Observation of a localized surface phonon on an oxide surface

The vibration modes of a clean Cr2O3(0001) surface have been studied under ultra high vacuum conditions with high-resolution electron energy loss spectroscopy. A mode which is confined to the first few atomic layers of the oxide was detected in addition to the Fuchs–Kliewer phonons. In contrast to the Fuchs–Kliewer phonons which extend deeply into the bulk, this mode is very sensitive towards the presence of adsorbates. Full-potential linearized augmented plane-wave calculations show that this mode is an in-phase oscillation of the second-layer oxygen atoms and the Cr atoms of the two layers below.