THICKNESS DEPENDENT LOW-FREQUENCY RELAXATIONS IN FERROELECTRIC LIQUID-CRYSTALS WITH DIFFERENT TEMPERATURE-DEPENDENCE OF THE HELIX PITCH

A low frequency relaxation mode 1 has been detected in ferroelectric p hases of three materials. The relaxation frequency and the dielectric strength of this mode depend strongly on the sample thickness, but the ir temperature dependences are qualitatively different in the material s studied, reflecting the behaviour of the helicoidal structure of the se materials. This mode has been attributed to a superposition of the Goldstone and thickness modes when the helicoidal structure exists and to the thickness mode only, when the helix is unwound. In addition, w ith one material, mode 2 has been detected at still lower frequencies, and this also exhibits a strong sample thickness dependence. It was a ttributed to fluctuations of the director field modified by a non-homo geneous ionic charge distribution across the sample. As both modes 1 a nd 2 are strongly sample thickness dependent, they do not represent bu lk properties of the materials studied, but reflect the structure in r eal samples, which is determined by surface conditions.