Lubor Lejček

Ferroelectric-like behaviour of the SmCP phase in liquid crystalline compounds with asymmetrical bent-core molecules

A new series of asymmetrical bent-shaped mesogens has been synthesised and their mesomorphic properties studied. All compounds exhibit the B2 (SmCP) phase. For some compounds we found a very low coercive field for switching to the saturated ferroelectric state and only one peak in the switching current. Due to the lack of SHG signal in the zero field state and the behaviour of the relaxation mode we cannot unambiguously assign ferroelectric or antiferroelectric character to the B2 phase observed. The observation of planar samples revealed a striped texture. Large optically active domains appear on very slow cooling, which are attributed to a non-homogeneous (twisted) in-plane structure imposed by surfaces. The twisted domains are suppressed under a critical electric field. A simple model of such a structure and its behavior in the electric field is proposed. The low temperature phase detected on further cooling is attributed to a crystalline-like smectic phase.

The new SmC* phase with periodic in-layer director modulation

Abstract Different types of 1 D and 2D periodic systems of defects were detected in free suspended films of chiral SmC and SmCA materials with the helical structure spontaneously unwound. Observed textures were attributed to the appearence of the phase with the periodic long range director modulations within the smectics layers.

Mechanism of inclusion chaining in SmC* free-standing films

The self-assembly of colloidal inclusions has recently been shown in smectic C* freely suspended films. In such 2D systems, the organization of the inclusions is qualitatively explained by elastic interactions induced by the disruption of the orientational order in the SmC* host phase. The interaction between resulting inclusion–defect pairs exhibits a dipolar character. We have developed a simplified model representing every inclusion and its companion hyperbolic defect by (+1)- and (−1)-wedge disclination lines, respectively. A finite anchoring energy has been introduced to explain the coalescence of the thinnest inclusions. Our model enables us to explain the chaining of the thickest inclusions and confirms the inclusion size dependence on the stability of the chains.

Communication: Experimental proof of symmetry breaking in tilted smectics composed of molecules with axial chirality.

For the first time, domains with twisted structures have been established in planar samples of achiral compounds in tilted smectic C phase. This evidences separation of molecular conformers differing in the sense of axial chirality and confirms polar C(2) symmetry of these domains. A simple model considering polar surface anchoring energy and bulk energy of the twist can account for this finding. Conditions for coexistence of twisted and homogeneous domains are discussed.

Twist deformation in anticlinic antiferroelectric structure in smectic B2 imposed by the surface anchoring

A simplified model is used to estimate the energies of observed structures in the smectic B2 phase composed of bent‐shaped molecules. An approximate twist solution connecting anticlinic antiferroelectric structure in the sample bulk with synclinic ferroelectric order induced at the sample surfaces by anchoring is proposed and the elastic and anchoring energies of this structure are determined. It is shown that uniform, twisted and partly twisted (mixed) anticlinic antiferroelectric structures can coexist with nearly the same energies.

A Model of π-Walls Between Domains of Opposite Chirality in B2 Phase of Bent-Shaped Molecules

Principally, domains of opposite chirality can exist either in anticlinic antiferroelectric structures in sample bulk or in synclinic ferroelectric structures induced near surfaces by strong polar surface anchoring or by the influence of an external electric field. Domains of opposite chirality are separated by π-walls. In the present contribution a simplified approximate theoretical model describing the molecular orientation in a π-wall is proposed. This model uses so called internal barrier energy introduced by M Nakata, et al. (Phys. Rev. Letters 2006;96:067802) and takes into account an external electric field acting along smectic layers. Electric field suppresses the component of spontaneous polarization perpendicular to field and layer normal and governs the wall width.

Approximate elastic model of the stripe texture in free-standing cheral smectic C films

Abstract In Ref. [4] (E. Gorecka et al.: Phys. Rev. Lett. 75 4047 (1995) the modulations of the director inside smectic layers of chiral smectic C freestanding film have been identified as a periodic system of domain walls on which the director orientation changes by π. Domain walls are terminated in the film bulk. An improved approximate model, which takes into account the system of ± π-disclinations on which domain walls are terminated, is proposed. Reasons, why the walls are terminated in the film bulk, are outlined. Model parameters are estimated from the experimental dependence of the periodicity d of the system of walls and the film thickness D.

Surface disclination cores in the B2 phase of bent-shaped molecules described by the Peierls–Nabarro model

The Peierls–Nabarro model, originally proposed to describe dislocation core in solids, is used to model core structures of 2π- and π-surface disclinations in the B2 phase of bent-shaped molecules. Structures of disclination cores are determined by surface anchoring. Core parameters as core widths (and positions of two partial π-disclinations as parts comprising 2π-disclination) are estimated using surface anchoring energy. Disclination core widths are usually narrow for strong anchoring. In such a case they are barely observable; nevertheless, they can exist.

Inclusions in Chiral and Non-Chiral Smectic C Free-Standing Films

Director distributions around small inclusions in chiral (Sm C*) and non-chiral (Sm C) smectic C free standing films are approximately analytically determined. These director distributions are equivalent to disclination configurations. Disclination models describing small inclusions can be also generalized to model greater inclusions. In Sm C films greater inclusions can be represented by (+1)-wedge disclination in its centre and two accompanying (−1/2)-wedge disclinations on its surface. Both (−1/2)-disclinations stay on inclusion surface because the anchoring energy is not enough high to expel them. Growing inclusions in Sm C* films can be approximately described by disclination dipole with (+1)-wedge disclination in the inclusion centre and (−1)-wedge disclination inside inclusion approaching the surface. When an inclusion is large enough that (−1)-disclination reaches the surface, it is usually expulsed from the surface to film bulk. The presence of disclination on the surface leads to the perturbation of the radial director anchoring and creation of local electric charge on the inclusion surface namely in Sm C* films with relatively high spontaneous polarisation. The disclination expulsion is then connected with the elimination of the surface electric charge.

Twist Disclinations in Ferroelectric Liquid Crystals as Sources of Induced Polarization Space Charges

The approximate evaluation of the electrostatic self-energy of ( - 2 ~ )-twist disclinations in ferroelectric liquid crystals is proposed. An effective disclination charge per unit length of disclination together with so called depolarization factor will be introduced and discussed in the case of two ( - 2 ~ )-twist disclination configurations typical for ferroelectric liquid crystals.