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

Abstract

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.

Free volume, molecular grains, self-organisation, and anisotropic entropy: machining materials

“…The data was taken from refs. [10,11,20,[22][23][24][25][26][27][28], and recent publications also support this analysis [29][30][31][32][33].…”

Fluctuation Modes of a Twist-Bend Nematic Liquid Crystal

“…The behavior of both modes, as well as the presence of a single hydrodynamic mode in the N TB phase (associated with splay fluctuations of the helical pitch axis), is quantitatively explained by a theoretical model based on two components: (1) a Landau-de Gennes freeenergy density, which is expanded in the director and polarization fields, and (2) a coarse-graining of this free energy that analogizes the heliconical structure to a smectic-A-like system characterized by a pseudolayer displacement field and an effective director normal to the layers. The model specifically describes how the distortions of the pseudolayers couple to the polarization field-a result useful not only to interpret our DLS measurements but also, as reported very recently [38], to account for second harmonic generation from defects in the N TB pseudolayer structure.…”

Does Topology Dictate the Incidence of the Twist‐Bend Phase? Insights Gained from Novel Unsymmetrical Bimesogens

“…The observation of a lower‐temperature mesophase lacking lamellar or positional molecular order (i.e., a nematic‐like mesophase), later identified as the “twist‐bend nematic” phase (N TB ), has motivated many studies of the properties of this intriguing phase of matter through a wide range of techniques . Although the helicoidal model of the N TB phase, proposed independently by Meyer and Dosov, is supported by experimental data, alternate models have also been suggested .…”

“…The observation of al ower-temperature mesophase lacking lamellar or positional molecular order( i.e.,anematic-likem esophase), [1] later identified as the "twist-bend nematic" phase (N TB ), [2][3][4][5][6][7][8][9][10] has motivatedm any studies of the properties of this intriguing phase of matter through aw ide range of techniques. [11][12][13][14][15][16][17][18] Although the helicoidal modelo ft he N TB phase, proposed independently by Meyer and Dosov, [19,20] is supported by experimental data, alternate modelsh ave also been suggested. [21][22][23][24] With the exception of Vanakaras et al, [23] theoretical treatments have not accounted for direct isotropic to twistbend nematic phase transitions, as cenario first reported in am ixed dimer/chiral-dopant system.…”