Nematic liquid crystals are characterized by their orientational order without positional order: Their typically rod-shaped molecules all align along, say, the z–axis. Nevertheless, they’re generally nonpolar. Even if the individual molecules have permanent electric dipole moments, the bulk liquid is a homogeneous mix of molecules oriented in the +z and −z directions.
A liquid crystal that breaks that rule could be of technological importance: Spontaneous electric polarization in a liquid could enable the molecular orientations to be switched quickly and with low power. But so far, ferroelectricity in liquid crystals has been limited to smectic phases, which, because of their partial positional order, aren’t fully fluid.
Now Nerea Sebastián, Alenka Mertelj (both at the Jožef Stefan Institute in Ljubljana, Slovenia), and their colleagues have shown experimentally that a nematic liquid crystal can become ferroelectrically ordered while remaining nematic. Their molecule of choice, first investigated three years ago by Richard Mandle, John Goodby, and colleagues at the University of York in the UK, is both polar and slightly wedge shaped. By measuring its heat capacity as a function of temperature, the York researchers had found that the system undergoes a phase transition, but they couldn’t determine the nature of the transition: Above and below the transition temperature, the nematic material looked exactly the same.
Through a combination of measurements, the Ljubljana researchers figured out what was going on. The high-temperature phase was the usual uniform nematic one, and the low-temperature phase was organized into periodic regions, shown in yellow and blue in the figure, that were slightly splayed in alternating directions.
Because of the molecules’ wedge-like shape, such splay could arise from their spontaneous alignment, not just along the z-axis, but in either the +z or −z direction. Second-harmonic-generation imaging, which is sensitive to electrical polarization, confirmed that picture: Each domain was electrically polarized. Furthermore, the liquid’s electric susceptibility diverged at the transition temperature, so the transition must be a ferroelectric one.
Nematic liquid crystals with unusual symmetry breaking have been observed before. For example, in the so-called twist–bend phase, banana-shaped molecules spontaneously organize into helical structures. But whereas the size scale of the orientation modulations in that phase is on the order of nanometers, the new splay modulations are each several microns wide. Unfortunately, the Ljubljana researchers’ experiments so far are limited to right around the transition point; cool the liquid too far, and the modulations reorient and are hard to measure. But they’re enough to start to get a theoretical handle on the subtle interplay between molecular polarity, shape, and symmetry breaking. (N. Sebastián et al., Phys. Rev. Lett. 124, 037801, 2020.)
