Low-frequency excitations in a molecular glass: Single-particle dynamics

The low-frequency dynamics of a molecular glass former has been investigated by means of inelastic neutron scattering from a mostly incoherent sample as well as by computer molecular-dynamics simulation. The mode assignments have been carried out using the neutron data for the polycrystal as well as by means of the analysis of the density of states (DOS) computed from a lattice-dynamics calculation. The polycrystalline sample is then taken as the reference state and the deviations that are a known characteristic of the glassy phase are discussed in detail. In particular, the enhancement of the low-frequency part of the DOS corresponding to the glassy phase that gives rise to a well-defined low-frequency inelastic peak in the neutron S(Q,ω) dynamic structure factors is found to arise from translational modes that are dominant over the rotational contribution. Such modes are also found to give rise to a bump in the temperature dependence of the ${\mathit{C}}_{\mathit{v}}$/${\mathit{T}}^3$ specific-heat curves at 2<T<10 K.