Spontaneous helielectric nematic liquid crystals: Electric analog to helimagnets

Recently, a type of ferroelectric nematic fluid has been discovered in liquid crystals in which the molecular polar nature at molecule level is amplified to macroscopic scales through a ferroelectric packing of rod-shaped molecules. Here, we report on the experimental proof of a polar chiral liquid matter state, dubbed helielectric nematic, stabilized by the local polar ordering coupled to the chiral helicity. This helielectric structure carries the polar vector rotating helically, analogous to the magnetic counterpart of helimagnet. The helielectric state can be retained down to room temperature and demonstrates gigantic dielectric and nonlinear optical responses. This matter state opens a new chapter for developing the diverse polar liquid crystal devices.

In nature , a new matter state usually arises as a result of unexpected combinations of hierarchical orderings. Helicity is one of the most essential nature of matter states for organizing superstructures in soft matters, spanning many length scales from the atomic to the macroscopic biological levels. When constructed from building blocks with inherent polarity, three hierarchical orderings could coexist in a helical structure: 1) the head-to-tail or polar symmetry of each building block (e.g., Fig. 1C), 2) the orientational order of a swarm of building blocks (Fig. 1A), and 3) the emergent helicity (Fig. 1B). While a simultaneous realization of these three orderings could lead to extraordinary material properties, such highly hierarchical structures are often challenging to achieve in man-made systems. Probably the most familiar example is the chiral magnet or helimagnet (Fig. 1B) in quantum systems, where the magnetic spins form two- or three-dimensional spiral structures (1, 2). The polar magnetic helical structures are considered mainly to originate from either the breaking of the space-inversion symmetry in crystal structures (3) or the magnetic frustration (1, 4, 5). Their strong magnetism-chirality coupling triggers enormous interests in condensed matter physics, leading to many unique quantum and information functionalities (6–9). From the mirror relationship between the magnetism and electricity, we anticipate the incidence of a possible electric version of the helimagnets, namely helielectrics. However, the diverse magnetic topological states rarely show up in electric systems, except a few recent breakthroughs (e.g., the observation of the electric skyrmions, polar vortices, and merons in metal-organic crystals) (10–12). The special electric states at nanoscale exhibit extraordinary properties such as local negative dielectric permittivity (13) and strain-polarization coupling (14, 15). Nevertheless, nearly all the aforementioned chiral magnet or electric-analog systems are based on elaborately fabricated inorganics. It is expected that the revolutionary realization of these topologies in a soft matter system would bring the advantages of flexibility, simple preparation, large-area film formation, and ease of integration into electric devices.Open in a separate windowFig. 1.Topological analogy: electric versus magnetic states. (A) Uniform magnetization or polarization. (B) Helimagnet or helielectric states. Possible helicoidal (top) and heliconical (bottom) textures are shown. (C) Molecular structure of the polar anisotropic entity, RM734. The molecular polar dipole is nearly parallel to the long molecular axis. (D) The ferroelectric nematic state with spontaneous polarization. (E) HN* state with heli realized by adding chiral generators into the polar chiral nematic state. One-dimensional polarization fields are also depicted in D and E for clarity. (F) The molecular structures of the chiral generators S1 and S2. (G) The state diagram of the two HN* materials by mixing RM734 with S1 or S2.Among the soft matter systems, liquid analogs of ferromagnet and helimagnet have been reported in liquid crystal (LC) colloids recently (16–20). For the electric versions, there already exist a category of materials possessing all the aforementioned three hierarchical orderings (i.e., the ferroelectric smectic LCs) (21–26). The smectic C* (SmC*) has layered heliconical structure with its local polarity aligning perpendicular to the long molecular axis. Confinement to thin LC cells leads to the unwound ferroelectric state of SmC* with microsecond switching time, thereby being a promising candidate for LC display applications. However, the unavoidable defect generation in the devices originated from the crystal-like structure has been one of the main technical difficulties. Moreover, the SmC* has intrinsically low fluidity and polarity (spontaneous polarization P_s < 1 μC). Here, we report a discovery of a helimagnetic analog state in polar LC materials, dubbed helielectric nematic (HN*). The spontaneous polar nematic ordering is coupled to the chiral orientational helicity (Fig. 1B), taking the form with a nearly helicoidal orientational field. Thanks to its much higher fluidity than the traditional SmC* ferroelectrics, uniform structures can be easily obtained by the typical thermal annealing process. The simultaneous observation of the traditional nonlinear second-harmonic generation (SHG) and SHG interferometry microscopies, as well as the optical observations of the selective reflection from HN* state, allow us to directly visualize the helical polar field. In contrast to the traditional nanoscopic helimagnetic or helielectric inorganics, a wide tunability of the periodic distance ranging from micrometers to near ultraviolet wavelength is achieved in the fluidic structure. Besides, the ability of switching between the polar and nonpolar helical LC states enables complementary physics study for the topology features in HN*. As gifts of the chirality–polarity interaction, the matter state uniquely expresses giant dielectric and SHG optical response, especially interesting SHG amplification when the SHG wavelength coincides with the reflection band of the HN* state.