EXPERIMENTAL PROBES OF BOND-ORIENTATIONAL ORDER MODELS OF BLUE PHASE-III OF CHIRAL LIQUID-CRYSTALS

Recently, a cubic bond orientational order model was proposed for blue phase (BP) III [Phys. Rev. Lett. 71, 2757 (1993)]. It was suggested t hat in blue phase III, although there is no periodic translational sym metry, a cubic orientational order persists. A spatially independent f ourth-rank. tenser B-alpha beta mu nu(4) was used to describe the cubi c bond orientational order. In this paper, we show that since BP III i s chiral, the bond orientational order parameter is expected to vary i n space. Depending on the relative magnitudes of the elastic constants in the free energy, the cubic bond orientational order will twist alo ng either a fourfold ([001]) or a threefold ([111]) symmetry direction . Due to the twist, the cubic symmetry of the bond orientational order is replaced by a lower point symmetry group, D-4(422) or D-3(32). Usi ng Landau theory and dimensional analysis, we find that if there is a chiral cubic bond orientational order in BP III, there should be a mea surable birefringence even in the absence of an external held. This is inconsistent with the knowledge that BP III is optically isotropic wi thout a field. We also point out that a phase with cubic bond orientat ional order can be distinguished from an isotropic phase using field i nduced birefringence experiments.