Polar order in bent-core nematics: An overview

The possible existence of a ferroelectric nematic phase has been challenging liquid crystal scientists since it was first envisaged about 30 years ago. At present, an undisputable proof of proper ferroelectricity in low-molecular weight thermotropic nematics is still missing. However, over the last decade several groups have reported the observation of a ferroelectric-like switching behavior in the nematic phase of a number of bent-core mesogens. This effect is generally ascribed to polar clusters of molecules, known as cybotactic groups, permeating the nematic phase. In the unperturbed state the cluster dipoles are randomly oriented, so that the phase is macroscopically uniaxial and apolar; however, an external field exceeding a threshold is able to align the cluster dipoles, inducing the transition to a switchable polar and biaxial state. Even though this behavior clearly differs from that expected in a proper ferroelectric liquid crystal, it still deserves considerable interest for its potential implications from both a fundamental and a technological point of view. Here we review the studies devoted to this extraordinary effect, both experiments and simulations, and discuss the open questions that still remain unanswered.