title: X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals.
abstract: 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.
authors.affiliation: Department of Physics, Kent State University, Kent, Ohio 44242, USA.|Department of Applied Physics, Amity Institute of Applied Sciences, Amity University, Noida, UP 201313, India.
authors.forename: Gautam
authors.initials: G
authors.lastname: Singh
authors.affiliation: School of Materials Science and Engineering, School of Chemistry and Biochemistry, and Center for Advanced Research on Optical Microscopy, Georgia Institute of Technology, Atlanta, Georgia 30332-0295, USA.
authors.forename: Jinxin
authors.initials: J
authors.lastname: Fu
authors.affiliation: Department of Physics, Kent State University, Kent, Ohio 44242, USA.
authors.forename: Dena M
authors.initials: DM
authors.lastname: Agra-Kooijman
authors.affiliation: School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon, Republic of Korea.
authors.forename: Jang-Kun
authors.initials: JK
authors.lastname: Song
authors.affiliation: School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon, Republic of Korea.
authors.forename: M R
authors.initials: MR
authors.lastname: Vengatesan
authors.affiliation: School of Materials Science and Engineering, School of Chemistry and Biochemistry, and Center for Advanced Research on Optical Microscopy, Georgia Institute of Technology, Atlanta, Georgia 30332-0295, USA.
authors.forename: Mohan
authors.initials: M
authors.lastname: Srinivasarao
authors.affiliation: College of Applied Engineering Sustainability and Technology, Kent State University, Kent, Ohio 44242, USA.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Fisch
authors.affiliation: Department of Physics, Kent State University, Kent, Ohio 44242, USA.|Division of Research and Department of Physics, University at Albany, Albany, New York 12222, USA.
authors.forename: Satyendra
authors.initials: S
authors.lastname: Kumar