Polar switching in the smectic-$A_d{P}_A$ phase composed of asymmetric bent-core molecules

We have studied mesogenic properties in the smectic-$A_d{P}_A$ phase of an asymmetric bent-core liquid crystal by means of polarizing optical microscopy, second-harmonic generation (SHG), electro-optical (EO), and dielectric measurements. In homeotropically aligned cells, EO switching is clearly observed from a schlieren texture with both four- and two-brush defects to a uniform bright domain under the application of very low in-plane electric field below $1\text{V}/\mu \text{m}$. The phase is SHG active under an electric field, i.e., SHG gradually increases without a threshold and saturates with the increasing field. In homogeneous cells, by applying triangular wave field at saturated voltage, the splitting of the polarization switching current peak is observed, indicating that the mesophase is antiferroelectric. The dielectric studies indicate a Debye-type relaxation of the transverse dipoles associated with the rotation of the molecules about their long axis. The dc bias-dependent dielectric relaxation time and the dielectric strength suggest that a field-induced antiferroelectric-ferroelectric phase transition occurs continuously beyond $1.2\text{V}/\mu \text{m}$ and is reversible. The field-dependent texture observation is consistent with the dielectric measurements. Two possible models are proposed to interpret the continuous phase transition.

Figure 1

(Color online) (a) Chemical structure and phase sequence of the compound used in the experiment. (b)–(e) Planar texture of a $12.8\text{−}\mu \text{m}$-thick cell at $145°\text{C}$ under various vertical dc fields. (f)–(i) Homeotropic texture of a $5.6\text{−}\mu \text{m}$-thick cell at $138°\text{C}$ under various in-plane dc fields.

Figure 2

(a) SHG intensity as a function of applied voltage at $140°\text{C}$. (b) SHG intensity as a function of temperature in both cooling (squares) and heating (open circles) runs in a homogeneous sample at an applied rectangular voltage of $80{\text{V}}_{\text{pp}}$ and at a frequency of 10 Hz. Cell thickness is $8.9\mu \text{m}$.

Figure 3

(a) Switching current in the $\text{Sm-}{A}_d{P}_A$ phase by applying a triangular wave voltage of $120{\text{V}}_{\text{pp}}$ and with the frequency of 9 Hz at $138°\text{C}$. (b) Spontaneous polarization $\left(P_s\right)$ as a function of temperature. Cell thickness is $4.8\mu \text{m}$.

Figure 4

Dielectric permittivity in the $\text{Sm-}{A}_d{P}_A$ phase. (a) Real part $\left({\epsilon }^{\prime }\right)$ and (b) imaginary part $\left({\epsilon }^{″}\right)$ as functions of frequency at various relative temperatures $\left(T-T_{I-\text{Sm}}\right)$. (c) The effect of dc bias field on the dielectric constant $\left({\epsilon }^{\prime }\right)$ at $T-T_{I-\text{Sm}}=-10°\text{C}$. Cell thickness is $12.8\mu \text{m}$.

Figure 5

Cole-Cole plot of the dielectric permittivity at $138°\text{C}$.

Figure 6

Variation of (a) dielectric strength $\left(\Delta \epsilon \right)$ and (b) relaxation time $\left(\tau \right)$ as a function of relative temperatures.

Figure 7

Variation of relaxation time $\tau$ (squares) and the dielectric strength $\Delta \epsilon$ (open circles) as a function of dc bias field.

Figure 8

Simple model of the field-induced antiferroelectric-ferroelectric transition. Black arrows connected to the arms of bent cores stand for the stronger dipole of $\text{C}\text{N}$.

Figure 9

(Color online) (a) and (b) Homeotropic textures under polarizers decrossed in the opposite direction. White arrows represent polarizer and analyzer. The dark regions with their brightness interchanged by rotating the analyzer clockwise or counterclockwise are chiral domains.