References of "Walba, David M"
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See detailThe peculiar optic, dielectric and x-ray diffraction properties of a fluorinated de vries asymmetric-diffuse-cone-model ferroelectric liquid crystal
Lagerwall, Jan UL; Coleman, D.; Körblova, E. et al

in Liquid Crystals (2006), 33(1), 17-24

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong ... [more ▼]

A new semi-fluorinated chiral smectic liquid crystal, W504, is investigated by electro-optic, dielectric and X-ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle theta; typical characteristics of a SmA - SmC transition following the de Vries asymmetric diffuse cone (ADC) model in which the non-zero director tilt in SmC arises through an ordering of tilting directions rather than an actual increase in average molecule tilt <theta(mol)>. In W504 a small increase in <theta(mol)> of about 4 degrees is however detected in the SmC phase. Although the increase in molecule inclination is much less than the increase in director tilt h, saturating close to 30 degrees, it leads to a shrinkage of the smectic layers by about 1 angstrom, a result of the large initial molecule tilt in the SmA phase, <theta(mol)>(SmA) approximate to 30 degrees. The tilting transition in W504 is thus mainly an ADC model disorder - order transition, but it also has a component of a structural transition. The semi- fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (001) X-ray scattering peak. In thin films the (001) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head - tail symmetry of the liquid crystal phase at the LC - air interface. [less ▲]

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See detailA chameleon chiral polar liquid crystal: Rod-shaped when nematic, bent-shaped when smectic
Lagerwall, Jan UL; Giesselmann, Frank; Wand, Michael D. et al

in Chemistry of Materials (2004), 16(19), 3606-3615

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the ... [more ▼]

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the same time represents a fundamental contradiction in terms of translational order, was recently reported by Nishiyama and co-workers. We have investi- gated this chiral twin dimer by optic, electrooptic, and dielectric methods and conclude that it is not an ordinary AFLC material, but one where the peculiar properties of bent-core smectics are combined with those of ordinary rod-shaped liquid crystals. The compound exhibits a new type of nematic-smectic phase transition, connected with a change of molecule conformation from rod- to bent-shaped. This also has an important impact on the chiral interactions in the system. Toward the high-temperature end of the smectic phase, the energy balance between bent conformation smectic and straight conformation nematic can be shifted by an electric field such that the transition to the nematic phase with stretched-out molecules can be field-induced. [less ▲]

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See detailPolarity-directed analog electrooptic switching in a low-polarization chiral smectic liquid crystal with positive dielectric anisotropy
Lagerwall, Jan UL; Kane, Alexander; Clark, Noel A. et al

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2004), 70(3), 031703

We describe an analog electro-optic (EO) switching mechanism occurring in thin cells filled with a low- polarization ferroelectric liquid crystal mixture with positive dielectric anisotropy. The mixture ... [more ▼]

We describe an analog electro-optic (EO) switching mechanism occurring in thin cells filled with a low- polarization ferroelectric liquid crystal mixture with positive dielectric anisotropy. The mixture is composed of an achiral nonpolar smectic-C 􏰐Sm-C􏰋 host doped with a small amount of a commercially available unichiral compound. The switching mechanism provides analog EO behavior, and thus could be attractive for informa- tion display applications. The process is polarization-driven for weak fields, while for higher field strength the dielectric coupling dominates the process. [less ▲]

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See detailThe case of thresholdless antiferroelectricity: Polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response
Rudquist, Per; Lagerwall, Jan UL; Buivydas, M. et al

in Journal of Materials Chemistry (1999), 9(6), 1257-1261

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem, 1996, 6, 671]. We find that the thresholdless or V-shaped ... [more ▼]

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem, 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in thf absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action. [less ▲]

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