MANIFESTATION OF BIQUADRATIC COUPLING IN THE SMECTIC-C-ASTERISK PHASESOFT MODE DIELECTRIC RESPONSE

The soft mode dielectric response of three ferroelectric chiral smecti c C substances is studied in the vicinity of the smectic C*-smectic A phase transition. For some substances the soft mode dielectric stren gth exhibits a pronounced anomaly in the C phase just below the phase transition. We show how this anomaly can be systematically correlated with a corresponding anomaly in the experimentally determined ratio o f spontaneous polarization over tilt. By comparing the experimental re sults with the theoretical predictions of an extended Landau model, we demonstrate how the crossover behaviour of the system can be interpre ted as being the consequence of the presence of a biquadratic coupling between tilt and polarization in the Landau free-energy of the system .