Dielectric biaxiality in smectic C* materials

Abstract

The dielectric biaxiality and dielectric anisotropy have been determined for helical smectic C* materials of varying pitch. The dielectric biaxiality was found to be positive, the dielectric anisotropy to be negative at 100 kHz. Thus, the permittivity component in the direction of the spontaneous polarisation (C₂ symmetry axis) has the largest value among the three principle components of the dielectric tensor, and ε₂ > ε₁ > ε₃. The dielectric biaxiality is further discussed as an order parameter of the tilted smectic phase. The critical behaviour is compared to that of the primary order parameter, the tilt angle, for a number of smectic C* compounds. It is verified that if the tilt increases according to θ∼(T_C–T)^β, the biaxiality increases according to δε(T_C–T)^2β.