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Articles 1 - 14 of 14
Full-Text Articles in Physical Sciences and Mathematics
Novel Surface State In A Class Of Incommensurate Systems, A. E. Jacobs, D. Mukamel, David W. Allender
Novel Surface State In A Class Of Incommensurate Systems, A. E. Jacobs, D. Mukamel, David W. Allender
David W Allender
We study the Landau model of the class of incommensurate systems with a scalar order parameter where the modulated phase is driven by a gradient-squared term with negative coefficient. For example, theoretical studies of cholesteric liquid crystals in a field (electric or magnetic) suggest that such an modulated phase should exist at high chirality. The bulk phase diagram in the presence of a bulk external field which couples linearly to the order parameter exhibits a modulated phase inside a loop in the temperature-field plane, and a homogeneous phase outside. On analyzing the same model for a semi-infinite system, we find …
Pseudo-Casimir Effect In Untwisted Chiral Nematic Liquid Crystals, F. Karimi Pour Haddadan, David W. Allender, S. Zumer
Pseudo-Casimir Effect In Untwisted Chiral Nematic Liquid Crystals, F. Karimi Pour Haddadan, David W. Allender, S. Zumer
David W Allender
We investigate theoretically the pseudo-Casimir force between parallel plates immersed in a chiral nematic liquid crystal. We focus on small-separation limit where the director configuration between the plates inducing strong homeotropic anchoring is uniform. We find that the force is attractive at separations smaller than the crossover distance and repulsive otherwise, and that it diverges logarithmically at the critical distance where the uniform structure is replaced by a distorted structure. We also analyze modifications to the force introduced by magnetic field and comment on the possible detection of the effect.
Wide Viewing Angle, Homeotropic Nematic Liquid-Crystal Display Controlled By Effective Field, S. H. Lee, H. Y. Kim, Y. H. Lee, I. C. Park, B. G. Rho, H. G. Galabova, David W. Allender
Wide Viewing Angle, Homeotropic Nematic Liquid-Crystal Display Controlled By Effective Field, S. H. Lee, H. Y. Kim, Y. H. Lee, I. C. Park, B. G. Rho, H. G. Galabova, David W. Allender
David W Allender
The electro-opticproperties of a liquid-crystal display associated with a homeotropic to multidomainlike transition for a liquid crystal with a positive dielectricanisotropy have been investigated. The cell plates are prepared in such a way that, in the absence of an electric field, the liquid-crystal alignment is homeotropic. an electric field created by interdigitated electrodes on both substrates causes a director deformation of a multidomain type. The display shows wide viewing angle, excellent color characteristics, and a fast response time. The molecular director configuration, together with the electro-optic characteristics of the device, are discussed in this letter.
Orientation And Surface Anchoring Of Nematic Liquid Crystals On Linearly Polymerized Photopolymers, H. G. Galabova, David W. Allender, J. Chen
Orientation And Surface Anchoring Of Nematic Liquid Crystals On Linearly Polymerized Photopolymers, H. G. Galabova, David W. Allender, J. Chen
David W Allender
The liquid crystal orientation on a poly(vinyl cinnamate) or poly(vinyl 4-methoxy-cinnamate) coated solid substrate exposed to linearly polarized UV light has been studied. The angular distribution of the polymer side chains and the photoreaction products was derived as a function of exposure time using a simple assumption about the probability for crosslinking of two side chains. The liquid crystal was assumed to be weakly anchored to the surface but have a degree of order and orientation determined from those of the polymer side chains. The polar and the azimuthal surface coefficients were estimated as a function of exposure time using …
Approximate Description Of The Three Dimensional Director And Electric Field In A Liquid Crystal Display At A High Voltage, G. Panasyuk, David W. Allender
Approximate Description Of The Three Dimensional Director And Electric Field In A Liquid Crystal Display At A High Voltage, G. Panasyuk, David W. Allender
David W Allender
An approximate analytical approach for describing properties of a liquid crystal display associated with a homeotropic to multidomainlike transition for a liquid crystal with a positive dielectricanisotropy was developed. The electrode and surface coating of the display are prepared in such a way that in the absence of an electric field (dark state) the liquid crystal alignment is homeotropic. The bright state corresponds to a situation when a high voltage is applied to the interdigitated electrodes located on both substrates, which causes a director deformation of a multidomain type. Approximate solutions for the three dimensional electric field and director configuration …
Model For The Director And Electric Field In Liquid Crystal Cells Having Twist Walls Or Disclination Lines, G. Panasyuk, David W. Allender
Model For The Director And Electric Field In Liquid Crystal Cells Having Twist Walls Or Disclination Lines, G. Panasyuk, David W. Allender
David W Allender
Two examples of the director structure and electric field in patterned electrode liquid crystal cells are studied using a recently developed calculational model. First, a display cell that exhibits a homeotropic to multidomainlike transition with twist wall structures has been considered for a liquid crystal with positive dielectric anisotropy. The model elucidates the behavior of the electric field. Calculations show good agreement between the model and direct computer solution of the Euler-Lagrange equations, but the model is at least 30 times faster. Second, the possibility that a cell has +/-1/2 disclination lines instead of a wall defect is probed. A …
Finite Molecular Anchoring In The Escaped-Radial Nematic Configuration: A 2-H-Nmr Study, G. P. Crawford, David W. Allender, J. William Doane, M. Vilfan, I. Vilfan
Finite Molecular Anchoring In The Escaped-Radial Nematic Configuration: A 2-H-Nmr Study, G. P. Crawford, David W. Allender, J. William Doane, M. Vilfan, I. Vilfan
David W Allender
The director-field configuration of a nematic liquid crystal confined to cylindrical cavities of polycarbonate Nuclepore membranes ranging from 0.3 to 0.05-mu-m in radius is determined using deuterium nuclear magnetic resonance (H-2 NMR). Spectral patterns from cavities of radius 0.3-mu-m reveal the escaped-radial configuration with singular point defects, but as the cylinder size is decreased, the elastic energy imposed by the curvature of the confining walls competes with the anchoring energy to tilt the directors away from their preferred perpendicular anchoring direction, preventing the expected transition to the planar-radial configuration. A surface fitting parameter is directly determined by simulating H-2-NMR line …
Surface States In Nearly Modulated Systems, A. E. Jacobs, D. Mukamel, David W. Allender
Surface States In Nearly Modulated Systems, A. E. Jacobs, D. Mukamel, David W. Allender
David W Allender
A Landau model is used to study the phase behavior of the surface layer for magnetic and cholesteric liquid-crystal systems that are at or near a Lifshitz point marking the boundary between modulated and homogeneous bulk phases. The model incorporates surface and bulk fields and includes a term in the free energy proportional to the square of the second derivative of the order parameter in addition to the usual term involving the square of the first derivative. In the limit of vanishing bulk field, three distinct types of surface ordering are possible: a wetting layer, a nonwet layer having a …
Surface-Layer Phase-Transitions In Nematic Liquid-Crystals, Natasha Kothekar, David W. Allender, R. M. Hornreich
Surface-Layer Phase-Transitions In Nematic Liquid-Crystals, Natasha Kothekar, David W. Allender, R. M. Hornreich
David W Allender
Surface-layer transitions in nematogenic materials characterized by a preferential planar surface interaction linear in the order parameter have been studied theoretically at temperatures above the bulk transition (T(NI)). The coupled Euler-Lagrange nonlinear differential equations obtained from the Landau-de Gennes free energy were solved exactly by numerical integration. This problem had been studied previously employing various limits and approximations with several differences in the phase diagram reported. The exact results allow one to determine which of these differences are artifacts of the approximations used and which are dependent upon the ratio of elastic constants. It is found, for physically relevant elastic …
Electro-Optic Characteristics Of A Liquid Crystal Cell In A Homogeneous To Twisted Planar Mode, S. H. Lee, H. Y. Kim, H. G. Galabova, David W. Allender
Electro-Optic Characteristics Of A Liquid Crystal Cell In A Homogeneous To Twisted Planar Mode, S. H. Lee, H. Y. Kim, H. G. Galabova, David W. Allender
David W Allender
The electro-optic properties of a novel liquid crystal device associated with a homogeneous to twisted-planar (HTP) transition have been investigated. The cell plates are prepared in such a way that in the absence of an electric field the liquid crystal orientation is uniform throughout the cell, and thus between crossed polarizers the cell appears black (off state). When an electric field of a particular configuration is applied, a 90° twist is imposed, and the cell transmits light (on state). It was found that the HTP device exhibits wide viewing angle and excellent color characteristics. The dependence of the electro-opticeffect on …
Charge Instabilities And Phase-Separation In 3-Band Extended Hubbard Models: Effect Of O-O Hopping, N. Kothekar, K. F. Quader, David W. Allender
Charge Instabilities And Phase-Separation In 3-Band Extended Hubbard Models: Effect Of O-O Hopping, N. Kothekar, K. F. Quader, David W. Allender
David W Allender
We explore the interplay between various interaction and hopping terms, in particular the effect of O-O hopping, tppo on the charge instabilities and phase separation in the three-band extended Hubbard models. Within a weak-coupling mean-field approach, we find that in two dimensions (2D), tppo moves the phase separation boundary appreciably to higher dopings. This would increase the window of doping over which charge-transfer-mediated superconductivity can occur. Considering a larger phase space, we also obtain a more complete picture of the coexistence lines and spinodal decompositions, and determine the charge-transfer-instability critical point. We also consider the 1D case: tppo is found …
Landau Theory Of Wall-Induced Phase Nucleation And Pretransitional Birefringence At The Isotropic-Nematic Transition, David W. Allender, G. L. Henderson, D. L. Johnson
Landau Theory Of Wall-Induced Phase Nucleation And Pretransitional Birefringence At The Isotropic-Nematic Transition, David W. Allender, G. L. Henderson, D. L. Johnson
David W Allender
The Landau-deGennes theory is used to study the isotropic-nematic phase transition of a semi-infinite system having a wall which induces local nematic order. Analytic solutions are obtained and it is found that (1) for sufficient surface order, the briefringence-induced optical phase shift diverges at the transition rather than at the supercooling limit, and (2) the regions of superheating and supercooling are severely restricted.
Theory Of A Symmetry-Breaking Phase-Transition In A Surface-Layer Of Nematic Liquid-Crystal, Y. L'Vov, R. M. Hornreich, David W. Allender
Theory Of A Symmetry-Breaking Phase-Transition In A Surface-Layer Of Nematic Liquid-Crystal, Y. L'Vov, R. M. Hornreich, David W. Allender
David W Allender
For a surface interaction linear in the order parameter and favoring an orientation in which liquid-crystalline molecules lie parallel to the surface (but which is independent of whether the orientation is uniaxial or isotropic in the bounding plane), a symmetry-breaking phase transition in a surface layer is possible at temperatures above that of the bulk isotropic to nematic transition. At the surface transition, the high-temperature in-plane isotropic state becomes unstable with respect to a biaxial phase, the transition being continuous and therefore of-the Berezinskii-Kosterlitz-Thouless (BKT) type, It is expected to occur for intermediate surface couplings but not at very weak …
Biaxial Order Parameters In Liquid-Crystals - Their Meaning And Determination With Nuclear-Quadrupole Resonance, David W. Allender, J. William Doane
Biaxial Order Parameters In Liquid-Crystals - Their Meaning And Determination With Nuclear-Quadrupole Resonance, David W. Allender, J. William Doane
David W Allender
A simple formulation i presented for the analysis of nuclear quadrupole spectra in biaxial liquid crystals based on orientational order parameters. The analysis contains no tilt angle or other model-dependent parameters not directly measured in NMR experiments. The observability and effect of various order parameters on the asymmetry of the electric field gradient η is discussed. It is shown that η is insensitive to rotational freeze-out of the variety expected to be observed in some biaxial phases. Nuclear quadrupole resonance experiments are described which are expected to yield rotational freeze-out parameters.