二维胶体玻璃中玻色峰与结构无序度的关联

本文从实验上研究了胶体玻璃在相同面密度下随着体系结构无序程度的增加, 振动态密度和玻色峰的变化规律. 通过调制两种不同粒径的温敏性水凝胶的数量比来改变体系的无序程度. 通过分析无序体系的声子模式得到体系的振动特性. 研究发现, 随着无序度的增加, 态密度在低频区域增强、玻色峰增高、玻色峰的峰值向低频区域移动. 不同无序程度的样品引起玻色峰的低频声子模式都表现出准局域的特点, 且低频准局域声子模式与样品中无序结构存在关联.

Boson peaks in doped colloid glasses

We experimentally investigated the correlation between local structures and phonon modes in quasi-2D colloidal glasses. The glass samples consist of thermo-sensitive poly-N-isopropylacrylamide microgel (PNIPAM) particles, whose diameter can be tuned by small changes of sample temperature. A binary mixture of these particles is confined between two coverslips and forms a monolayer of quasi-2D glass. By changing the number ratio between large and small particles, the structure or the overall degree of disorder of the samples can be systematically tuned. We employ a video microscopy to record the motion of the colloidal particles in the sample for 11 min at a rate of 60 fps. The trajectories of individual particles are obtained by particle tracking software. Dynamical matrix is constructed using covariance matrix analysis, from which the eigenfrequency and eigenvector of vibrations are extracted. In this study, we focus on the evolution of the low-frequency quasi-localized phonon modes in glasses, as the system becomes more and more disordered from the increased dopants. To compare the results from different samples, we choose those with packing fraction of 86%, and rescale the eigenfrequencies by the median frequency of each sample. For the four doping levels investigated (2%, 9%, 29%, 61%), the density of states at low frequencies increases with the doping level, suggesting that the fraction of low-frequency modes increases with disorder, which is corroborated by the higher boson peaks at higher dopant fractions. We have measured the participation ratio of the obtained phonon modes, and find that the boson peak corresponds to quasi-localized vibration modes, or soft modes. We also examine the correlation between the soft modes and local structural parameter. Specifically, we have calculated the local orientational order parameter in our samples, and computed the correlation coefficients between the relative amplitude and the local orientational order parameter for each mode. The soft modes are found to have a significantly negative correlation with the local orientational order parameter, which implies that the soft modes are concentrated in regions with poor local order. We therefore conclude that the local disorder is probably the structural origin of soft modes in glasses.