Collective dynamics and de Gennes narrowing in polymeric liquid Se: High-resolution inelastic x-ray scattering

Abstract

We have measured the dynamic structure factor, $S(Q,E)$, of liquid Se at 523 and 773 K, at the momentum $Q$ and energy $E$ transfer, from 1.9 to $42\text{ }{\text{nm}}^{\ensuremath{-}1}$, and over $\ifmmode\pm\else\textpm\fi{}40\text{ }\text{meV}$, respectively, using a high-resolution inelastic x-ray scattering technique. To deconvolute the observed $S(Q,E)$ including the stretching mode at approximately 30 meV, we chose a model function constituted of Gaussian peaks and calculated the intermediate scattering function. The excitation energy of the longitudinal sound mode at 523 K disperses as fast as $1600\text{ }{\text{ms}}^{\ensuremath{-}1}$, which is approximately 50% faster than the adiabatic sound velocity. Although the adiabatic sound slows down with increasing temperature, the dynamical sound velocity hardly depends on temperature and the positive dispersion becomes approximately 70% at 773 K. In addition, the narrowing of the quasielastic peak was observed near the static structure factor maximum. We found that the normalized second frequency moment sum rule is applicable for polymeric liquid Se when the stretching mode is exactly treated. However to obtain the first- and second-order memory functions reasonably depending on time from the model function, we had to neglect the component of the stretching mode. We discussed the quasiparticle dynamics in liquid Se under this approximation. The results obtained from the application of the normalized second frequency moment sum rule for the quasiparticle indicate that the existence of six-atom cluster is largely correlated with the narrowing.