Dielectric heating and relaxations in smectic a liquid crystals

The dependence of the temperature changes ΔT on frequency occurring in the smectic A, nematic and isotropic phase of a positive dielectric liquid crystal material initiated by dielectric heating is investigated. The same relaxation mechanism governing the strong increase of ΔT in nematics is shown to be responsible for inducing comparably strong dielectric heating effects in smectic A phases.     

Structure function of smectic liquid crystals

The behavior of the structure function is investigated for single-domain smectic A and C liquid crystals in the vicinity of the reciprocal lattice vectors in connection with the absence of translational long-range order. The influence of an external orienting magnetic field, the dimensions of the system, and the temperature is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 17–22, March, 1980.     

Long-range order in smectic liquid crystals

The instability of the long-range positional order in smectic A and C liquid crystals under thermal fluctuations is discussed, along with the nematic-smectic A phase transition. The correlation function of the density fluctuations is calculated. It is shown that the long-range orientational order of a layered structure is stable.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 32–36, August, 1978.     

Macroscopic properties of smectic liquid crystals

We discuss the macroscopic behavior of smectic C_G liquid crystals. Smectic C_G is the most general tilted smectic phase that is fluid in the layers. It is characterized by global C₁ symmetry. Consequently, it is ferroelectric, pyroelectric and piezoelectric, opening up a number of possible applications for such a phase. As smectic C_G-phase has a macroscopic hand due to its structure, it is a natural candidate to explain the recent experimental observations of left and right-handed helices in a system composed of achiral molecules. We also discuss critically to what extent smectic C_G could be important for liquid crystalline phases formed by banana-shaped molecules. Phase transitions involving a smectic C_G phase and defects of its in-plane director are briefly discussed. Received: 25 March 1998 / Revised: 15 June 1998 / Accepted: 15 July 1998     

Dielectric permittivity in the smectic a phase of liquid crystals

We propose a functional dependence of the dielectric constant with the smectic order parameter of liquid crystals in their smectic A phase. The proposal is verified by comparing with the published values of heptylazoxybenzene.     

On twinning in smectic crystals

It is shown that mechanical twinning in smectic crystals is possible. The structure of the boundary of twins for a small disorientation of crystallites is determined. The periodic twin structure, which should appear at the tension of the smectic layer, is proposed. Original Russian Text ? V.I. Marchenko, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, p. 841.     

Defects in smectic C* liquid crystals

We consider the structure and properties of various topological defects that can occur in smectic C* liquid crystals. The polarization field associated with disclinations, the effect of incommensuration on the structure of dispirations, some interesting situations in the interaction between dispiration and disclination and between dispirations themselves have been discussed in detail. The properties of cholesteric type disclinations and a possible model for the core structure of a wedge disclination have also been dealt with. The author felicitates Prof. D S Kothari on his eightieth birthday and dedicates this paper to him on this occasion.     

On twinning in smectic crystals

It is shown that mechanical twinning in smectic crystals is possible. The structure of the boundary of twins for a small disorientation of crystallites is determined. The periodic twin structure, which should appear at the tension of the smectic layer, is proposed.     

Wedge-screw dispiration in nonchiral smectic O liquid crystals

Using the approximative expression for the elastic energy density of the smectic O liquid crystal the elastic self-energy of (, b)- and (3, b)-wedge-screw dispirations is estimated. Using the elastic self-energy of (3, b)-wedge-screw dispiration the stability of this defect is discussed.The author expresses his thanks to Professor A. Fukuda, Professor H. Takezoe and Dr. Y. Takanishi for discussions on their observations of (3, b)-wedge-screw dispirations in liquid crystals with antiferroelectric and anticlinic molecular ordering. This work was supported by the grant of Acad. Sci. of the Czech Rep. No. 19062 and also benefited by the grant No. 202/93/155 from the Grant Agency of the Czech Republic.     

Charge ordering induces a smectic phase in oblate ionic liquid crystals

We report a computer simulation study of an electroneutral mixture of oppositely charged oblate ellipsoids of revolution with aspect ratio A=1/3. In contrast with hard or soft repulsive ellipsoids, which are purely nematic, this system exhibits a smectic-A phase in which charges of equal sign are counterintuitively packed in layers perpendicular to the nematic director.     

Discontinuity walls terminated by twist disclinations in smectic a liquid crystals

A discontinuity wall in smectic A liquid crystals was discussed in M. Kléman:Points, Lines and Walls in Liquid Crystals, Magnetic Systems and Various Ordered Media (J. Wiley, Chichester, 1983). Possible solutions describing a singularity — twist disclination which terminates the discontinuity wall in an infinite model smectic A liquid crystal are discussed. One of solutions proposed can be used to describe the so-called zigzag defect line in chiral smectic C liquid crystals on which opposite oriented chevrons are terminated. This work was supported by Grant No.: 202/96/1687 of the Grant Agency of the Czech Republic.     

Twist disclination loops in chiral smectic C liquid crystals

The notion of an infinitesimal twist disclination loop known from solids is introduced into a chiral smectic C liquid crystal with parallel layers. Finite rectangular and straight twist disclinations are described as a synthesis of a planar distribution of infinitesimal loops. Then a small impurity which disturbs only the molecular arrangement in one smectic layer is modelled by a pair of infinitesimal twist disclination loops. The interaction between a 2π-twist disclination and a small impurity is estimated using the interaction energy between a straight 2π-twist disclination and an infinitesimal twist disclination loop.     

Wedge-screw dispiration in smectic liquid crystals with antiferroelectric order

Using the approximative expression for the elastic energy density of the antiferroelectric (AF) liquid crystal the elastic self-energy of (, b)-wedge-screw dispiration is estimated. Then the elastic interaction between (, b)-wedge-screw dispiration and (2)-wedge dischnation in AF liquid crystals is investigated. The attraction of dislocation part of dispiration with (2)-wedge disclination can lead to the formation of (3)-wedge-screw dispiration. When (2)-wedge disclination is trapped by (2d)-screw dislocation the resulting (2, 2d)-dispiration can dissociate into a pair of two (, d)-dispirations.The authors express their thanks to Professors A. Fukuda, H. Takezoe, Dr. Y. Takanishi and to Dr. K. Miyachi for discussions concerning their experimental observations of (3, d)-wedge-screw dispirations and (, d)-twist-edge dispirations in AF structures. This work was supported by the grant No. 19062 from Acad. Sci. Cz. R. and also benefited by the grant No.: 202/93/155 from the Grant Agency of the Czech Republic.     

Observation of edge dislocations in helical smectic liquid crystals

We report the observation of elementary edge dislocations in smectic liquid crystals possessing helical structure. The dislocations were observed in the entire temperature range of helical phases, including ferroelectric, ferrielectric and antiferroelectric phases. The mechanism for visualizing the dislocations is based on the phenomenon of selective reflection of circularly polarised light. The performed observations of dislocations deliver not only information on the mechanisms of defect creation in various chiral smectic phases, but also on the structure and properties of the investigated smectics, often inaccessible using standard methods.     

Theoretical model of the transition between C1 and C2 chevron structures in smectic liquid crystals

We present a study of the effect of weak anchoring on the transition between C1 and C2 chevron structures in smectic-C liquid crystals. The coexistence of C1 and C2 chevron structures within a single cell causes zigzag defects to occur and may affect the optical characteristics of the cell. By standard Euler-Lagrange minimisation of the total energy of the system, we obtain analytical expressions for the equilibrium director cone angle in the two chevron states. These in turn allow us to compare the total energies of the states and determine the globally stable chevron profile. We show that analytical predictions for the critical transition temperature, which depends on anchoring strength and pretilt angle, are in good agreement with those obtained numerically.     

Correlation between the molecular chirality index and the spontaneous polarisation in series of smectic C* liquid crystals

A correlation between spontaneous polarisation and the chirality index of the constituent molecules, multiplied by the molecular transverse dipole, has been considered for five series of ferroelectric smectic C* liquid crystals. A good correlation has been found within each series, and it has been shown that the scaled chirality index may be used to estimate the spontaneous polarisation in a series of smectic C* mesogens with small systematic changes of the molecular structure. At the same time one cannot expect any significant correlation between the polarisation and the chirality index for smectic C* materials of sufficiently different structure because the spontaneous polarisation may be significantly effected by some details of the molecule's structure including the orientation of the transverse dipole with respect to the primary short molecular axis.     

Vortex liquid crystals in anisotropic type II superconductors

In an isotropic type II superconductor in a moderate magnetic field, the transition to the normal state occurs by vortex lattice melting. In certain anisotropic cases, the vortices acquire elongated cross sections and interactions. Systems of anisotropic, interacting constituents generally exhibit liquid crystalline phases. We examine the possibility of a two step melting in homogeneous type II superconductors with anisotropic superfluid stiffness from a vortex lattice into first a vortex smectic and then a vortex nematic at high temperature and magnetic field. We find that fluctuations of the ordered phase favor an instability to an intermediate smectic-A in the absence of intrinsic pinning.