The thickness mode contribution to the permittivity of ferroelectric liquid crystals

Dielectric properties of a material prepared of valine in the ferroelectric SmC* phase are studied in the frequency range 100 Hz divided by 1 MHz and for the sample thickness d between 6 and 100 mu m for both helicoidal and twisted sample structure. In all studied samples a low frequency relaxation has been found with the relaxation frequency proportional to 1/d(2) and with the contribution to permittivity proportional to d(2). we suggest this relaxation to be assigned to a 'thickness mode', which is a fluctuation of the molecular twist existing within the smectic layers in the direction of the sample plane normal. This twist deformation existing in both helicoidal and twisted samples is fixed by the anchoring on the sample surfaces. It is shown that the thickness mode is an important source of permittivity in the SmC* phase of the studied material.