Phonons in two-dimensional soft colloidal crystals

TL;DR

This study measures vibrational modes in 2D colloidal crystals, introduces error correction methods for phonon analysis, and links low-frequency modes to structural defects, advancing understanding of phonon behavior in soft materials.

Contribution

It develops error correction techniques for phonon measurements in colloidal crystals and demonstrates their effectiveness in revealing phonon features and defect correlations.

Findings

Debye relation observed at low frequencies

Error correction enhances detection of van Hove singularities

Low-frequency modes correlate with structural defects

Abstract

The vibrational modes of pristine and polycrystalline monolayer colloidal crystals composed of thermosensitive microgel particles are measured using video microscopy and covariance matrix analysis. At low frequencies, the Debye relation for two dimensional harmonic crystals is observed in both crystal types; at higher frequencies, evidence for van Hove singularities in the phonon density of states is significantly smeared out by experimental noise and measurement statistics. The effects of these errors are analyzed using numerical simulations. We introduce methods to correct for these limitations, which can be applied to disordered systems as well as crystalline ones, and we show that application of the error correction procedure to the experimental data leads to more pronounced van Hove singularities in the pristine crystal. Finally, quasi-localized low-frequency modes in polycrystalline two-dimensional colloidal crystals are identified and demonstrated to correlate with structural defects such as dislocations, suggesting that quasi-localized low-frequency phonon modes may be used to identify local regions vulnerable to rearrangements in crystalline as well as amorphous solids.