title: Phonons in two-dimensional soft colloidal crystals.
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, quasilocalized low-frequency modes in polycrystalline two-dimensional colloidal crystals are identified and demonstrated to correlate with structural defects such as dislocations, suggesting that quasilocalized low-frequency phonon modes may be used to identify local regions vulnerable to rearrangements in crystalline as well as amorphous solids.
authors.affiliation: Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA and Beijing National Laboratory for Condensed Matter Physics and Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Ke
authors.initials: K
authors.lastname: Chen
authors.forename: Tim
authors.initials: T
authors.lastname: Still
authors.forename: Samuel
authors.initials: S
authors.lastname: Schoenholz
authors.forename: Kevin B
authors.initials: KB
authors.lastname: Aptowicz
authors.forename: Michael
authors.initials: M
authors.lastname: Schindler
authors.forename: A C
authors.initials: AC
authors.lastname: Maggs
authors.forename: Andrea J
authors.initials: AJ
authors.lastname: Liu
authors.forename: A G
authors.initials: AG
authors.lastname: Yodh