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Effect of a quenched random field on a continuous symmetry breaking transition: Nematic to smectic-A transition in octyloxycyanobiphenyl-aerosil dispersions

Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN: 1063-651X, Vol: 67, Issue: 2, Page: 13-null
2003
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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-[Formula presented] 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-[Formula presented] 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 [Formula presented] becomes progressively smaller. For the samples with the largest concentrations of aerosil particles the [Formula presented] peak becomes highly smeared and shifts smoothly to lower temperatures. © 2003 The American Physical Society.

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