Fluctuation Modes of a Twist-Bend Nematic Liquid Crystal

Parsouzi, Zeinab; Shamid, Shaikh M.; Borshch, Volodymyr; Challa, Pavan Kumar; Baldwin, A. R.; Tamba, Maria-Gabriela; Welch, Chris; Mehl, Georg H.; Gleeson, J. T.; Jákli, Antal; Lavrentovich, Oleg D.; Allender, David W.; Selinger, Jonathan V.; Sprunt, Samuel

doi:10.1103/physrevx.6.021041

Cited by 34 publications

(86 citation statements)

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“…A linear coupling of P to the curl ofn drives the heliconical modulation ofn, and, on the nematic side of the transition, renormalizes the bend elastic constant to zero, destabilizing the nematic phase in agreement with the theory of Dozov [7] and with experiment [16]. While studies of fluctuations in the N TB phase are consistent with the presence of a short-pitch polarization wave [17], more direct confirmation, by a method sensitive to broken centrosymmetry due to P, is clearly desirable. Given the absence of any mass density wave (Bragg peaks) in X-ray diffraction measurements [4,18,19], confirmation of P would establish the N TB state as the first example of a locally polar LC phase without smectic-like molecular layering -a prospect of fundamental significance.…”

Section: Introductionsupporting

confidence: 69%

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Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

et al. 2016

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The nematic twist-bend (NTB) phase, exhibited by certain thermotropic liquid crystalline (LC) dimers, represents a new orientationally ordered mesophase -the first distinct nematic variant discovered in many years. The NTB phase is distinguished by a heliconical winding of the average molecular long axis (director) with a remarkably short (nanoscale) pitch and, in systems of achiral dimers, with an equal probability to form right-and left-handed domains. The NTB structure thus provides another fascinating example of spontaneous chiral symmetry breaking in nature. The order parameter driving the formation of the heliconical state has been theoretically conjectured to be a polarization field, deriving from the bent conformation of the dimers, that rotates helically with the same nanoscale pitch as the director field. It therefore presents a significant challenge for experimental detection. Here we report a second harmonic light scattering (SHLS) study on two achiral, NTB-forming LCs, which is sensitive to the polarization field due to micron-scale distortion of the helical structure associated with naturally-occurring textural defects. These defects are parabolic focal conics of smectic-like "pseudo-layers", defined by planes of equivalent phase in a coarse-grained description of the NTB state. Our SHLS data are explained by a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a linear coupling between the two.

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Section: Introductionsupporting

confidence: 69%

“…Since for small β the polarization magnitude p 0 in the N TB phase is [17] p 0 ≈ K 3 q 0 β/Λ, Eq. (30) produces Eq.…”

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confidence: 99%

Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

et al. 2016

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“…behavior mirrors broadly that of analogues with shorter and longer internal spacers and 7 mesogens with negative dielectric anisotropy [24,25,26,27,28]. The clearing temperature of DTC5C9 increases in strong magnetic fields, with a rate ~0.6 o C/T [29];…”

Section: Chemical Structure Phase Diagram and Alignmentmentioning

confidence: 61%

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

et al. 2016

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We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay ( 11 K ), twist ( 22 K ), and bend ( 33 K ) elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behavior to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rodlike mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behavior, described by Haller. The elastic constants also follow the standard behavior, with their ratios being practically temperature-independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N-Ntb phase transition. 33 K decreases strongly to 0.4 pN near the N -Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase. Keywords: dimeric nematic; elastic constants; birefringence; negative dielectric anisotropy; twist-bend nematic. N phase. Uniform bend, however, cannot be realized in space without other types of deformations, either splay or twist. Thus two different variations of the nematic with spontaneous bend have been proposed by Meyer [8] and Dozov [9]: a twist-bend nematic and a splay-bend nematic (yet to be discovered experimentally). The relative stability of the two is controlled by the ratio of the splay 11 K to twist 22 K constants. Namely, in the twist-bend phase, 11 22 /2 KK  , while in the splay-bend case, 11 22 /2 KK  [9, 10]. As for the ChOH state, Meyer [11] predicted that it can be induced by an external electric or magnetic field in a cholesteric liquid crystal only when 33 22 KK  . In calamitic nematics formed by rod-like molecules, the latter condition is not satisfied; the expected and universally observed trend follows the inequalities 33 22 11 K K K  [13]. Prior research indicates that the dimeric materials feature elastic properties very different from those of rod-like nematogens. Atkinson et al. [14] demonstrated a strongodd-even effect in the behavior of 33 K . Two dimers, one with an even number of alkyl groups in the spacer chain and another one with an odd number, were characterized by the splay Frederiks transition technique. In this approach, 11 K is determined from the threshold field of reorientation, and 33 K is obtained by extrapolating the voltage dependence of capacitance far above the threshold. 33 K of the odd homolog was about (3-4) pN, much smaller (by a factor 10 ) than the corresponding value for the even homolog; the results were in good agreement with the theoretical consideration of the odd-even effect by...

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“…It needs to be accompanied, at least, by the nonvanishing λ-coupling (λ = 0) in F . The nonvanishing λ is also needed for a proper explanation of the fluctuation modes in N TB , as suggested by recent light scattering experiments [39].…”

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confidence: 88%

Nematic twist–bend phase in an external field

Paja̧k

Longa

Chrzanowska

2018

Proc. Natl. Acad. Sci. U.S.A.

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The response of the nematic twist-bend (N TB ) phase to an applied field can provide important insight into structure of this liquid and may bring us closer to understanding mechanisms generating mirror symmetry breaking in a fluid of achiral molecules. Here we investigate theoretically how an external uniform field can affect structural properties and stability of N TB . Assuming that the driving force responsible for the formation of this phase is packing entropy we show, within Landau-de Gennes theory, that N TB can undergo a rich sequence of structural changes with field.For the systems with positive anisotropy of permittivity we first observe a decrease of the tilt angle of N TB until it transforms through a field-induced phase transition to the ordinary prolate uniaxial nematic phase (N ). Then, at very high fields this nematic phase develops polarization perpendicular to the field. For systems with negative anisotropy of permittivity the results reveal new modulated structures. Even an infinitesimally small field transforms N TB to its elliptical counterpart (N TBe ), where the circular base of the cone of the main director becomes elliptic.With stronger fields the ellipse degenerates to a line giving rise to a nonchiral periodic structure, the nematic splay-bend (N SB ), where the two nematic directors are restricted to a plane. The three structures, N TB , N TBe and N SB , with a modulated polar order are globally nonpolar. But further increase of the field induces phase transitions into globally polar structures with nonvanishing polarization along the field's direction. We found two such structures, one of which is a polar and chiral modification of N SB , where splay and bend deformations are accompanied by weak twist deformations (N * SBp ). Further increase of the field unwinds this structure into a polar nematic (N p ) of polarization parallel to the field.

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Fluctuation Modes of a Twist-Bend Nematic Liquid Crystal

Cited by 34 publications

References 35 publications

Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

Nematic twist–bend phase in an external field

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