Multiphonon response in corrugated two-dimensional systems

The vibrational density of states associated with the in-plane dynamics of intercalants in graphite intercalation compounds obtained from inelastic-neutron-scattering (INS) measurements shows a characteristic two-peak structure. Using molecular-dynamics simulations of screened Coulomb systems on a strongly corrugated substrate we show that the high-frequency peak is due to two-phonon response, thus providing a correct understanding of the INS experiments. The temperature and q dependence of the incoherent dynamic structure factor confirms our picture.