Search results for "ferroelectric"
showing 10 items of 374 documents
Dielectric and electro-optical studies of a ferroelectric copolysiloxane.
1994
This paper reports an extensive study of the static (spontaneous polarization, ${\mathit{P}}_{\mathit{s}}$, tilt angle \ensuremath{\theta}, and pitch) and dynamic properties of a ferroelectric liquid-crystalline polymer. It is shown that a ferroelectric liquid-crystalline polymer (diluted polysiloxane) exhibits physical behavior similar to that of a low-molar-mass liquid crystal in terms of contrast ratio, pitch of the helix, and relationship between ${\mathit{P}}_{\mathit{s}}$ and \ensuremath{\theta}. The degenerated collective modes for the director rotation are shown to split into two modes at the Sm-A--Sm-${\mathit{C}}^{\mathrm{*}}$ phase transition temperature using dielectric spectros…
Poincaré sphere analysis of a ferroelectric liquid crystal optical modulator: application to optimize the contrast ratio
2008
The Poincare sphere representation is used to analyze the polarization transformation achieved with a ferroelectric liquid crystal (FLC) optical modulator. This device acts as a switchable wave-plate, in which the orientation of the principal axes rotates under the action of an applied bipolar voltage. In the standard operational mode for intensity switching, the rotation angle of the principal axes is �θ = π/4 and the phase shift is φ = π (half-wave-plate). However, for wavelengths different from the design one, the FLC deviates from the half-wave-plate performance and the optical contrast is diminished. We use the Poincare sphere representation to perform a theoretical analysis of the int…
Theory and modeling of polarization switching in ferroelectrics
2005
Abstract Kinetics of polarization response in ferroelectrics is reproduced within Langevin, Fokker–Planck and imaginary time Schrodinger equation techniques for energy functionals of growing complexity modeling an assembly of coarse grained particles with attractive first neighbor interaction. Symplectic integration based numerical approach captures dynamic hysteresis, polarization switching, and spatially extended stationary polarization. Solution of relevant nonstationary problem is adapted to large scale parallel computing.
Mapováni spinů povrchových a bulkových Rashba stavů v tenkých vrstvách feroelektrického α-GeTe(111)
2015
Rozbíjení inverzní symetrie ve fereeleRashba efekt; Fotoemisse; DFTktrickém polovodiči způsobuje děleni stavů, tzv Rashba efekt. V tomto článku ukazujeme kompletně mapování spinové polarizace těchto Rashba stavů za pomoci spinovo rozlišené fotoemisse. The breaking of bulk inversion symmetry in ferroelectric semiconductors causes a Rashba-type spin splitting of electronic bulk bands. This is shown by a comprehensive mapping of the spin polarization of the electronic bands in ferroelectric α- GeTe(111) films using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10 000 data points. The experiment reveals an opposite…
Ferroelectric Liquid Crystalline Elastomers
2002
Ferroelectric LC-elastomers represent an interesting class of material because they combine the ordering of liquid crystalline ferroelectric phases and the rubber elasticity of polymer networks. Switching of the electric polarization leads to deformation of the polymer network, equivalent to stretching a spring, and creates a stress in the network of polymer chains. The interaction of mesogens and the network can be varied by using different topologies of net points: Crosslinking is carried out either within the siloxane sublayers (leading to fast switching elastomers) or between the siloxane sublayers (resulting in an elastomer that favors the ferroelectric switching state in which the cro…
Director orientation in chevron surface-stabilized ferroelectric liquid crystal cells. Verification of orientational binding at the chevron interface…
1990
Abstract We present results of theoretical modelling and experimental study of director distributions and the associated optical properties of chevron surface stabilized ferroelectric liquid crystal (SSFLC) cells. Chevron cells are modelled as being two stacked FLC slabs, described by distinct orientation distributions of the director [ncirc]-polarization P([ncirc] – P) couple. In each slab the [ncirc] – P distribution is governed by bulk Frank elastic and electric field-induced torques and by surface torques at the FLC-solid interface and at the chevron interface, where the two distributions are coupled. The optical properties of such structures are calculated numerically and the results t…
Ordering and mobility of ferroelectric liquid crystal dimer as studied by FT-IR spectroscopy with 2D-IR correlation analysis
1997
Both a conservative rapid-scan FT-IR technique and a novel step-scan FT-IR technique with 2D correlation analysis were used to study the orientation and the mobility ofa ferroelectric liquid crystal dimer during switching under an electric field. The detailed mutual arrangements of different molecular segments (mesogen, poly(methylene) chain, polysiloxane chain) in a smectic C* phase were derived from the static spectra. It was shown that the long mesogen axis, the average poly(methylene) and the average polysiloxane chain axes do not coincide with each other. The hindered rotation of the carbonyl group is confirmed. Time-resolved FT-IR technique was used to follow the segmental motion with…
Manipulation of the ferroelectricity in LC polymers via photomechanical isomerization of azobenzene moieties
1996
We report the synthesis and characterization of a new ferroelectric liquid crystalline side chain copolymer containing photoisomerizable chromophores and non-photoisomerizable mesogenic units. On illumination with UV-light a suppression of the polar order is obtained as indicated by a substantial drop of the spontaneous polarization (Ps) compared to the dark state. We interpret this as the phase order being disrupted by the cis-isomer of the chromophore. After illumination with VIS-light only the major part of Ps is restored. The photochemical cis-trans back-isomerization (restoration of the phase) leads to a different stationary state than the dark state of the material. The thermal reisom…
Ferroelectric liquid crystalline polysiloxanes designed for high second-order nonlinear susceptibilities
1994
Ferroelectric liquid crystalline (FLC) copolysiloxanes containing suitable comonomers («NLO-chromophores») for second harmonic generation (SHG) have been prepared. Their important feature is a donor-acceptor x-system sited perpendicular to the long axis of the side-group moieties in order to adopt the polar ordering of the ferroelectric LC phase. The influcences of the mesogen structure and of the content of NLO-chromophores on the liquid crystalline and ferroelectric properties are studied. The desired chiral smectic C * phase is retained up to 30 mol-% of chromophore. Also, first measurements of second harmonic generation have been performed
Quantum chemical modelling of point defects in KNbO3 perovskite crystals
2000
Abstract We present results of semi-empirical quantum chemical calculations for several perovskite KNb x Ta 1−x O 3 (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO 3 . Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO 3 – unlike Ta impurities in KNbO 3 – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (…