Excitonic Effects on the Silicon Plasmon Resonance

We present an ab initio calculation of the electron energy loss spectrum of silicon including local-field, self-energy, and excitonic effects. When self-energy corrections are added to the standard random phase approximation (RPA) the line shape of the plasmon resonance worsens. The electron-hole interaction cancels this correction and improves the result both compared to the RPA and to the self-energy one, yielding very good agreement between theory and experiment provided that the mixing of interband transitions of both positive and negative frequencies is included.