X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals

Gautam Singh, Jinxin Fu, Dena M. Agra-Kooijman, Jang Kun Song, M. R. Vengatesan, Mohan Srinivasarao, Michael R. Fisch, Satyendra Kumar

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    20 Scopus citations

    Abstract

    The temperature dependence of the orientational order parameters (P2(cosβ)) and (P4(cosβ)) in the nematic (N) and twist-bend nematic (Ntb) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The (P2(cosβ)) obtained from both techniques are the same, while (P4(cosβ)), determined by Raman scattering is, as expected, systematically larger than its x-ray value. Both order parameters increase in the N phase with decreasing temperature, drop across the N-Ntb transition, and continue to decrease. In the Ntb phase, the x-ray value of (P4(cosβ)) eventually becomes negative, providing a direct and independent confirmation of a conical molecular orientational distribution. The heliconical tilt angle α, determined from orientational distribution functions in the Ntb phase, increases to ∼24 at ∼15 K below the transition. In the Ntb phase, α(T) -(T∗-T)λ, with λ=0.19±0.03. The transition supercools by 1.7 K, consistent with its weakly first-order nature. The value of λ is close to 0.25 indicating close proximity to a tricritical point.

    Original languageBritish English
    Article number060701
    JournalPhysical Review E - Statistical, Nonlinear, and Soft Matter Physics
    Volume94
    Issue number6
    DOIs
    StatePublished - 13 Dec 2016

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