The behavior of isolated inclusions (nematic droplets, smectic islands) and formation of chains and clusters from inclusions in oriented smectic membranes have been studied. Investigations of inclusions were performed in membranes in which the molecular ordering was oriented by an external magnetic field. At planar boundary conditions on the interface between the membrane and inclusions different configurations of the c-director field were observed: Coulombic, dipolar, quadrupolar, and mixed. The observed orientation of inclusions and their interactions and self-organization correlate with the predictions of the theory based on the electromagnetic analogy. Chaining and formation of superstructures differ in oriented and nonoriented membranes.
Recently Wang et al. [Phys. Rev. Lett. 104, 027801 (2010)] reported the discovery of a novel multilayer phase in polar liquid crystals. The phase was unambiguously assigned to a six-layer antiferroelectric structure (Sm-C(d6)(*)) by resonant x-ray diffraction. This discovery lead to essential progress in understanding the nature of polar phases. However, more recently, Chandani et al. [Liq. Cryst. 38, 663 (2011)] in the same material clearly identified the novel phase as a ferrielectric five-layer structure (Sm-C(d5)(*)) by the electric-field-induced birefringence. This contradiction seemed to be a mystery. In this paper we show that the two experiments are in agreement. Phenomenological Landau theory of the phase transitions shows that both phases (Sm-C(d6)(*) and Sm-C(d5)(*)) exist and transform into each other in a relatively low electric field.
Recently Wang [Phys. Rev. Lett. 104, 027801 (2010)] discovered a novel smectic-C* liquid-crystal commensurate structure with six-layer period. Challenged by this discovery, we show that the observed novel structure and the unusual sequence of polar phases can be explained in the framework of the discrete Landau model of phase transitions with a two-component order parameter. Peculiarities of the six-layer phase and the influence of short-range and long-range interactions on the formation of different phases and phase sequences are discussed.
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