Soft Mode Relaxation Frequency Research Articles

Effect of smectic A temperature width on the soft mode in ferroelectric liquid crystals

The behavior of soft mode range with respect to the temperature width of smectic A (Sm A) phase has been studied in four different ferroelectric liquid crystal (FLC) materials in the frequency range 10Hz–10MHz. The studies have been carried out in a planarly well aligned cells at different temperatures and different bias fields in Sm C* and Sm A phases. Dielectric studies of these FLCs near Sm C*-Sm A phase transition show that the temperature range of soft mode relaxation frequency phenomenon varies with the temperature width of Sm A phase. The dependence of tilt angle on temperature shows the nature of the order of transition at Sm C*-Sm A phase. The coupling between order parameters of Sm C* and Sm A phase influences the soft mode and phase transition in Sm C* and Sm A phases.

Time-resolved measurements of the dynamics of the photoinduced smectic-C* alpha-smectic-A transition.

We present the study of the dynamics of the recently reported photoinduced smectic-C(*)(alpha)-smectic-A transition. High resolution time-resolved dielectric dispersion measurements carried out during the photoisomerization process demonstrate that the magnitude of the uv intensity mimics the role played by temperature in determining the behavior of the soft mode relaxation. We also show that the uv intensity dependence of the soft mode relaxation frequency f(R) in the photoinduced smectic-A phase can be described with a functional form similar to that derived for the temperature dependence of f(R) and compare the experimentally determined critical exponent with the theoretically predicted for the Ising and three-dimensional XY universality classes. Our study illustrates an interesting feature, namely, the magnitude of light intensity can be treated like a thermodynamic variable such as temperature and pressure to study phase transitions in general.

Comparative x-ray and dielectric measurements of smectic A-smectic-C* transition in bulk and confined geometries.

Comparative x-ray and dielectric measurements have been made on a liquid crystal exhibiting a smectic-A-chiral-smectic-C (smectic-C*) transition in bulk and confined geometries. It is observed that confining the material in Anopore membranes having 200-nm pore size leads to the following features: (1) the temperature dependence of the x-ray layer spacing shows a qualitatively different behavior, (2) in the smectic-A phase the soft mode relaxation frequency increases by a factor of 2.5, and (3) in the smectic-C* phase the relaxation frequency of the Goldstone mode increases dramatically by as much as 400 times, perhaps owing to a partial unwinding of the helix by the surface induced field.

Mean-Field Behavior of the Paraelectric-Ferroelectric Phase Transition of a Fluorinated Compound

The partially fluorinated MHPOBC analogue (MHPO(F)PBC) shows an attractive phase sequence: SmA*, SmC f *, SmC* and SmC A *. The SmC f * phase was only observed by reversal current switching method. Measurements of spontaneous polarization P s (T) reveal the existence of the SmC f * sub-phase between the SmA* and SmC* phases. Linear dielectric spectroscopy studies corroborate almost model behavior of the soft mode relaxation frequency - obtained under bias field of 20V/20 w m -in the vicinity of the SmA*-SmC* phase transition.

Soft Mode Dielectric Relaxation Under the Influence of Bias Electric Field of a Ferroelectric Liquid Crystal Mixture

Temperature dependent soft mode part of the complex dielectric constant has been measured as a function of bias electric field both in the smectic C* and smectic A* phases for a ferroelectrc liquid crystal (FLC) mixture. From the experimental data, soft mode relaxation frequency and dielectric strength have been determined both in smectic C* and smectic A* phases. In smectic A* phase, relaxation frequency and inverse of dielectric stmngth of the FLC mixture does not follow the Curie-Weiss law. The critical exponent for the studied FLC mixture is 1.43. Dielectric results shows a dependence of soft mode dielectric strength and relaxation frequency with bias field near the tmudion temperature in smectic A* phase to an extent of 1°C above the transition tanperature and above 1°C bias field does not have any effect on relaxation frequency and dielectric strength It is observed that relexation frequency increases with increasing bias field while the compounding dielectric strength decreases.

Dielectric and electrooptical study of a new antiferroelectric liquid crystal with the thiobenzoate group

Abstract A new thiobenzoate compound exhibits polar phases SmCα*, ferroelectric SmC*, ferrielectric SmC* and antiferroelectric SmC*γ. From the electrooptic intensity and the permittivity measured in the frequency range 10 ÷ 106 Hz the soft mode relaxation frequency in the paraelectric SmA phase and the Goldstone mode relaxation frequency in the polar phases have been determined. In the polar phases the temperature dependences of both the spontaneous polarizarion and the tilt angle are measured under an electric field high enough to bring about the saturated ferroelectric state. The dependencies are smooth within the entire temperature range studied indicating all the studied phases are indistinguishable under high electric field.

Electrooptic properties of chiral smectic liquid crystals with a dipolar order

Abstract The compound 4-(1-methylheptyloxycarbonyl)phenol 4′-octyloxy biphenyl-4-carbonate exhibits ferroelectric S*C, paraelectric SA, antiferroelectric S*CA and two ferrielectric S*Cα and S*Cγ phases. From the electrooptic response measured in the frequency range 100 Hz to 100 kHz the soft mode relaxation frequency in the paraelectric phase and the Goldstone mode relaxation frequency in the ferrielectric and ferroelectric phases have been determined. In the S*Cγ phase the shape of the real and imaginary parts of the electrooptic frequency dispersion reflects dipolar disorder. In the ferrielectric, ferroelectric and antiferroelectric phases the temperature dependencies of both the spontaneous polarization and the tilt angle are measured under an electric field high enough to enforce the saturated ferroelectric state. The dependencies are smooth within the entire temperature range studied indicating all the studied phases are indistinguishable under high electric field.

The effect of biasing electric field on relaxations in FLC investigated by the dielectric and optical methods

Abstract The dielectric and electrooptical responses within the frequency range 30 Hz–100 kHz are used to study the properties of the soft and the Goldstone mode in dependence of bias electric field. It is shown that the soft mode relaxation frequency is linearly bias dependent in both SmA and SmC* phases, the Goldstone mode relaxation frequency is constant for small bias. After the modulated structure is unwound a low frequency mode persists a relaxation frequency of which also linearly increases with increasing bias. This mode is connected with vibration of molecules around the layer normal keeping the constant tilt angle, which could take place in surface layers. The Cole—Cole diagrams constructed from the optical response are obtained for all studied modes in the shape of semicircles, showing all the modes have relaxation character with well defined relaxation frequencies. The bias field dependencies of both optical and dielectric responses in the SmC* phase reflect the unwinding process of the helico...

Dielectric studies of the soft mode and Goldstone mode in ferroelectric liquid crystals

Abstract Different contributions to the dielectric permittivity in ferroelectric liquid crystals are discussed, with emphasis on the soft mode and the Goldstone mode and their location in the dielectric spectrum. Experimentally, the complex dielectric permittivity has been studied as a function of temperature and frequency in the range 5 Hz - 13 MHz for three different ferroelectric liquid crystal materials. The main problems encountered in dielectric measurements at low and high frequency are discussed in some detail. The soft mode dielectric behaviour has been studied as a function of temperature, frequency and bias electric field. The applicability of the Curie-Weiss law for the soft mode dielectric contribution in the A* phase was analyzed. In the C* phase the temperature dependence of the dielectric contribution of the Goldstone mode has been measured. By applying a bias electric field, we have been able to study the soft mode dielectric behaviour also deep into the C* phase. In the A* and C* phases the relaxation frequency and the inverse of the dielectric strength of the soft mode vary linearly with |T-TC|, which can be used to determine the A*-C* transition temperature to a good accuracy. The ratio of the slopes of the soft mode relaxation frequency vs temperature line and the inverse of dielectric strength vs temperature line in the C* and A* phases has been calculated for the three compounds and compared with solid ferroelectric systems. The soft mode rotational viscosity in the A* phase has been calculated for one of the compounds.

Soft Relaxations Near the Ferroelectric Phase Transition in Liquid Crystals

Abstract The dispersion of dielectric and optical properties found in experiment yields the relaxation frequencies of the polarization P and the tilt angle θ. The temperature dependences of these frequencies are compared to those obtained from the free energy F(P, θ) describing the ferroelectric phase transition. This comparison shows that order parameters P and θ are strongly coupled in the whole temperature region studied, both showing the same soft-mode relaxation frequency in the SA phase. In the ferroelectric SC . phase the soft-mode relaxation frequency has been detected only when the helicoidal structure has been unwound by a d.c. electric field. On the other hand this d.c. field influences the soft-mode relaxation frequency in the vicinity of the phase transition and the soft-mode dielectric strength even in the whole Sc phase.

The effect of biasing electric field on the soft mode in the vicinity of the ferroelectric phase transition in liquid crystals

Abstract The complex permittivity of a room-temperature ferroelectric liquid crystal 4-n-octyloxy benzoic acid 4′-[(2-methylbutyloxy)carbonyl]phenyl ester, has been measured in the vicinity of the phase transition in the frequency range 40 Hz-300 kHz. In the para-electric phase the contribution εs of the soft mode to the permittivity and the soft-mode relaxation frequency f s satisfy the Curie-Weiss law. Under a biasing field E, the helicoidal structure is unwound, and εs and f s can then be measured even in the ferroelectric phase. On the other hand, the phase transition is smeared under the influence of E. This smearing results in deviations from the Curie-Weiss law for both εs and f s in the vicinity of the transition. On increasing E, the maximum of the permittivity, εmax, is lowered and shifted to higher temperatures. Both the shift and ε−1 max are proportional to E 2/3. From experimentally found dependences, some constants in the free energy are determined.

Soft mode behaviour of a room temperature ferroelectric liquid crystal

Abstract The complex dielectric permittivity was studied in the frequency region 100 Hz ÷ 100 kHz in the vicinity of the SA→ SC* phase transition temperature Tc = 32°C for benzoic acid 4-octyloxy 4–|(2-methyl butyloxy) carbonyl | phenylester. The d.c. electric field of the intensity E = 4 kV/cm was used to unwind the helicoidal structure in S C* phase, therefore the contribution to ϵ from the Goldstone mode was eliminated. It was found that at 100 Hz the temperature dependence of the soft node contribution had the form of the Curie-Weiss law in both SA and SC* phases. At higher frequencies the maximum of ϵ at TC is lowered and at frequencies 10 ÷ 40 kHz a minimum appears. This behaviour was explained by the temperature dependence of the soft mode relaxation frequency fS which was evaluated by fitting the complex permittivity data to the Debye relaxation formula. It was established that fS depended linearly on temperature in SA as well as in SC* phases with a minimum of 1 kHz at TC.