THEORY OF LONGITUDINAL FERROELECTRIC SMECTICS

Recently, it was discovered that a-chiral smectic layers can undergo a phase transition into a polar smectic. The new phase is fundamentally different from that of the existing chiral Sm-C* ferroelectrics. In this paper we examine the underlying mechanism responsible for the new form of ferroelectricity. A Landau theory is presented to discuss the competition between ferroelectricity and other forms of ordering such as anti-ferroelectricity. An effective free energy is constructed to describe the coupling between layer undulations and the polarizability of the sample. We find that in the new phase the Landau-Peierls instability is absent and that second sound can propagate along the layers. Our considerations call for a number of experiments which we believe are crucial for a clarification of the true nature of the observed phases.