INVESTIGATION OF THE TGBA-ASTERISK PHASE IN A FERROELECTRIC LIQUID-CRYSTAL USING DIELECTRIC-SPECTROSCOPY

The dielectric response of a ferroelectric liquid crystal with the TGBA* phase has been investigated over the frequency range 0.1-10(6) Hz. The dielectric properties as a function of temperature and direct bias voltages of the TGBA* phase exhibit behaviour different from that in the smectic A (SmA) phase. From the dielectric spectra, two relaxation processes, instead of only one process (the soft mode) previously known, have been found for the helical TGBA* phase in the absence of the direct bias voltage. These are assigned to (a) the soft mode and (b) a relaxation process in the grain boundaries. The latter process is found for the first time. The dielectric response for the SmC*-TGBA* transition suggests a weakly first-order phase transition. The effect of direct bias voltage on the dielectric spectra of the TGBA* phase differs from that of the SmA phase in terms of (a) the relaxation frequency of the soft mode decreases instead of increasing with increase in the bias voltage-suggested to arise because the TGB helix is unwound by the electric field, and (b) a relaxation process is found to emerge for higher bias voltages. The latter is assigned to the deformation process of the structure because the TGB helix is disturbed by the electric field.