Spontaneous polarization of helicoidal ferroelectric smectic C* liquid crystals

Measurements of the spontaneous polarization of DOBAMBC in helicoidal smectic C* phase is presented. It is shown that the polarization shows an anomalous temperature dependence characterized by one square root behaviour close to Tc and by another one far from Tc. This behaviour is explained theoretically to be a consequence of a nonchiral biquadratic coupling between the polarization and the tilt of molecules in the Landau expansion of the free-energy density. Also the tilt is predicted to exhibit a (less pronounced) crossover region of the same type as the polarization within the proposed model. It is also noted that this crossover region coincides with the temperature interval where the pitch of the helix displays anomalous properties.