Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration. We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought.     

漏缺数据平稳时间序列下的谱密度估计

在天文学和医学领域,非均匀抽样数据的谱什计已获得广泛研究.这些研究通常是在周期性的探测和决定性的信号环境里进行的.本文从另外的角度考虑估计漏缺数据平稳时间序列的谱密度,提出了一种渐近无偏的估计方法.使用模拟方法把它与古典周期图、LOMB周期图以及基于SVD的周期图进行对比,结果显示这种新方法很大程度上降低了偏差.