Thickness dependent phase behavior of antiferroelectric liquid crystal films

Resonant x-ray diffraction study of an unusually large phase coexistence in smectic liquid-crystal films

The recent discovery of the new smectic-C(d6)(*) (SmC(d6)(*)) phase [S. Wang et al., Phys. Rev. Lett. 104, 027801 (2010)] also revealed the existence of a noisy region in the temperature window between the SmC(d6)(*) phase and the smectic-C(d4)(*) (SmC(d4)(*)) phase. Characterized by multiple resonant peaks spanning a wide region in Q(Z), the corresponding structure of this temperature window has been a mystery. In this Letter, through a careful resonant x-ray diffraction study and simulations of the diffraction spectra, we show that this region is in fact an unusually large coexistence region of the SmC(d6)(*) phase and the SmC(d4)(*) phase. The structure of the noisy region is found to be a heterogeneous mixture of local SmC(d6)(*) and SmC(d4)(*) orders on the sub-μm scale.

Surface-surface interaction in smectic liquid crystal films

Null transmission ellipsometry was employed to study the field induced transition of the surface arrangements in freestanding films of smectic liquid crystals. The interlayer interaction between the two surfaces obtained from the threshold voltage for the transition is found to be antiferroelectric and is quasilong ranged. The possible microscopic origins of the measured interaction and its relevance to the interlayer interaction in antiferroelectric liquid crystal materials are discussed.

Crossover from tricritical to critical end point behavior in free-standing smectic films

We study the smectic to nematic (SmA-N) phase transition taking place at the center of a free-standing film that exhibits enhanced surface order due to the anchoring promoted by a surrounding gas. The usual McMillan mean-field approach predicts that the SmA-N transition in bulk samples can be continuous or discontinuous (first or second order) depending on the molecular geometry, with a tricritical point separating these two regimes. Here we show that the additional orientational order imposed by the surface anchoring stabilizes the surface-induced smectic and nematic phases, leading to the breakdown of the tricritical point and to the emergence of a critical end point. We report the full phase diagram, which depicts four distinct structures as the film thickness is reduced.

Surface-aligning field in smectic liquid-crystal films

Two modified mean-field J(1)-J(2) models are studied to explain the surface reduction of twisting power in the helical smectic-C*(α) phase in free-standing liquid crystal films. Profiles of the surface interlayer interaction are calculated from the experimental results. The calculations reveal the existence of a strong surface field and indicate that the surface field is the reason for the observed reduced twisting power near the surface region. Our results provide a quantitative study of the interlayer interactions through surface effects in smectic liquid crystals.

Surface-induced reduction of twisting power in liquid-crystal films

Null transmission ellipsometry was employed to study the temperature evolution of the helical structure of the smectic-C(α)* phase. Free-standing films with thickness ranging from 31 to more than 400 layers were prepared and studied. The experimental results show a reduced twisting power in thin films. A simple model was constructed to explain the results. Surface effects were found to be the key reason for this phenomenon. Our findings are consistent with the theoretical studies of helically ordered magnetic films.

Surface and bulk uniaxial to biaxial smectic-A transition in a bent core liquid crystal

A null transmission ellipsometer was employed to study the uniaxial to biaxial smectic-A phase transition of a bent core liquid crystal material. Free-standing films of thicknesses ranging from 5 molecular layers to more than 300 were prepared and studied. Critical exponents for both the surface and interior biaxiality were obtained. The results were discussed in the general framework of phase transition in lower dimensions.