DIELECTRIC RESPONSE OF FERROELECTRIC-S(C)ASTERISK LIQUID-CRYSTALS IN A BIAS ELECTRIC-FIELD

The dielectric response of the ferroelectric chiral smectic C phase in a bias electric field is studied theoretically within the Landau mode l using the constant amplitude approximation. It is argued that the re sponse consists of two modes, one of which is related to the unwinding of the helix (the unwinding mode), whereas the other mode is associat ed with the distortion of the polarization distribution at constant pi tch of the helix (the Goldstone mode). The relaxation frequency of the unwinding mode (f(U)), which is inversely proportional to the square of the sample dimension along the helical axis, is estimated to be of the order of 10(-4) Hz for a sample of length l almost-equal-to mm alo ng the helical axis. Consequently, in dielectric experiments performed usually at frequencies higher than f(U), only the Goldstone mode cont ribution is detected. In this way, we can explain the behaviour of the measured static dielectric susceptibility which goes to zero as the c ritical field is approached from below, although the model predicts th at the total static dielectric susceptibility, being a sum of the Gold stone mode and the unwinding mode contribution, diverges at the critic al field.