Propagating and localized vibrational modes in Ni-Zr glasses

Abstract

Numerical investigations of the vibrational eigenmodes of amorphous Ni_xZr_100-x alloys are presented. Structural models are prepared by molecular dynamics simulations of the quenching processes, based on interatomic forces derived using a tight-binding-bond approach. The vibrational properties are investigated via a direct diagonalization of the dynamical matrix for N=729-atom models, and via recursion calculations of the vibrational spectral functions, partial and total dynamical structure factors and vibrational densities of states for large N=2916-atom models. The static structure of the Ni_xZr_100-x glasses is characterized by a pronounced chemical and topological short-range order (SRO). We investigate in detail the manifestation of the SRO in the partial dynamic spectral functions and structure factors S_IJ(k, omega ). We discuss the possibility of measuring partial dynamic structure factors using inelastic neutron scattering and demonstrate that our results are in good agreement with the existing experimental data on the total dynamical structure factors. We show that, although most eigenmodes are extended, localized modes can be found at the upper and lower edges of the frequency spectrum. Of particular interest is the prediction of low-energy localized modes, which have a profound influence on the low-temperature thermodynamic properties.