Effect of the nematic range on the critical behavior and anisotropy of the heat transport parameters at the smectic- A –nematic phase transition

The photopyroelectric technique has been used to study the specific heat, the thermal conductivity, and the thermal diffusivity at the smectic-A--nematic phase transition of homeotropic and planar samples of 4-n{{-}pentylphenylthiol{-}4}^{{'}}$-$n-octyloxythiolbenzoate (8S5) and 4-n-nonyl-4^'-cyanobiphenyl (9CB). It has been shown that, as in the case of 4-n{{-}octyl{-}4}^{{'}}$-cyanobiphenyl (8CB), the thermal conductivity remains approximately constant at the smectic-$A$--nematic $(A{-}N)$ phase transition and therefore the specific heat and the thermal diffusivity have the same critical exponent. Though the $A{-}N$ transition in 9CB is quite close to the tricritical point, it turns out that tricriticality does not affect the critical behavior of the thermal conductivity. The temperature dependence of the thermal conductivity anisotropy, which can be considered as a macroscopic order parameter, has been reported for all the samples investigated. It has been shown that the influence of smectic ordering on the order parameter becomes progressively more important when the A-N tricritical point is approached.