Quasi-two-dimensional Ising critical behavior of de Vries liquid crystals observed in the heat capacity and dielectric response.

Heat capacity and dielectric measurements have been made on a liquid crystal compound exhibiting de Vries type of smectic-A{*} (Sm-A{*}) phase. Heat capacity shows a significant anomaly which is almost symmetric for above and below the Sm-A{*}-Sm-C{*} transition temperature. The transition was found to be very weakly first order. The critical exponent gamma determined from the dielectric data lies 1.8+/-0.2. The present heat capacity data as well as former data for another compound of de Vries type have been analyzed in detail. It was found that the heat capacity data for both compounds are fitted well with a logarithmic divergence except in the immediate vicinity of the transition. These results agree with an expectation that de Vries Sm-A{*}-Sm-C-{*} transition can exhibit quasi-two-dimensional Ising critical behavior.

Appearance of a liquid crystalline nematic-isotropic critical point in a mixture system of rod- and bent-shaped molecules.

Heat capacity measurements have been made on a liquid-crystal mixture system formed from bent-shaped molecule 1,3-phenylene bis[4-(4-8-alkoxyphenyliminomethyl)benzoates] (P-8-O-PIMB) and rod-shaped molecule n-pentyl-cyanobiphenyl (5CB). The obtained results can be understood assuming that the addition of P-8-O-PIMB molecules to the 5CB system affects as a field conjugate to the nematic order parameter. The B(X)-B(4) transition can be viewed as the nematic-isotropic transition of 5CB, which is embedded in a framework of the B(4) structure of P-8-O-PIMB molecules. The present mixture system offers a rare example of the nematic-isotropic critical point, whose critical behavior has not been studied yet in detail.

Polarization-modulated smectic liquid crystal phases.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or "splayed." However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

Patterns in hybrid nematic liquid-crystal films: topography and topology.

Spontaneous pattern formation in hybrid nematic liquid-crystal films on glycerol was studied with both polarized transmission and unpolarized monochromatic reflected light microscopy. These observations reveal that the patterns that are found in 1 to 10-microm-thick films are due to a combination of film topography, gradients in film thickness that act as an aligning field, and topology, connectivity requirements on the vector field. The patterns studied include cellular patterns, disclinations connected by 2pi-walls, and 2pi-walls that narrow to a point at the edge of the films.

Holes and disclinations in hybrid nematic liquid crystal films.

Defect patterns in hybrid nematic liquid crystal films of pentylcyanobiphenyl (5CB) on a glycerol surface were studied with polarized transmission and monochromatic reflected light microscopy. We report that disclinations of apparent topological strength zero, stable pairs of +1 and -1 disclinations, and higher strength defects found in these films are stabilized by holes in the liquid crystal film. These holes are produced by a monolayer of surfactant on the glycerol surface. In addition, we describe the topology of the director field in the film during the coalescing of two such holes.

Observation of a soft smectic liquid-crystal phase in a mixture showing V-shaped switching.

We present the experimental observation of a soft smectic liquid crystal phase in a three-component liquid crystal mixture reported to exhibit V-shaped switching. Unlike the layer compression modulus B of the usual smectic phase, B of the soft smectic phase has a peculiar temperature and frequency dependence. It was found that this characteristic feature is one of the main driving force to realize an ideal V-shaped switching.

Molecular orientation near the surface of a smectic liquid crystal cell showing V-shaped switching by means of attenuated total internal reflection ellipsometry.

Attenuated total internal reflection ellipsometry was used to probe the molecular orientation and switching near the surface of a smectic liquid crystal cell showing V-shaped switching. We find that the switching occurs collectively near the surface as in the bulk. The molecules form a twisted state, but the twist angle relative to the bulk layer normal is small because of compensating twist of the smectic layer normal. As a result, a rather uniform molecular orientation is produced, resulting in high extinction and a high contrast ratio in the absence of a field.

Observation of a pretransitional effect near a virtual smectic-A--smectic-C* transition.

Unusual softening of the layer compression modulus B has been observed near the phase boundary where the smectic-C* phase vanishes in a naphtalene-based liquid crystal mixture. From the systematic study of x-ray and layer compression measurements, this unusual effect is attributed to the pretransitional softening near a virtual smectic-A-smectic-C* phase transition in the smectic-A phase, which no longer appears on the thermoequilibrium phase diagram. This phenomenon is similar but not equivalent to supercritical behavior.

Ferroelectric phases in a chiral bent-core smectic liquid crystal: dielectric and optical second-harmonic generation measurements

We report unambiguous evidence for ferroelectricity in a chiral bent-core liquid crystal. This evidence includes (i) a single current peak in the polarization reversal current, (ii) a strong dielectric response that is suppressed by applying a bias electric field, and (iii) strong optical second-harmonic generation in the absence of electric field even at normal incidence. We propose a phase structure that is anticlinic in tilt and ferroelectric in polar order, i.e., smectic-C(A)P(*)(F).

Effect of optical purity on the critical heat capacity at the smectic-A-(chiral)-smectic-C transition in an antiferroelectric liquid crystal

High resolution ac calorimetric measurements have been carried out near the smectic-A-chiral-smectic-C phase transition in the antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4(')-octyloxybiphenyl-4-carboxylate (MHPOBC). Data on samples with different optical purities have been analyzed in detail using a renormalization-group expression with corrections-to-scaling terms. The chiral-smectic-C(alpha)-chiral-smectic-C phase transition is first order, while the smectic-A-chiral-smectic-C(alpha) phase transition is second order. A direct smectic-A-chiral-smectic-C phase transition, which occurs in near-racemic mixtures, was found to be quasitricritical and weakly first order. This implies that the smectic-A-smectic-C transition in the racemate locates at a special point where four critical lines intersect.

Critical behavior of layer compression modulus near the smectic- A-smectic- C(*)(alpha) transition

The layer compression modulus was measured near the smectic- A-smectic- C(*)(alpha) phase transition in a chiral smectic liquid crystal. The layer compression modulus B shows marked pretransitional effects above the phase transition. The critical softening of B can be fit to a simple power law. It is described by the theoretical description based on the renormalization-group method and seems to suggest a crossover behavior. These effects clearly show that the transition is not of the Landau mean-field type.

Induced antiferroelectric smectic- C(*)(A) phase by doping ferroelectric- C* phase with bent-shaped molecules

Doping of the ferroelectric Sm-C(*) phase with bent-shaped molecules induces the antiferroelectric Sm-C(*)(A) phase. The effect was observed by means of electro-optic and dielectric measurements in systems with weak interlayer interactions in which the relative strength of anticlinic-synclinic order between molecules in adjacent layers is easily controlled by external factors. Fourier-transform infrared spectroscopy studies suggest that the bent-shaped molecules are not flat. They reorient upon the electric field-induced antiferroelectric-ferroelectric transition to adopt a position in which the average direction of the carbonyl groups is in the smectic plane and a bending tip along the C2 symmetry axis.

Critical fluctuations at the untilted-tilted phase transition in chiral smectic liquid crystals

The layer compression modulus B was measured near the Sm-A-Sm-C* and Sm-A-Sm-C(*)(A) transitions of chiral smectic liquid crystals. B shows a marked pretransitional softening due to the order parameter fluctuations above the phase transition. The width of the critical region of the Sm-A-Sm-C* and Sm-A-Sm-C(*)(A) transitions is smaller than that of the Sm-A-Sm-C(*)(alpha) transition. The critical exponents of B are in agreement with the theoretical description based on renormalization-group method. These results reveal that these transitions are not of the Landau mean-field type.

Spontaneous Enantiomeric Resolution in a Fluid Smectic Phase of a Racemate.

Right- and left-handed homochiral domains segregate in a fluid smectic phase of the racemic (R*,S*)-beta-Me-TFMHPOBC analogue shown. This CF(3)-containing liquid crystalline compound exhibits an electrooptic response in a homogeneous cell, although no macroscopic dipole is expected to exist. Moreover, the homeotropic cells of this material exhibit striped domains, which are associated with fine stripes having two opposite tilt senses with respect to the predominant stripes.