Surface-induced ordering effect in one antiferroelectric liquid crystal compound

Null transmission ellipsometer was employed to study the surface-induced tilt transition of an antiferroelectric liquid crystal compound in the bulk smectic-$A$ $\left(\text{Sm}A\right)$ temperature range. The data of the surface tilt transition can be fitted with an extended mean-field model. These results indicate the transition is near a mean-field tricritical point. The surface layers’ tilt directions are found to be in different planes several degrees above the bulk $\text{Sm}A\text{-Sm}{C}_{\alpha }^{\ast }$ transition.

Figure 1

(Color online) $\Delta$ and $\Psi$ as a function of temperature upon cooling from an 81-layer film, with $\alpha =90°$ (black dots) and $270°$ (red triangles). Inset shows raw data obtained from a cooling run while switching the direction of $E$. On the top is the chemical structure of MHPBC

Figure 2

(Color online) ${\Delta }_{270}-{\Delta }_{90}$ as a function of $T_s-T$ from 20- (black dots), $81\pm 2$- (red stars), and $800\pm 30$- (green triangles) layer films obtained from cooling runs. $T_s=100.65°\text{C}$ for the films shown.

Figure 3

(Color online) Extended mean-field theory fitting of ${\left({\Delta }_{270}-{\Delta }_{90}\right)}^2$ from the 800-layer film.

Figure 4

(Color online) (a) $\Delta$, $\Psi$ vs $\alpha$ data (symbols) and fits (solid lines) from a 20-layer film at (a) $81.5°\text{C}$ (black dots) and (b) $76°\text{C}$ (red triangles). (b) $\Delta$ vs $\Psi$ plot for the rotation data. (c) Cartoons and top views of the corresponding layer structures from the fitting. In the top view, black (longer) rods represent the outermost surface layers while red rods (shorter) represent the layers adjacent to the surfaces. Note: the change in $\Psi$ over the whole range of $\alpha$ is extremely small (approximately $0.02°$). Our resolution in $\Psi$ is about $0.002°$. This is the main reason for the scattering of the $\Psi$ vs $\alpha$ data.