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 phases of three materials. The relaxation frequency and the dielectric strength of this mode depend strongly on the sample thickness, but their temperature dependences are qualitatively different in the materials studied, reflecting the behaviour of the helicoidal structure of these 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, with one material, mode 2 has been detected at still lower frequencies, and this also exhibits a strong sample thickness dependence. It was attributed to fluctuations of the director field modified by a non-homogeneous ionic charge distribution across the sample. As both modes 1 and 2 are strongly sample thickness dependent, they do not represent bulk properties of the materials studied, but reflect the structure in real samples, which is determined by surface conditions.