Observation of Vibrational Surface Modes in the Acousto-optical Bulk Gap of TiN

Abstract

Phonon frequency distributions of bulk [$g^b\left(\omega \right)$] and microcrystalline [$g^m\left(\omega \right)$] titanium nitritde are obtained from neutron scattering experiments. The surface excess distribution $g^s\left(\omega \right)=g^m\left(\omega \right)-g^b\left(\omega \right)$ shows, at low energies, a positive region due to acoustic surface modes. A second positive region in $g^s\left(\omega \right)$ proves, for the first time experimentally, the existence of a band of surface vibrational modes bridging the acousto-optical gap of the bulk modes; the double-peak structure indicates two branches of surface modes.