Low-energy excitation in glassy solids

The disordered microstructure of glassy solids gives rise to a population of soft excitations whose density determine various mechanical, dynamic and thermodynamic properties of the glass. Despite their fundamental importance, much is unknown about the abundance of these `glassy modes', their spatial structure and degree of universality. In this seminar I will present results from numerical simulations of several model glasses showing that the low-frequency tail of the density of states follows a universal law D(ω)∼ω^4 up to the vicinity of the largest wavelength shear-wave frequency. I will discuss the localization properties of soft glassy modes, and the effects of preparation protocol, internal stresses, and dimensionality on their statistics. Finally, I will move away from the harmonic regime and introduce an alternative definition of low-energy excitations in glasses that properly accounts for anharmonicities of the potential energy. I will show that these nonlinear excitations, unlike soft quasi-localized harmonic modes, do not hybridize with extended excitations. Nonlinear excitations therefore coexist with plane-waves in phononic frequency bands, making them good candidates as the fundamental low-energy excitations in glassy solids