二维胶体玻璃中的振动模式

发布时间：2018-10-10 10:41

【摘要】：本文研究了二维胶体玻璃中玻色峰随着无序度增加的变化规律，分析了引起玻色峰的声子模式的特点，并且探讨了声子模式和体系局域结构的相关性。样品是温敏性的微凝胶，混合直径大小不同的两种微凝胶放在两层盖玻片中间使其成单层，通过调节大小粒子的数目比来改变体系的无序程度。实验过程中用视频显微镜拍摄粒子的运动，拍摄速度是60帧每秒，总共拍摄时间是11分钟。拍摄到的视频用粒子跟踪技术得到粒子的位移随时间变化的数据，接下来用动力学矩阵的方法求解本征频率和本征矢量。本文关注二维胶体体系中随着无序程度的增加玻色峰的变化规律，所以不同无序程度的样品的面密度都选取86%。研究发现随着体系无序程度的增加态密度在低频区域增强、玻色峰增高。为了研究引起玻色峰的声子模式的特点，本文计算了体系中所有振动模式的参与比，发现玻色峰产生的频率范围对应的模式是低频准局域的。为了研究引起玻色峰的低频准局域模式的结构特点，本文计算了不同频率下粒子的相对振幅和局域方向序参数的关联，发现低频准局域模式与局域无序结构有很强的负相关，即低频准局域模式往往产生于局域结构无序的位置。接着本文计算了体系中局域结构（包括自由体积、近邻数目和局域熵）的涨落（当地粒子与其周围近邻粒子的结构的差异），同样也计算了粒子的相对振幅与局域结构涨落的关联，，研究发现局域结构涨落大的位置与低频准局域模式有很强的相关性。所以低频准局域模式产生于局域结构无序的位置和局域结构不均匀的位置。
[Abstract]:In this paper, the variation law of boson peak with the increase of disorder in two-dimensional colloidal glass is studied, the characteristics of phonon mode which causes the boson peak are analyzed, and the correlation between phonon mode and local structure of the system is discussed. The samples are thermo-sensitive microgels. The mixed microgels with different diameters are placed in the middle of the two layers of glass slide to form a monolayer. The disorder degree of the system is changed by adjusting the ratio of the number of particles. During the experiment, the motion of particles was captured with a video microscope. The shooting speed was 60 frames per second, and the total shooting time was 11 minutes. The video recorded by particle tracking technique is used to obtain the data of the displacement of particles with time, and then the eigenfrequency and eigenvector are solved by the method of dynamic matrix. In this paper, we focus on the change of boson peak with the increase of disorder in two-dimensional colloidal system, so the surface density of samples with different degree of disorder is 86%. It is found that the density of states increases in the low frequency region and the boson peak increases with the increase of the disorder degree of the system. In order to study the characteristics of phonon modes which cause the boson peak, the participation ratio of all vibration modes in the system is calculated. It is found that the mode corresponding to the frequency range of the boson peak is low frequency quasi-local. In order to study the structural characteristics of the low-frequency quasi-local mode that causes the boson peak, the correlation between the relative amplitude of particles and the local directional order parameters at different frequencies is calculated. It is found that the low-frequency quasi-local mode has a strong negative correlation with the local disordered structure. That is, the low frequency quasi-local mode often comes from the disordered position of the local structure. Then the local structures (including free volume) in the system are calculated. The fluctuation of the number of nearest neighbors and the local entropy (the difference between the structure of the local particle and the nearby particle) is also calculated, and the correlation between the relative amplitude of the particle and the fluctuation of the local structure is also calculated. It is found that there is a strong correlation between the location of local structure fluctuation and the low frequency quasi-local model. So the low frequency quasi-local mode comes from the disordered position of the local structure and the non-uniform position of the local structure.
【学位授予单位】：北京理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ171.1;O648.1

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