A. V. Emelyanenko

Degeneracy lifting near the frustration points due to long-range interlayer interaction forces and the resulting varieties of polar chiral tilted smectic phases

To gain a clear understanding of ferroelectricity and antiferroelectricity together with their frustration in chiral tilted smectic liquid crystals, we have constructed E–T phase diagrams by drawing field-induced birefringence contours in the prototype binary mixture system of MHPOCBC and MHPOOCBC. The results obtained are discussed in terms of the theoretical model proposed by Emelyanenko and Osipov; we have insisted on the appropriateness of specifying the biaxial subphases, which emerge sequentially in the temperature-induced transition, by the relative ratio of ferroelectric and antiferroelectric orderings in the superstructure unit cell, such as , where qT  = [F]/([A] + [F]). Additional subphases other than the ordinary subphases with three- and four-layer superstructures, and , have been established to exist, firmly for and less adequately for and . Likewise, we have observed several stable superstructures during the field-induced transition from the biaxial subphases to unwound SmC*, and have tried to specify them using qE  = |[R] − [L]|/([R] + [L]), where [R] and [L] refer to the numbers of smectic layers with directors tilted to the right and to the left, respectively, in a unit cell of the superstructure. We have also found the characteristic field-induced deformation of the uniaxial subphase and attempted to understand it in terms of the devils staircase due to soliton condensation reported recently by Torikai and Yamashita ( 41 ).

Influence of dimerization on the nematic-isotropic phase transition in strongly polar liquid crystals

A molecular theory of the nematic-isotropic phase transition is developed for the fluid composed of rod-like particles with large longitudinal dipoles. The equilibrium between the monomers and dimers with antiparallel dipoles is explicitly taken into account and the concentration of dimers is determined self-consistently together with the nematic order parameters. We show that for small central dipole moments the nematic-isotropic transition temperature increases with the increasing dipole. This is in accordance with the results of previous approaches. By contrast, for large values of the dipole the transition temperature decreases due to the growing concentration of dimers. This result enables one to explain the results of recent computer simulations that reveal a destabilization of the nematic phase in a system of hard rods with large central longitudinal dipoles. The temperature variation of the concentration of dimers is also analysed and the sign of the concentration discontinuity at the transition p...

Theoretical studies of the structure of intermediate chiral smectic phases with increasing periodicity

Different approaches to the description of the structure of intermediate chiral smectic C* phases are discussed and the concept of the “discrete” flexoelectric effect is introduced that describes a polarization induced in a smectic layer by a difference in director orientation in the two other layers adjacent to it. It is shown that the “discrete” flexoelectric effect is determined by electrostatic dipole-quadrupole interaction between positionally correlated molecules located in adjacent smectic layers. It is also shown that a simple discrete model proposed in the literature can be used to describe the whole sequence of intermediate chiral smectic C* phases with increasing periods, and to determine the non-planar structure of each phase without additional assumptions. In this sequence the phases with three and four layer periodicity have the same structure, as observed in the experiment. The theory predicts also the structure of intermediate phases with longer periods that have not been studied experimentally so far. The influence of long-range interaction between polarization fluctuations in different layers, proposed by Prost and Bruinsma, is also considered. Several intermediate phases are shown to be suppressed by this interaction, while the stability of the phases three and four layer periodicity is increased.

Smooth transitions between biaxial intermediate smectic phases

The possibility of smooth transitions between biaxial smectic subphases via defect propagation is considered on the basis of our theoretical approach reported previously [Emelyanenko et al., Phys. Rev. E, 2006, 74, 011705]. The extrapolation of our results suggests an infinite number of biaxial intermediate phases, each one existing stably only at a particular temperature, and not over a range. Three- and four-layer subphases are confirmed to be important due to the minor differences between them and their neighboring subphases. Some other subphases (including a six-layer subphase) are also found to be very similar to the neighboring subphases. Several sets of intermediate phases are recognized. Each set represents a combination of two typical fragments of the smectic structure in various proportions.

Induction of new ferrielectric smectic phases in the electric field

ABSTRACT A theoretical approach for the description of intermediate phase sequences (observed in the temperature range between Sm-C* and Sm-CA*) in the presence of electric field is derived. The induction of long-periodicity ferrielectric smectic phases by the electric field is discussed. In particular, we acknowledge the induction of the 12-layer and 15-layer periodicity subphases, whose structure coincides with the ones observed by resonant x-ray scattering in [A. Iida, I. Nishiyama, and Y. Takanishi, Phys. Rev. E 89, 032503 (2014)].

Molecular-statistical approach to a behavior of ferroelectric, antiferroelectric and ferrielectric smectic phases in the electric field

The fundamental theoretical approach derived in A.V. Emelyanenko et al., Phys. Rev. E 74, 011705 (2006) is complemented by a consideration of the influence of the homogeneous electric field on Sm- CA* , biaxial intermediate phases, and Sm-C* . The crucial role of the induced polarization is investigated for the first time. The evolution of any tilted smectic phase in the electric field is found to meet the two thresholds. The first threshold corresponds to the unwinding process, and the second one corresponds to the phase transition into the bi-domain structure of Sm-C* , where the tilt plane has some contribution either along or against the electric field, while the average direction may still be perpendicular to the electric field. The tilt plane in the monodomain (conventional) structure preceding the second threshold is the same in every unwound phase, and is perpendicular to the electric field. No 3D distortion in Sm- CA* is predicted on application of the electric field. The entire electric-field-temperature phase diagrams including the possibility of existence of the maximal number of tilted smectic phases are plotted and compared with the experimental ones. The numerical calculations in the framework of this fundamental study are done with help of AFLC Phase Diagram Plotter software developed by the author and available at his web page.

Antiferroelectric and ferrielectric liquid-crystal display: Electrically controlled birefringence color switch as a new mode

— A mixture with intermediate biaxial (ferrielectric) smectic phases existing in a broad temperature range has been developed. At any temperature within this range, as well as in the antiferroelectric phase range, several birefringence color states can be switched stepwise by application of an electric field, and therefore a LC cell placed between crossed polarizers can display form several colors without the use of color filters. A very small time switching between color states (about 10 μsec) can be a basis for this new mode in display technology because several full-color optical states can be realized in the same material (or in the mixture of materials). These possibilities were investigated both theoretically and experimentally.

Communication: Orientational structure manipulation in nematic liquid crystal droplets induced by light excitation of azodendrimer dopant

Reversible orientational transitions in the droplets of a nematic liquid crystal (NLC) caused by the change of boundary conditions under the low intensity diode illumination are investigated. Photosensitivity of NLC is achieved by the addition of the dendrimer compound with azobenzene terminal groups. Two types of NLC droplets in glycerol are considered: the spherical droplets in the bulk of glycerol and the droplets laid-down onto the solid substrate. In the second case, the first order phase transition is revealed. The effects described can be useful for the development of highly sensitive chemical detectors and microsized photo-tunable optical devices.

Multistability in the mixtures of smectic-C^{*} materials with compensated twisting power.

The effect of multistability in the mixtures of smectic-C^{*} materials with compensated twisting power, i.e., the existence of a large number of almost equiprobable states in the same mixture under the same conditions, is analyzed in the framework of elastic continuum theory. A simple molecular model is also considered. It is shown that multistability can follow from the bulk properties of the smectic-C^{*} materials with compensated twisting power, but with high spontaneous polarization. Multistability leads to the formation of ferroelectric domains, in which the director oscillates in space. The length and amplitude of this oscillation is tunable smoothly by an electric field. Theoretical results for the domain length agree completely with the experimental data. A suggestion is made as to why each domain structure is remembered without the energy consumption, when the electric field is abruptly switched off. The structural dependence on material parameters, such as the spontaneous polarization, the elastic constant, and the equilibrium wave number, is predicted.

Simple theory of transitions between smectic, nematic, and isotropic phases

The transitions between smectic, nematic, and isotropic phases are investigated in the framework of a unified molecular-statistical approach. The new translational order parameter is different from the one introduced in K. Kobayashi [Phys. Lett. A 31, 125 (1970)] and W. L. McMillan [Phys. Rev. A 4, 1238 (1971)]. The variance of the square sine of intermolecular shift angle along the director is introduced to take self-consistently into account the most probable location of the molecules with respect to each other, which is unique for every liquid crystal (LC) material and is mainly responsible for the order parameters and phase sequences. The mean molecular field was treated in terms of only two parameters specific to any intermolecular potential of elongated molecules: (1) its global minimum position with respect to the shift of two interacting molecules along the director and (2) its inhomogeneity/anisotropy ratio. A simple molecular model is also introduced, where the global minimum position is determined by the linking groups elongation Δ/d, while the inhomogeneity/anisotropy ratio Gβ/Gγ is determined by the ratio of electrostatic and dispersion contributions. All possible phase sequences, including abrupt/continuous transformation between the smectic and nematic states and the direct smectic-isotropic phase transition, are predicted. The theoretical prediction is in a good agreement with experimental data for some simple materials correlating with our molecular model, but it is expected to be valid for any LC material.