Excitations of back-folded graphite bands in KC8

Abstract

High-resolution (0.1 eV) electron-energy-loss spectroscopy shows seven energy-loss features between 15 and 40 eV in stage-1 potassium graphite, K${\mathrm{C}}_8$. Features due to separate potassium and graphite excitations are identified, none of which occur in pristine graphite. The new graphite excitations are due to back-folded bands imposed by the two-dimensional K superlattice. The energies and oscillator strengths of the new transitions were determined from $\epsilon \mathrm{}\left(E\right)\mathrm{}$, obtained by a Kramers-Kronig analysis of the energy-loss data. The strength of the carbon-intercalant interaction was computed in a model calculation which fits both energy splittings and oscillator strengths. This interaction modulates the carbon-carbon transfer integral by about 20%, yet the oscillator strength of the back-folded band excitations is only ∼2% of the total $\pi$-electron oscillator strength.