Non-phononic density of states of two-dimensional glasses revealed by random pinning

TL;DR

This study uses random pinning to suppress phonons in two-dimensional glasses, enabling accurate measurement of non-phononic vibrational density of states and revealing their localized nature and hybridization effects.

Contribution

The paper introduces a random pinning technique to isolate non-phononic modes in 2D glasses, providing clear evidence of their ω^4 density of states and localization properties.

Findings

Non-phononic density of states follows ω^4 in 2D glasses.

Low-frequency non-phononic modes are truly localized.

Hybridization causes excess density of states over Debye prediction.

Abstract

The vibrational density of states of glasses is considerably different from that of crystals. In particular, there exist spatially localized vibrational modes in glasses. The density of states of these non-phononic modes has been observed to follow $g(\omega) \propto \omega^4$, where $\omega$ is the frequency. However, in two-dimensional systems, the abundance of phonons makes it difficult to accurately determine this non-phononic density of states because they are strongly coupled to non-phononic modes and yield strong system-size and preparation-protocol dependencies. In this article, we utilize the random pinning method to suppress phonons and disentangle their coupling with non-phononic modes and successfully calculate their density of states as $g(\omega) \propto \omega^4$. We also study their localization properties and confirm that low-frequency non-phononic modes in pinned systems are truly localized without far-field contributions. We finally discuss the excess density of states over the Debye value that results from the hybridization of phonons and non-phononic modes.