INDUCED POLARIZATION IN THE SmA*-PHASE: A COMPARISON OF THEORETICAL MODELS AND EXPERIMENTAL METHODS

Abstract

When measuring the po larization of a ferroelectric liquid crystal one usually obtains quite a pronounced tail P(T) going well into the SmA *-phase. If this is a manifestation of the elcctrodinic effect one would, at first thought, expect it to scale as P-(T-Tct and as P-E. In reality, however, it scales like P-( T-Tc ( and P-£ 3 . In order to find out whether this is consistent with the Landau description of the transition one has to check the precise character of the quantity P actually furnished by the measurement. A recent theoretical model based on the triangle-wave technique combined with the Landau theory for the temperature dependence of the induced polarization in the SmA *-phase is experimentally verilied in a number of different compounds. Measurements with the bridge method also confirm the model, indicating the equivalence of the bridge and triangle-wave techniques. The concept of measured (observed) polarization as distinguished from polarization as order parameter in the Landau theory, is pointed out and their relation discussed.

Thermodynamics of the photoferroelectric effect in smectic-C* liquid crystals

“…discussion outlined in the previous section indeed clearly applies to the experimental situation: In the nonirradiated state P s (T) continuously decreases to zero, reflecting the second-order nature of the ferroelectric smectic C* -A* transition at about T c Ϸ327 K. The slight tailing of the P s (T) curve into the paraelectric Sm-A* phase originates from the nonlinearity of the electroclinic effect that is induced by the electric field which we applied to measure the spontaneous polarization. 26 Upon irradiation the phase transition is considerably shifted through the secondary photoferroelectric effect to about T c ЈϷ325 K. If we now shift the P s (T) curve of the nonirradiated state to the new T c Ј ͑dotted line in Fig. 3͒, we still observe a clear difference compared to the P s (T) actually measured in the irradiated state.…”

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives

“…47 Due to the coupling between P S and h, the temperature dependence of P S near T C should also reflect the nature of the SmA*-C* phase transition. However, P S measurements by integration of polarization reversal current peaks are complicated by induced polarization contributions near T C due to the electroclinic effect (soft mode), 48 although the latter can often be minimized by using low triangular ac fields. The P S (T 2 T C ) plot for the 5 mol% mixture of 3a in 2-PhP (Fig.…”

Simulations of helix unwinding in ferroelectric liquid crystals

“…To compare our results with experimental measurements of the polarization as a function of electric field, we must take into account one subtlety of the experiments. As shown by Ruth et al [19,20], the polarization observed experimentally (by the triangle-wave technique or other techniques) is not the total polarization conjugate to the electric field. Rather, it is a specific nonlinear component of the polarization, which can be written as…”

“…As shown by Ruth et al [19,20], the polarization observed experimentally (by the triangle-wave technique or other techniques) is not the total polarization conjugate to the electric field. Rather, it is a specific nonlinear component of the polarization, which can be written as…”