X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals
Autor: | Dena Mae Agra-Kooijman, Michael R. Fisch, Jang-Kun Song, Mohan Srinivasarao, Satyendra Kumar, M. R. Vengatesan, Jinxin Fu, Gautam Singh |
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Rok vydání: | 2016 |
Předmět: |
Materials science
Biaxial nematic Condensed matter physics Dimer X-ray Order (ring theory) 02 engineering and technology 010402 general chemistry 021001 nanoscience & nanotechnology 01 natural sciences 0104 chemical sciences symbols.namesake chemistry.chemical_compound Tricritical point chemistry Liquid crystal Phase (matter) symbols 0210 nano-technology Raman scattering |
Zdroj: | Physical Review E. 94 |
ISSN: | 2470-0053 2470-0045 |
Popis: | The temperature dependence of the orientational order parameters 〈P_{2}(cosβ)〉 and 〈P_{4}(cosβ)〉 in the nematic (N) and twist-bend nematic (N_{tb}) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The 〈P_{2}(cosβ)〉 obtained from both techniques are the same, while 〈P_{4}(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-N_{tb} transition, and continue to decrease. In the N_{tb} phase, the x-ray value of 〈P_{4}(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 N_{tb} phase, increases to ∼24^{∘} at ∼15 K below the transition. In the N_{tb} 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. |
Databáze: | OpenAIRE |
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