Poisson ratio and excess low-frequency vibrational states in glasses

TL;DR

This paper links the Poisson ratio in glasses to the nature of low-frequency vibrational states, proposing that strong glasses exhibit breathing-like modes while fragile glasses show shear-like modes, with experimental verification in silica and polymeric glasses.

Contribution

It introduces a novel connection between Poisson ratio, density fluctuations, and vibrational mode character in glasses, supported by experimental evidence.

Findings

Strong glasses have predominantly breathing-like low-frequency modes.

Fragile glasses exhibit shear-like vibrational modes.

Hydrostatic pressure shifts modes from breathing-like to shear-like.

Abstract

In glass, starting from a dependence of the Angell's fragility on the Poisson ratio [V. N. Novikov and A. P. Sokolov, Nature 431, 961 (2004)], and a dependence of the Poisson ratio on the atomic packing density [G. N. Greaves et al., Nat. Mater. 10, 823 (2011)], we propose that the heterogeneities are predominantly density fluctuations in strong glasses (lower Poisson ratio) and shear elasticity fluctuations in fragile glasses (higher Poisson ratio). Because the excess of low-frequency vibration modes in comparison with the Debye regime (boson peak) is strongly connected to these fluctuations, we propose that they are breathing-like (with change of volume) in strong glasses and shear-like (without change of volume) in fragile glasses. As a verification, it is confirmed that the excess modes in the strong silica glass are predominantly breathing-like. Moreover, it is shown that the excess breathing-like modes in a strong polymeric glass are replaced by shear-like modes under hydrostatic pressure as the glass becomes more compact.