Vibrational states of glassy and crystalline orthoterphenyl

Low-frequency vibrations of glassy and crystalline orthoterphenyl are studi ed by means of neutron scattering. Phonon dispersions are measured along th e main axes of a single crystal, and the corresponding longitudinal and tra nsversal sound velocities are obtained. For glassy and polycrystalline samp les, a density of vibrational states is determined and cross-checked agains t other dynamic observables. In the crystal, low-lying zone-boundary modes lead to an excess over the Debye density of states. In the glass, the boson peak is located at evert lower frequencies. With increasing temperature, b oth glass and crystal show anharmonicity.