Direct smectic A-smectic C-A* phase transition

The anticlinic smectic C-A phase belongs to the class of tilted smectic pha ses with an azimuthal angle alternating from one direction (phi = 0) to the other (phi = pi) in successive layers. It occurs in general at lower tempe rature than the uniformly tilted smectic C phase, but may be obtained direc tly from the untilted smectic A phase. We use the chiral nCTBB9* series syn thesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperatur e behaviour of the birefringence and of the optical rotatory power across t he transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C-A* soft modes. The main conclusion is that the only order parameter governing the critical behaviour of the phase transition is the tilt angle theta, as we get a common divergence of both soft modes at the s ame temperature. This confirms previous high resolution calorimetric studie s by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructur ing transitions between the tilted subphases.