Chen Ke, Still Tim, Schoenholz Samuel, Aptowicz Kevin B, Schindler Michael, Maggs A C, Liu Andrea J, Yodh A G
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.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Aug;88(2):022315. doi: 10.1103/PhysRevE.88.022315. Epub 2013 Aug 26.
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.
利用视频显微镜和协方差矩阵分析测量了由热敏微凝胶颗粒组成的原始和多晶单层胶体晶体的振动模式。在低频下，在两种晶体类型中均观察到二维谐波晶体的德拜关系；在较高频率下，声子态密度中的范霍夫奇点的证据被实验噪声和测量统计数据显著模糊。使用数值模拟分析了这些误差的影响。我们介绍了校正这些限制的方法，这些方法可应用于无序系统以及晶体系统，并且我们表明将误差校正程序应用于实验数据会导致原始晶体中出现更明显的范霍夫奇点。最后，在多晶二维胶体晶体中识别出准局域低频模式，并证明其与诸如位错等结构缺陷相关，这表明准局域低频声子模式可用于识别晶体以及非晶固体中易发生重排的局部区域。
