Random-field effects on the nematic–smectic-A phase transition due to silica aerosil particles

A high-resolution calorimetric study has been made on the smectic-A to nematic phase transition in liquid crystals perturbed by the dispersion of 70-Å-diam silica spheres ~aerosils!. The specific heat peak of this transition decreases in amplitude and shifts to lower temperatures with increasing silica density. However, it remains sharp in comparison to the effects of aerogel confinement at the same silica densities. The bulk transition in octylcyanobiphenyl ~8CB! exhibits a crossover critical exponent a50.30, whereas on addition of aerosils the specific heat peak takes on an increasingly asymmetric appearance reminiscent of threedimensional ~3D! XY criticality. Power-law fits reveal an effective a continuously decreasing from 0.30 to 20.03 with increasing silica density. No changes in the critical behavior are found when the bulk liquid crystal already possesses a small negative a, i.e., 3D XY criticality, as shown in octylphenylthiol-pentyloxybenzoate. Apparently, the silica-dopant-induced random field drives the critical behavior towards 3D XY universality in 8CB by changing the nature of the coupling between the nematic and smectic order parameters. The nematic to isotropic transition in 8CB plus aerosils has also been observed; it exhibits a bimodal character that deviates from observations in aerogel systems. @S1063-651X~97!09503-2#