Collective dynamics at high wave vector in the glass-forming liquid Ca0.4K0.6(NO3)1.4

Abstract

The nature of the phonons at high wave vectors (0.2<k<7.0 Å−1) in the glass-forming liquid Ca0.4K0.6(NO3)1.4 has been investigated by using molecular dynamics (MD) simulations. The MD simulations have been performed with a previously proposed polarizable model for the nitrate anion [M. C. C. Ribeiro, Phys. Rev. B 61, 3297 (2000)]. Time correlation functions of mass and charge current fluctuations have been calculated in order to obtain the spectra of the acoustic and the optic modes, respectively, both the longitudinal and the transverse ones (LA, TA, LO, and TO). Phonon dispersion curves, ω(k), and the damping, Γ(k), of the LA modes are shown to oscillate out of phase with the static structure factor, S(k), in a wide range of wave vectors. Corresponding dynamic structure factors, S(k,ω), are calculated as k-dependent, ω-constant cuts of the LA spectra. It is shown that S(k,ω) oscillates in phase with S(k) for low energy modes. A similar analysis is applied to the LO spectra, the corresponding dynamic structure factor following the charge static structure factor, Sq(k).