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

The nematic twist-bend (\(\mathrm{N_{TB}}\)) 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 \(\mathrm{N_{TB}}\) phase is distinguished by a heliconical win...

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Bibliographic Details
Published in:arXiv.org 2016-03
Main Authors: Pardaev, Shokir A, Shamid, S M, Tamba, M G, Welch, C, Mehl, G H, Gleeson, J T, Allender, D W, Selinger, J V, Ellman, B, Jakli, A, Sprunt, S
Format: Article
Language:English
Subjects:
Broken symmetry
Conics
Coupling (molecular)
Crystal defects
Defects
Dimers
Domains
Flux density
Free energy
Light scattering
Liquid crystals
Mesophase
Nematic crystals
Order parameters
Polarization
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Summary:The nematic twist-bend (\(\mathrm{N_{TB}}\)) 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 \(\mathrm{N_{TB}}\) 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 \(\mathrm{N_{TB}}\) 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, \(\mathrm{N_{TB}}\)-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 \(\mathrm{N_{TB}}\) 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.
ISSN:2331-8422
DOI:10.48550/arxiv.1603.00911