Quasi-two-dimensional Ising critical behavior of de Vries liquid crystals observed in the heat capacity and dielectric response

Heat capacity and dielectric measurements have been made on a liquid crystal compound exhibiting de Vries type of smectic-$A^{*}$ $\left(\mathrm{Sm}\text{−}{A}^{*}\right)$ phase. Heat capacity shows a significant anomaly which is almost symmetric for above and below the $\mathrm{Sm}\text{−}{A}^{*}\text{−}\mathrm{Sm}\text{−}{C}^{*}$ transition temperature. The transition was found to be very weakly first order. The critical exponent $\gamma$ determined from the dielectric data lies $1.8\pm 0.2$. The present heat capacity data as well as former data for another compound of de Vries type have been analyzed in detail. It was found that the heat capacity data for both compounds are fitted well with a logarithmic divergence except in the immediate vicinity of the transition. These results agree with an expectation that de Vries $\mathrm{Sm}\text{−}{A}^{*}\text{−}\mathrm{Sm}\text{−}{C}^{*}$ transition can exhibit quasi-two-dimensional Ising critical behavior.

Figure 1

Molecular structure of TSiKN65 [7, 17].

Figure 2

Temperature dependence of heat capacity $C_p$ of TSiKN65. The data are acquired in a cooling run.

Figure 3

Comparison of heat capacity data for TSiKN65 in the vicinity of $T_c$ obtained in the NAS mode (open circles) and in the AC mode (closed circles).

Figure 4

Typical Cole-Cole plot obtained from the dielectric measurement for TSiKN65. The solid line is a fitting curve with Eq. (2).

Figure 5

Temperature dependence of reciprocal of the relaxation strength $\Delta ϵ$ obtained for TSiKN65. The solid line is a fitting curve obtained using Eq. (4) with $\gamma$ fixed at 1.75 (see text).

Figure 6

Molecular orientation in smectic phases. Thick arrow represents the liquid crystal molecule.

Figure 7

(Color online) Semilogarithmic plot of $\Delta C_p$ data for TSiKN65. Straight lines show calculated values with Eq. (6).