Effect of optical purity on phase sequence in antiferroelectric liquid crystals

We use the discrete phenomenological model to study theoretically the phase diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical purity and temperature. Recent experiments have shown that in some systems the number of phases is reduced if the optical purity is extremely high. In some materials the SmC$_{A}^{\star}$ phase is the only stable tilted smectic phase in the pure sample. In the scope of the presented model this high sensitivity of the phase sequence in the AFLCs to optical purity is attributed to the piezoelectric coupling which is reduced if optical purity is reduced. We limit our study to three topologically equal phases - SmC$^{*}$, SmC$_{\alpha}^{*}$ and SmC$_{A}^{*}$ and show that the reduction of optical purity forces the system from the antiferroelectric to the ferroelectric phase with a possible SmC$_{\alpha}^{\star}$ between them. The effect of the flexoelectric and quadrupolar coupling is considered as well. If the phase diagram includes only two phases, SmC$^{\star}$ and SmC$$, the flexoelectric coupling is very small. The materials which exhibit the SmC$_{\alpha}^{\star}$ in a certain range of optical purity and temperature, can be expected to have a significant flexoelectric coupling that is comparable with the piezoelectric coupling. And finally, when temperature is lowered the phase sequence SmA $\to$ SmC$$ $\to$ SmC$^{\star}$ $\to$ SmC$$ is possible only in materials in which quadrupolar coupling is very strong.Comment: 17 pages including 6 figures, submitted to PR

Spin Caloritronics

This is a brief overview of the state of the art of spin caloritronics, the science and technology of controlling heat currents by the electron spin degree of freedom (and vice versa).Comment: To be published in "Spin Current", edited by S. Maekawa, E. Saitoh, S. Valenzuela and Y. Kimura, Oxford University Pres

Mechanoluminescence Color Conversion by Spontaneous Fluorescent-Dye-Diffusion in Elastomeric Zinc Sulfide Composite

Color conversion, long-wavelength light emission by absorbing short-wavelength light, is an attractive approach for developing a broad-color expression technology and is widely used in solid-state lighting, dye-lasers, and colorful displays. Up to now, many papers have been published reporting various mechanoluminescent materials emitting color of ultraviolet, blue, green, orange, and red. However, the strategies of previous reports have focused on color-tuning of mechanoluminescent material itself through newly developing inorganic mechanoluminescent compounds. Here, a new strategy for the color manipulation of mechanoluminescence (ML) is introduced by physically combining fluorescent dyes with existing mechanoluminescent materials. An elastomeric zinc sulfide (ZnS) composite is prepared in a polydimethylsiloxane framework with spontaneously diffused 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), and red luminescence by complete color conversion via DCJTB is demonstrated, which fully absorbed green ML from ZnS. Based on this approach, color-tuning of ML from red to green is successfully achieved and color expression range is expanded by employing electroluminescence (EL). Various-color-emitting EL/ML electromechanical display is demonstrated using color discrepancy between DCJTB employed EL and ML. As the implementation is fairly straightforward, it is believed that present color conversion is a viable and common method to manipulate broader color expression for future ML applications. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

POLARIZED RAMAN SCATTERING AND ITS ELECTRIC FIELD EFFECTS IN DOBAMBC

On observe la diffusion Raman polarisée dans les phases smectique A et smectique C* du (S)-2-méthylbutyl-p-(-p-décyloxybenzylidène)-amino-cinnamate, qui est un cristal liquide ferroélectrique. Les rapports de polarisation de l'intensité Raman dépendent fortement de la température dans la phase SC* mais très peu dans la phase SA. Cette dépendance en température est expliquée par les variations de l'angle d'inclinaison de l'axe moléculaire. Sous champ électrique, on constate une variation anormale avec la température du rapport de polarisation à la transition SA-SC*.Polarized Raman scattering is observed in aligned smectic A and smectic C* phases in (S)-2-methylbutyl-p-(p-decyloxybenzylidene)-amino-cinnamate, which is known as a ferroelectric liquid crystal. The polarization ratios of the Raman intensity strongly depend on temperature in the smectic C* phase but scarcely in the smectic A phase. This temperature dependence is qualitatively explained by the change in the tilt angle of the molecular axis. Under an electric field, an anomalous variation with temperature in the polarization ratios has been found at the smectic A-smectic C* phase transition point

Induced and spontaneous deracemization in bent-core liquid crystal phases and in other phases doped with bent-core molecules

Recently discovered chiral properties of several bent-core smectic liquid crystal phases are summarized and discussed in detail under the assumption that typical bent-core molecules may exist in chiral conformational states and are achiral only on average. Results of atomistic computer simulations are presented which indicate the existence of strongly chiral conformational states for typical bent-core mesogens. A theory is developed which describes a possible shift of equilibrium between left- and right-handed conformations in a macroscopically chiral phase. The theory describes a chirality induction in the B2 bent-core phase and a reduction of the helical pitch in cholesteric and chiral SmC* phases doped with bent-core molecules. Finally, the possibility of spontaneous deracemization in bent-core smectic phases is discussed in detail

Lowring lasing threshold in chiral nematic liquid crystal structure with different anisotropies

Cholesteric liquid crystal (CLC) cells were fabricated using three kinds of nematic liquid crystal (NLC) materials with different optical birefringent anisotropies Delta n. A larger Delta n of NLCs results in a larger photonic band gap (PBG) width and higher density of states (DOS) at the PBG edge for each CLC cell. We were able to lower the threshold energy by a factor of more than two using CLC cells with a broader PBG width.close121