Surface-induced multiple reentrant transitions

Evolution of a rare sequence of surface transitions with temperature and film thickness

In free-standing smectic films, layers near the surfaces of the film often contain molecules tilted away from the layer normal, while in the bulk of the film the magnitude of the tilt decays exponentially with distance from the surface. We have identified the detailed molecular tilt orientations in the surface layers of films for one antiferroelectric liquid crystal compound. A series of five surface structures exists with different nonplanar tilt arrangements for each structure. The molecular orientations in the surface layers evolve with temperature. The polarization of the film also evolves with temperature, corresponding to the tilt arrangements. Using ellipsometric data, we reconstruct the changes in the magnitude and azimuthal direction of the tilt as functions of temperature. We have also studied films of several different thicknesses. We present a phase diagram for the five surface structures showing the dependence on temperature and film thickness.

Thickness dependent phase behavior of antiferroelectric liquid crystal films

Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-C(alpha)* phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-C(FI2)* phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-C(alpha)* phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems.

Theory of the non-planar surface phase in tilted smectic films under electric field

We present a phenomenological theory of the non-planar tilted surface phase recently observed in freely suspended smectic films. We show that the three phases --Synclinic, Anticlinic, and non-planar (Axial)- stable in non-chiral systems, merge into a single phase in chiral systems. We discuss some experimental consequences of this specific chiral feature. We predict the effects of electric field application parallel to the smectic layers on the stability of the various zero-field surface phases, and determine the conditions for the observation of field-induced chiral Anticlinic/Synclinic crossovers. In thin films they depend only on the ratio between the bulk electroclinic coupling energy and the surface electric-field-tilt coupling.

Nonplanar tilts in very thin smectic films of one liquid-crystal compound

Surface effects cause tilted molecular arrangements in smectic layers near the surface of a free-standing liquid-crystal film in which the bulk of the film is in the smectic- A phase. One recent work has shown that the tilt directions in adjacent surface layers may be nonplanar. In this paper we study films with thicknesses of two to six smectic layers. Surface effects dominate in these very thin films. We show that the molecular tilts are nonplanar even in these very thin films.

Surface-induced ordering effect in one antiferroelectric liquid crystal compound

Null transmission ellipsometer was employed to study the surface-induced tilt transition of an antiferroelectric liquid crystal compound in the bulk smectic- A (SmA) temperature range. The data of the surface tilt transition can be fitted with an extended mean-field model. These results indicate the transition is near a mean-field tricritical point. The surface layers' tilt directions are found to be in different planes several degrees above the bulk SmA-SmC_{alpha};{ *} transition.

Effects of doping on an unusual smectic- C*alpha-smectic-C*FI2-smectic-C* phase sequence

The compound 10OHF has a partially inverted phase sequence, unique among the series of nOHF homologous compounds and all other known liquid crystals, with the smectic-C*FI2 (SmC*FI2) phase occurring at higher temperature than the smectic-C* (SmC*) phase. We present ellipsometric data to identify the phase sequences of 9OHF, 10OHF, 11OHF, and 12OHF. Binary mixtures of 10OHF with C11, a compound with the typical phase sequence among the smectic phases, show that the unusual phase sequence of 10OHF stabilizes upon mixing and that SmC*FI2 predominates over SmC* throughout the entire mixing phase diagram. In thin films of some mixtures, surface interactions induce a reentrant SmC*FI2-SmC*-SmC*FI2 transition in the rest of the film.

Optical studies on the surface-induced tilted layers in freestanding films of two no-layer-shrinkage liquid crystal compounds

Null transmission ellipsometry has been employed to study the molecular arrangements in freestanding films of two no-layer-shrinkage liquid crystal homologous compounds above the bulk smectic A-smectic C* (Sm C*) transition temperature. An unusual nonplanar-parallel or nonplanar-antiparallel-parallel transition has been observed in both compounds under a proper electric field. With the addition of one CH2 group, while the SmC* phase is more stable thermally, the magnitude of the critical field needed to induce a parallel-antiparallel transition decreases dramatically.

Investigations of nanoscale helical pitch in smectic-C*alpha and smectic-C* phases of a chiral smectic liquid crystal using differential optical reflectivity measurements

Differential optical reflectivity (DOR) was used to study the temperature dependence of the short helical pitch in freestanding films of a liquid crystal compound. The experimentally measured DOR signal was fitted using Berreman's 4 x 4 matrix method to get the pitch value in the smectic-C*alpha (sMC*alpha) phase. The results show continuous evolution of the pitch between the smectic-C*alpha and sMC*alpha phases. In sMC*alpha, the pitch decreases as temperature increases and is found to level off at 16+/-1 smectic layers at the sMC*alpha to smectic-A* transition.