Nonlinear dielectric relaxation spectroscopy of the antiferroelectric liquid crystal 4-(1-trifluoromethyl-pheptyloxycarbonyl) phenyl 4'-octyloxybiphenyl-4-carboxylate

The linear and third-order nonlinear dielectric constants of the antiferroelectric liquid crystal 4-(1-trifluoromethyl-pheptyloxycarbonyl) phenyl 4'-octyloxybiphenyl-4-carboxylate were experimentally studied in its smectic-A (Sm-A) and smectic-C-A* (Sm-C-A*) phases. Both dielectric constants showed a critical behavior near the Sm-A-Sm-C-A* phase transition corresponding to the softening of ferroelectric soft mode and the increase of the fluctuation in the tilt direction. In the Sm-Ca-A* phase, no relaxation was observed in the linear spectrum, but the third-order nonlinear dielectric spectrum showed a Debye-type relaxation with a relaxation frequency of a few kHz corresponding to the Goldstone mode in the Sm-C-A* phase. This nonlinear response was theoretically analyzed and ascribed to a field-induced biaxiality, i.e., the deformation of the helical structure due to a coupling between the dielectric anisotropy and the electric field. The temperature dependence of the nonlinear spectrum was also discussed.