Second-harmonic dielectric response of an antiferroelectric liquid crystal under dc electric fields

Second-harmonic dielectric response under dc fields in an antiferroelectric liquid-crystal 4-(1-methyl-heptyloxycarbonyl)phenyl 4-octylcarbonyloxybiphenyl- 4-carboxylate (MHPOCBC) has been investigated. The second-harmonic response from the amplitude mode in the smectic- Calpha* phase was observed due to the symmetry breaking by the application of dc electric fields. A Landau theory was developed in order to analyze experimentally obtained frequency dispersions. From the analysis we found a softening of the amplitude mode with increasing the dc field at a temperature close to the smectic- Calpha*-smectic- A phase transition point. It was turned out that the relaxation frequency of the amplitude mode decreases linearly with the applied electric field.