Prof. Taras BRYK¹

Talk: "What is different in propagation of collective excitations in glasses and melts?"

¹Yukhnovskii Institute for Condensed Matter Physics of NAS of Ukraine, Lviv, Ukraine

Propagation of collective excitations in liquids and glasses is different, especially in the long-wavelength region. For liquids long-wavelength transverse sound cannot propagate (although on nanoscales transverse shear waves can emerge) and longitudinal sound usually has so-called "positive dispersion". In glasses acoustic longitudinal and transverse excitations have specific damping related to the glass structure, as well as a puzzle of the origin of the Boson peak on the reduced by Debye law vibrational density of states in glasses puts questions about the dispersion of longitudinal and transverse excitations.

I will talk about a methodology of estimation of dynamic eigenmodes in melts, which is possible to extend to glassy systems. Analysis of collective dynamics in liquids from scattering experiments or computer simulations is usually based on application of a memory function ansatz, which assumes several channels of correlation decay in the second-order memory function. However, such an approach allows one to recover experimental/simulated dynamic structure factors but does not shed light on the processes responsible for propagation of collective excitations. Traditional phonons known from the solid state theory do not exist in liquids, therefore it is important to have a methodology which enables estimation of the leading processes contributing to the propagation of collective excitations. In particular, for the case of one-component liquids it is imporant to estimate how the origin of acoustic modes is changing with increasing wave numbers from macroscopic viscous to atomic-scale elastic regime.