotropies i n the momentum distribution of graphite and are discussed in view of the th eoretical results.