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

Phonon frequency distributions of bulk [gᵇ(ω)] and microcrystalline [gᵐ(ω)] titanium nitritde are obtained from neutron scattering experiments. The surface excess distribution gˢ(ω)=gᵐ(ω)-gᵇ(ω) shows, at low energies, a positive region due to acoustic surface modes. A second positive region in gˢ(ω) 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.