Effect of a quenched random field on a continuous symmetry breaking transition: nematic to smectic-A transition in octyloxycyanobiphenyl-aerosil dispersions.
Effect of a quenched random field on a continuous symmetry breaking transition: nematic to smectic-A transition in octyloxycyanobiphenyl-aerosil dispersions.
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猝灭随机场对连续对称破缺转变的影响：辛氧基氰基联苯-气相二氧化硅分散体中向列相到近晶-A 的转变。
DOI：
10.1103/physreve.67.021703
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发表时间：
2003
期刊：
影响因子：
--
通讯作者：
G. Iannacchione
中科院分区：
文献类型：
--
作者：
Paul S. Clegg;Chris Stock;R. Birgeneau;Carl W. Garland;A. Roshi;G. Iannacchione
对分散有气溶胶粒子的液晶辛氧基氰基联苯进行了高分辨x射线衍射和交流量热实验。测量是在体相到近晶-A转变温度附近的温度范围内进行的。在该转变处，液晶在单个方向上打破平移对称性。氢键结合在一起形成极低密度凝胶的二氧化硅颗粒提供淬灭无序。随机凝胶导致近晶层的X射线反射的可观察到的增宽。结构因子是很好地描述了模型的效果，aerosils作为一个淬火的随机场。分散的气溶胶被认为是钉的平移顺序和层的位置的方向。后者似乎对X射线线形的影响最大。我们表明，aerosil表面积，小角度散射验证，等同于随机场的方差。量热的结果表明，在比热峰与近晶-A过渡的显着变化。随着气溶胶浓度的增加，比热峰保持尖锐，但幅度减小，温度以非单调的方式变化。在这种情况下，临界指数α逐渐变小。对于具有最大浓度的气溶胶颗粒的样品，C（p）（N-A）峰变得高度模糊，并平滑地向较低温度移动。
High-resolution x-ray diffraction and ac-calorimetric experiments have been carried out on the liquid-crystal octyloxycyanobiphenyl in which aerosil particles are dispersed. The measurements were made over a temperature range around the bulk nematic to smectic-A transition temperature. At this transition the liquid crystal breaks translational symmetry in a single direction. The silica particles, which hydrogen bond together to form a very low density gel, provide the quenched disorder. The random gel leads to observable broadening of the x-ray reflection from the smectic layers. The structure factor is well described by modeling the effect of the aerosils as a quenched random field. Dispersed aerosils are thought to pin both the direction of the translational ordering and the position of the layers. The latter appears to have the greatest effect on the x-ray line shape. We show that the aerosil surface area, as verified by small-angle scattering, equates to the variance of the random field. Calorimetric results reveal substantial change in the specific heat peak associated with the nematic to smectic-A transition. As the concentration of aerosil increases, the specific heat peak remains sharp yet decreases in magnitude and shifts in temperature in a nonmonotonic fashion. In this regime, the critical exponent alpha becomes progressively smaller. For the samples with the largest concentrations of aerosil particles the C(p)(N-A) peak becomes highly smeared and shifts smoothly to lower temperatures.