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

Flexoelectricity and piezoelectricity - reason for rich variety of phases in antiferroelectric liquid crystals

The free energy of antiferroelectric liquid crystal which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric and flexoelectric interactions to the nearest layers and dipolar electrostatic interactions to the nearest and to the next nearest layers induce indirect tilt interactions with chiral and achiral properties, which extend to the third and to the fourth nearest layers. Chiral indirect interactions between tilts can be large and induce helicoidal modulations even in systems with negligible chiral van der Waals interactions. If indirect chiral interactions compete with chiral van der Waals interactions, the helix unwinding is possible. Although strength of microscopic interactions change monotonically with decreasing temperature, effective interlayer interactions change nonmonotonically and give rise to nonmonotouous change of modulation period through various phases. Increased enatiomeric excess i.e. increased chirality changes the phase sequence.Comment: 4 pages, 1 figur

Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals

We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters

Dielectric response of a ferroelectric nematic liquid crystalline phase in thin cells

We studied dielectric properties of a polar nematic phase (NF) sandwiched between two gold or ITO electrodes, serving as a cell surfaces. In bulk, NF is expected to exhibit a Goldstone mode (phason), because polarization can uniformly rotate with no energy cost. However, because the coupling between the direction of nematic director and polarization is finite, and the confinement, even in the absence of the aligning surface layer, induces some energy cost for a reorientation of polarization, the phason dielectric relaxation frequency is measured in a kHz regime. The phason mode is easily quenched by a bias electric field, which enables fluctuations in the magnitude of polarization to be followed in both, the ferronematic and nematic phases. This amplitude (soft) mode is also influenced by boundary conditions. A theory describing the phase and amplitude fluctuations in the NF phase shows that the free energy of the system and, consequently, the dielectric response are dominated by polarization-related terms with the flexoelectricity being relevant only at a very weak surface anchoring. Contributions due to the nematic elastic terms are always negligible. The model relates the observed low frequency mode to the director fluctuations weakly coupled to polarization fluctuations

Introduction of current scientific results into education: metastudy and towards a theoretical framework

Inspired by our experiences during an introduction of current research results into education for a case of liq-uid crystals, we tried to find similarities or differences in existing approaches to a transfer of new scientific knowledge into education. There were not many reports on introductions of new topics. Reports mainly con-sidered designs of new laboratory experiment (in advanced optics and similar), only few modules were found (on tribology, semiconductors), sometimes topics aimed for teachers are presented (nanotechnology, super-conductors) and only one example on semiconductors was found where a module was developed and evaluat-ed [2]

Identification of gifted students with underprivileged background by doing experiments

A new approach for identification of the gifted coming from underprivileged background having weaker reading, writing, math and artistic skills. An example of the activity on screen colours incorporated into the regular classroom setting that provides hints on abilities of such students as well as the others. The methodology for developing such units is briefly discussed

Identification of gifted students with underprivileged background by doing experiments

A new approach for identification of the gifted coming from underprivileged background having weaker reading, writing, math and artistic skills. An example of the activity on screen colours incorporated into the regular classroom setting that provides hints on abilities of such students as well as the others. The methodology for developing such units is briefly discussed

Liquid crystals in school

Liquid crystals are modern materials, present everywhere and are also a topic in current research. Therefore they are good candidates for being interesting and motivating. In the workshop we will explain what liquid crystals are, introduce an additional phase and show special optical properties on which liquid crystal display (LCD) technology is built