Deciphering helix assembly in the heliconical nematic phase via tender resonant X-ray scattering

Abstract

Being a link between a uniaxial nematic and a chiral nematic, the twist bend nematic (N_TB) has been an intriguing topic for research over the last decade as a key to understanding chirality generation. Accurate description and understanding of resonance effects in helical structures provide crucial knowledge on the phase behavior beyond positional ordering. We examined the manifestation of resonance effects in N_TBvia tender resonant X-ray scattering (TReXS) at the sulfur K-edge. For the first time we demonstrate quantitatively that the energy dependence of the scattering peak in the N_TB phase follows the energy dependence of the complex refractive indices measured by X-ray absorption. Taking advantage of molecular energy calculation, we decipher the distinct helical pitch variation trend for symmetric/asymmetric dimers from the perspective of hybridization and reveal the strong effect of substituting oxygen for sulfur atoms on the helical pitch and the thermal helix extension.