Skip to main content

Soft modes of the dielectric response in the twist-bend nematic phase and identification of the transition to nematic splay bend phase in the CBC7CB dimer

Merkel, Katarzyna; Kocot, Antoni; Mehl, Georg H; Welch, Chris

Authors

Katarzyna Merkel

Antoni Kocot

Georg H Mehl

Chris Welch



Contributors

Chris Welch
Researcher

Abstract

The dielectric spectra of the twist–bend nematic phase (NTB) of (the bent-shaped) achiral liquid-crystal dimer 1”-,7”-bis(4-cyanobiphenyl-4’-yl) heptane (CB7CB) are studied for the determination of the different relaxation modes. Two molecular processes and one collective process were observed in the megahertz frequency range. Two molecular processes were assigned: one to the precessional rotation of the longitudinal components of the cyanobiphenyl groups and the second to the spinning rotation of the transverse component of the CB7CB dimer. The peak, at a frequency about 1MHz, shows a peculiar temperature behavior at the NTB to N transition, reminiscent of the soft mode at the transition from the SmA to the SmC phase. This peak can be assigned to a collective fluctuation of the tilt angle of the coarse grained director N with respect the pseudo-layer normal. This corresponds well with the electro-clinic effect observed as a response to an electric field in electro-optic experiments. The low frequency relaxation process, observed in the frequency range 1 Hz – 102 Hz can be identified as a Goldstone mode, related to long-scale fluctuation of the cone phase. The frequency drop of the mode on increasing the bias field is interpreted as unwinding of a helix and an indication of the formation of a field induced nematic splay bend phase (NSB). This finding is also confirmed by birefringence data in the presence of strong bias fields.

Journal Article Type Article
Publication Date Sep 30, 2019
Journal Physical Chemistry Chemical Physics
Print ISSN 1463-9076
Electronic ISSN 1463-9084
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
APA6 Citation Merkel, K., Kocot, A., Mehl, G. H., & Welch, C. (2019). Soft modes of the dielectric response in the twist-bend nematic phase and identification of the transition to nematic splay bend phase in the CBC7CB dimer. Physical chemistry chemical physics : PCCP, https://doi.org/10.1039/c9cp04952e
DOI https://doi.org/10.1039/c9cp04952e
Keywords Physical and Theoretical Chemistry; General Physics and Astronomy
Additional Information : This document is Similarity Check deposited; : Single-blind; : Received 5 September 2019; Accepted 29 September 2019; Accepted Manuscript published 30 September 2019
;