Dynamics of the nematic-electroclinic effect

Li, Zili; Ambigapathy, R.; Petschek, Rolfe G.; Rosenblatt, Charles

doi:10.1103/physreva.43.7109

Cited by 17 publications

(7 citation statements)

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“…One such possible effect is the nematic electroclinic effect (NECE) [14,15], which might be expected to contribute to the observed behavior as the director is neither parallel nor perpendicular to the electric field over large regions of the sample. However, measurements of the time response of the nematic electroclinic effect [16] show the NECE to be much more rapid (TNECE< 10"^ s) than the effect reported herein, and with a response time independent of the magnetic field. We thus dismiss this possibility.…”

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Surface-induced polarization at a chiral-nematic–substrate interface

Tripathi

Terentjev

et al. 1991

Phys. Rev. Lett.

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A novel linear electro-optic effect is observed in a surface-stabilized chiral nematic cell above the Freedericksz transition threshold field. The slow dynamic response indicates that this effect arises from the interfacial region which elastically couples to the bulk, rather than directly from the bulk. Moreover, evidence suggests that the primary effect is a spontaneous electric polarization very near the interface. Other possible contributions to this effect are discussed.PACS numbers: 61.30.Cz, 6l.30.EbThe relationship between symmetry and spontaneous polarizations in liquid crystals has long been the subject of intense study. In 1969 Meyer showed [1] that in the presence of a bend or splay distortion the nematic symmetry is reduced to C2, giving rise to a spontaneous electron polarization P along the twofold axis. Several years later Meyer et al. proposed and demonstrated the existence of "ferroelectricity" in liquid crystals [2], an effect which exists in the absence of elastic deformations. In simple mean-field theory, the polarization scales as the polar tilt angle 9. (Although deviations from this linear relationship have been observed [3], they turn out to be of only minor importance in the work described herein.) Finally, in addition to its presence in bulk tilted phases, ferroelectricity exists at a symmetry-breaking interface in samples composed of nonchiral molecules. The surface destroys inversion symmetry, allowing for the existence of a spontaneous polarization normal to the interface [4,5]. In this paper we report on experiments in a system which has macroscopic, as opposed to local, C2 symmetry. We argue that this allows for the observation of a chiral polarization which is concentrated at the surfaces of the cell.Inversion symmetry is broken at a liquid-crystalsubstrate interface. If, in addition, the molecular director is tilted by an angle 0 with respect to the surface normal (which can be accomplished with an external magnetic field), then a chiral nematic would exhibit C2 symmetry at the interface. This symmetry group is identical to that of the chiral smectic-C* phase. In consequence, we would expect a spontaneous two-dimensional electric polarization which resides very close to the surface and is oriented perpendicular to the director and in the plane of the interface. In order to probe this polarization, an inplane ac electric field of magnitude |E| and frequency v is applied perpendicular to the dipole, thereby inducing a small periodic azimuthal rotation of the interfacial molecules. This surface-driven motion then propagates elastically into the sample's interior by a combination of bend and twist elasticity. Finally, the overall azimuthal motion of the bulk sample can be detected by observing the depolarized component of transmitted light in the classical electroclinic geometry [6]. The expected optical signal would thus be proportional to |E|, a behavior which is observed experimentally.An indium-tin-oxide-(ITO-) coated glass slide, approximately 20 n/n, was chemically etched t...

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confidence: 53%

Surface-induced polarization at a chiral-nematic–substrate interface

Tripathi

Terentjev

et al. 1991

Phys. Rev. Lett.

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“…where ω = 2πν and As the bulk nematic ECE [16] has much faster (submicrosecond) response times, the sharp fall-off of |e c (ω)| with ω clearly suggests a surface ECE that drags the bulk along elastically [11,14]. Noting in Fig.…”

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confidence: 99%

Macroscopic Torsional Strain and Induced Molecular Conformational Deracemization

et al. 2011

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“…Here we report on temperature-and frequency-dependent measurements of an electroclinic effect in this thin paranematic region of a chiral liquid crystal above the bulk isotropic to chiral nematic -this is sometimes referred to as "cholesteric" -phase transition temperature. Our central results are: An ECE is observed and grows continuously with decreasing temperature before being cut-off by a first order phase transition at T IN ; the rate of growth with decreasing temperature, viz., / c de dT , is much more gradual than was observed in a thin chiral parasmectic region by Lee, et al [37]; and a relaxation frequency is in the neighborhood of several hundred Hz, which is slower by many orders of magnitude than the frequency response in the bulk nematic electroclinic effect, yet faster than a surface-driven ECE into a bulk nematic phase [40]. wide Nem* phase.…”

Section: Introductionmentioning

confidence: 76%

Section: Introductionmentioning

confidence: 99%

“…Single relaxation Debye behavior often is observed in a bulk ECE in the Sm-A* and chiral nematic phases [26,40,45]. The relaxation time τ bulk for the bulk chiral nematic electroclinic effect is fast, typically τ bulk < 10 -6 s [40]. However, surface nematic electroclinic effects due to chirality localized at a surface tend to be much slower [45,46,47].…”

Section: Introductionmentioning

confidence: 99%

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Electroclinic effect in a chiral paranematic liquid-crystal layer above the bulk nematic-to-isotropic transition temperature

2016

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Electroclinic measurements are reported for two chiral liquid crystals above their bulk chiral isotropic-nematic phase transition temperatures. It is found that an applied electric field E induces a rotation θ [∝Ε] of the director in the very thin paranematic layers that are induced by the cell's two planar-aligning substrates. The magnitude of the electroclinic coefficient dθ/dE close to the transition temperature is comparable to that of a bulk chiral nematic, as well as to that of a parasmectic region above a bulk isotropic-to-chiral smectic-A phase. However, dθ/dE in the paranematic layer varies much more slowly with temperature than in the parasmectic phase, and its relaxation time is slower by more than three orders of magnitude than that of the bulk chiral nematic electroclinic effect.

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Dynamics of the nematic-electroclinic effect

Cited by 17 publications

References 5 publications

Surface-induced polarization at a chiral-nematic–substrate interface

Surface-induced polarization at a chiral-nematic–substrate interface

Macroscopic Torsional Strain and Induced Molecular Conformational Deracemization

Electroclinic effect in a chiral paranematic liquid-crystal layer above the bulk nematic-to-isotropic transition temperature

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