Singh Gautam, Fu Jinxin, Agra-Kooijman Dena M, Song Jang-Kun, Vengatesan M R, Srinivasarao Mohan, Fisch Michael R, Kumar Satyendra
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.
Phys Rev E. 2016 Dec;94(6-1):060701. doi: 10.1103/PhysRevE.94.060701. Epub 2016 Dec 13.
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.
利用X射线和偏振拉曼散射测量了液晶二聚体CB7CB向列相（N）和扭曲弯曲向列相（Ntb）中取向序参量〈P2(cosβ)〉和〈P4(cosβ)〉的温度依赖性。通过这两种技术获得的〈P2(cosβ)〉是相同的，而通过拉曼散射确定的〈P4(cosβ)〉正如预期的那样，系统地大于其X射线值。在向列相中，随着温度降低，这两个序参量均增大，在N-Ntb转变处下降，并继续减小。在扭曲弯曲向列相中，〈P4(cosβ)〉的X射线值最终变为负值，这直接且独立地证实了锥形分子取向分布。从扭曲弯曲向列相中的取向分布函数确定的螺旋倾斜角α在转变温度以下约15 K时增加到约24°。在扭曲弯曲向列相中，α(T)∝(T*-T)λ，其中λ = 0.19±0.03。转变过冷1.7 K，与其弱一级性质一致。λ值接近0.25，表明接近三临界点。
