Critical behavior of the optical birefringence at the nematic to twist-bend nematic phase transition

Abstract

The critical behavior at the transition from a uniform nematic to a twist-bend modulated nematic phase is revealed and shown to be well explained by the mean field approximation. The study was performed on a group of materials that exhibit an unusually broad temperature range of the nematic phase above the twist-bend modulated nematic phase, so the critical range in which the order parameter fluctuations are strong and can be experimentally observed is wide. The formation of instantaneous helices is observed already several degrees above the transition temperature, strongly influencing the birefringence of the system. The analysis of a critical part of the birefringence changes provides a set of critical exponents that are consistent with the mean field approximation, indicating a large correlation length of helical fluctuations in the nematic phase.

Twist‐Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene‐based Nonsymmetric Dimers

“…The tilting of the molecules and formation of a helical structure at the N‐N TB phase transition is accompanied by a small departure of the optical birefringence from the critical temperature behaviour observed in the N phase (Figure 9). Similar behaviour has been observed for other dimers [49] . Comparing the actual value of birefringence in the N TB phase with the line extrapolated from a critical increase in the N phase ( where is the maximum value of optical birefringence for a phase with ideal orientational order, S=1, is a critical temperature and is a critical exponent), to the temperature range of the N TB phase allows us to estimate the tilt angle of the molecules in the heliconical structure, [50] and it was found to saturate below 20°.…”

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

“…Interestingly, the measured birefringence departs from the critical dependence when approaching the N -NTB phase transition (Figure 8), such behaviour was attributed recently to the formation of instantaneous local heliconical states [30]. Among the dimers containing cyanoterphenyl (CT) unit, the highest no is found for the dimer with the methylene and ether linkages ( n o =0.383), consistent with the most linear shape of CTO6CB molecules.…”

“…Indeed, this approach underpinned the successful design of the first pure material to exhibit a direct NTB-I transition [29], and such transitions remain rare [22]. The nematic phase preceding a twist-bend nematic phase shows anomalously low values of the bend elastic constant, K33, allowing for the appearance of an instantaneous, local heliconical structure [30] and may be exploited in new effects such as the electrically controlled selective reflection of light [31] and electrically tuneable lasers [32].…”

The Chiral Twist‐Bend Nematic Phase (N*TB)

“…The Supporting Information includes a video of the N* TB –N* phase transition. We suggest that the textural changes on cooling reflect the unwinding of the long helix of the N* phase and the formation of instantaneous short helices as the transition to the N* TB phase is approached. The X‐ray diffraction patterns for the N* TB and N* phases of this mixture are shown in Figure S2 (Supporting Information), and these are identical to those seen for achiral systems (Figure ).…”