Observation of a Chiral-Symmetry-Breaking Twist-Bend Instability in Achiral Freely Suspended Liquid-Crystal Films

Freely suspended smectic-$C$ films, 2, 3, and 4 layers in thickness, of the achiral compound ${\mathrm{C}}_7{\mathrm{F}}_{15}{\mathrm{CH}}_2\mathrm{O}-\phi -\mathrm{C}\mathrm{O}\mathrm{O}-\phi -\mathrm{O}{\mathrm{C}}_6{\mathrm{H}}_{13}$ exhibit a thermotropic transition at which the uniform two-dimensional smectic-$C$ molecular orientation field becomes modulated by a periodic bend wave soliton. No such transition is found in single-layer films. This transition is interpreted as a chiral-symmetry-breaking transition, marking the spontaneous appearance of frustrated local bend.