HIGH-FREQUENCY DIELECTRIC BEHAVIOR OF A FERROELECTRIC LIQUID-CRYSTAL NEAR THE SMECTIC-C-ASTERISK-SMECTIC-A PHASE-TRANSITION

The temperature and frequency dependence of the complex dielectric susceptibility of a ferroelectric liquid crystal (FLC) near the smectic-C*-smectic-A phase transition has been calculated using the generalized Landau model. It is shown that, although the dielectric response of the Sm-C* phase consists generally of four modes (soft, Goldstone, and two high-frequency polarization modes), only three bands appear in the dielectric-loss spectrum of FLC's at the Sm-A -Sm-C* phase transition. The calculations based on the generalized Landau model show that the frequency split of the two high-frequency modes is too low to be detected as two separate relaxation processes. A special technique has to be used to split these modes. At the Sm-A - Sm-C* phase transition this process does not split or broaden. These results are in agreement with the recent experimental data. It is shown that in the light of these calculations a revision of the theory as proposed by Pleiner and Brand [Phys. Rev. A 43, 7064 (1991); Mol. Cryst. Liq. Cryst. Lett. 8, 11 (1991)] is rather unnecessary.