Flexoelectricity-driven softening of bend elasticity leads to spontaneous chiral symmetry breaking in a polar fluid

Abstract

The origin of recently observed spontaneous chiral symmetry breaking in polar fluids is an unsolved problem, and poses fundamental questions as to how heliconical structures emerge in systems composed of achiral molecules. We report on the softening of bend elasticity close to such phase transition, showing that flexoelectric coupling between the electric polarization and the bend deformation is the responsible mechanism, presumably arising from the bent shape of the constituent highly polar molecules.

Figures (8)

• Figure 1: Schemes and polarized microscope (POM) images of the material F7i in a 10 $\mu$m-thick cell with parallel rubbing. (a) N$_{\text{F}}$ at 32$^{\circ}$C and (b) N$_{\text{TBF}}$ just below the N$_{\text{F}}$-N$_{\text{TBF}}$ phase transition. P, A, and R denote the polarizer, analyzer, and rubbing directions, respectively. Inset (b) shows a photograph of the cell exhibiting strong Bragg scattering.
• Figure 2: (a) 3D plot of the dielectric absorption spectra versus temperature. (b) Temperature dependence of the dielectric strengths ($\Delta \varepsilon$, empty symbols) and frequencies of maximum absorption ($f_{\text{a}}$, full symbols) of the different relaxation processes.
• Figure 3: Temperature dependence of the bend and twist diffusivities (top), normalized bend and twist elastic constants (middle), and ratio $K_3/K_2$ (bottom).
• Figure 4: (a) Dependence of $K_{3,\text{eff}}/K_{3,\text{eff}}(55^{\circ}$C$)$ on the amplitude of the mode $\Delta \varepsilon_{\text{NF,H}}$. (b) Schematics of the bend flexoelectric effect in the N$_{\text{F}}$ phase. Here, b is the vector related to shape polarity (see text).
• Figure 5: Temperature dependence of the amplitude of the X-ray scattering peak that corresponds to local smectic order.