Understanding the Low-Frequency Modes in Disordered Systems at Single-Particle Level

TL;DR

This study visualizes and analyzes low-frequency vibrational modes in disordered colloidal systems at the single-particle level, revealing their origins and correlations with dynamics, thus advancing understanding of disordered solids.

Contribution

First experimental visualization of low-frequency modes in disordered systems at single-particle resolution, clarifying their microscopic origin and relation to dynamics.

Findings

Low-frequency modes caused by collective resonance of disordered particles.

Observation of a plateau in the density of states due to isostaticity.

Intrinsic correlation between low-frequency modes and system dynamics.

Abstract

Normal modes provide a fundamental basis for understanding crucial properties of solids, such as the thermal conductivity, the heat capacity and the sound propagation. While the normal modes are excellently described by plane waves in crystals, they are far less understood in disordered systems, due to the great difficulties in characterizing the heterogeneous vibrational behaviors. Using charged colloids with long-range repulsion, we successfully make different disordered systems without any contact friction, whose normal modes can be visualized at single-particle level. In these systems, we directly tackle the long-time outstanding puzzle in condensed matter physics: the microscopic origin of the low-frequency modes in disordered systems. For the first time, we experimentally clarify that the low-frequency modes are caused by the collective resonance of relatively disordered particles (or soft structures) coupled with long-wavelength transverse excitations, settling this puzzle at single-particle level. Next to these low-frequency modes in the density of states, we also observe a plateau due to isostaticity, verifying the fundamental prediction of jamming model. Moreover, we reveal the intrinsic correlation between the low-frequency modes and the real dynamics, which may lead to a universal mechanism for aging, melting and yielding.