Abstract
Joint Inelastic Neutron and X Ray Scattering (INS and IXS)
measurements have been performed on heavy water across the melting
point. The spectra bear clear evidence on a low and a high frequency
inelastic shoulders related to a transverse and a longitudinal mode
respectively. Upon increasing the momentum transfer, the spectral
shape evolves from a viscoelastic regime, where the low frequency
mode is clearly overdamped, towards an elastic region where its
propagation becomes allowed. The crossover between the two regimes
occurs whenever both the position and the width of the low frequency
mode match the inverse of the structural relaxation time.
Furthermore a discontinuity in the solid phase in the the shear-mode
frequency, which tends to disappear upon increasing the exchanged
momentum, is observed.
