Instabilities across the isotropic conductivity point in a nematic phenyl benzoate under AC driving

We characterize the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes. The material, which possesses a negative dielectric anisotropy ε_a and an inversion temperature for electrical conductivity anisotropy σ_a, exhibits a monostable tilted alignment near T_IN, the isotropic-nematic point. On cooling, an anchoring transition to the homeotropic configuration occurs close to the underlying smectic phase. The field experiments are performed for (i) negative σ_a and homeotropic alignment, and (ii) weakly positive σ_a and nearly homeotropic alignment. Under ac driving, the Freedericksz transition is followed by bifurcation into various patterned states. Among them are the striped states that seem to belong to the dielectric regime and localized hybrid instabilities. Very significantly, the patterned instabilities are not excited by dc fields, indicating their possible gradient flexoelectric origin. The Can-Helfrich mechanism-based theories that take account of flexoelectric terms can explain the observed electroconvective effects only in part.