Observation of Vibrational Surface Modes in the Acousto-optical Bulk Gap of TiN
Phonon frequency distributions of bulk [${g}^{b}(\ensuremath{\omega})$] and microcrystalline [${g}^{m}(\ensuremath{\omega})$] titanium nitritde are obtained from neutron scattering experiments. The surface excess distribution ${g}^{s}(\ensuremath{\omega})={g}^{m}(\ensuremath{\omega})\ensuremath{-}{g}^{b}(\ensuremath{\omega})$ shows, at low energies, a positive region due to acoustic surface modes. A second positive region in ${g}^{s}(\ensuremath{\omega})$ 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.