Recent discovery of the ferroelectric nematic phase N_F resurrects a question about stability of the uniform N_F state with respect to the formation of either standard for solid ferroelectrics domain structure, or often occurring in liquid crystals space modulation of the polarization vector P (and naturally coupled to P nematic director. In this work within Landau mean-field theory we investigate the linear stability of the minimal model admitting the conventional paraelectric nematic N and N_F phases. Our minimal model, (besides the standard terms of the expansion over P and director gradients) includes, also standard for liquid crystals, director flexoelectric coupling term f and often overlooked in the literature (although similar by its symmetry to the director flexoelectric coupling) the flexo-dipolar coupling. We find that in the easy-plane anisotropy case the uniform N_F state loses its stability with respect to one-dimensional or two-dimensional modulation. For non-zero f the 2D modulation threshold is always higher than its 1D counterpart. No any instability at all if one neglects the dipole-flexoelectric coupling. In the easy-axis case the both instability thresholds are the same, and the instability can occur even without flexo-dipolr coupling. We speculate that the phases with 1D or 2D modulations can be identified with discussed in the literature [see M.P.Rosseto, J.V.Selinger, Physical Review E, volume 101, page 052707 (2020)] single splay or double splay nematics.
Nematics with a broken polar symmetry is one of the fascinating recent discoveries in the field of soft matter. High spontaneous polarisation and the fluidity of the ferroelectric nematic $N_{\mathrm{F}}$ phase make such materials attractive for future applications and interesting for fundamental research. Here we explore the polar and mechanical properties of a room-temperature ferroelectric nematic and its behaviour in a magnetic field. We show that $N_{\mathrm{F}}$ is much less susceptible to the splay deformation than to the twist. The strong splay rigidity can be attributed to the electro…
Recent experimental realization of ferroelectric nematic liquid crystalline phases stimulated material development and numerous experimental studies of these new phases, guided by their fundamental and applicative interest. In this Perspective, we give an overview of this emerging field by linking history and theoretical predictions to a general outlook of the development and properties of the materials exhibiting ferroelectric nematic phases. We will highlight the most relevant observations up-to-date, e.g., giant dielectric permittivity values, polarization values an order of magnitude large…
A thermal gradient-induced circular motion of particles placed on ferroelectric nematic liquid crystal sessile drops is demonstrated and explained. Unlike hurricanes and tornadoes that are the prime examples for thermal motors and where turbulent flows are apparent, here the texture without tracer particles appears completely steady indicating laminar flow. We provide a simple model showing that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become ele…
Ferroelectric ordering in liquids is a fundamental question of physics. Here, we show that ferroelectric ordering of the molecules causes formation of recently reported splay nematic liquid-crystalline phase. As shown by dielectric spectroscopy, the transition between the uniaxial and the splay nematic phase has the characteristics of a ferroelectric phase transition, which drives an orientational ferroelastic transition via flexoelectric coupling. The polarity of the splay phase was proven by second harmonic generation (SHG) imaging, which additionally allowed for determination of the splay m…
We present a generalization of the Maier-Saupe (MS) theory that incorporates the description of the ferroelectric nematic phase (NF) within its scope of application. The extension of the theory is carried out in a natural way, by adding to the nematic potential of MS a term proportional to cosθ and to the polar order parameter <cosθ >, where θ is the angle between the long molecular axis and the spontaneous polarization. The NF phase can be reached by cooling from the isotropic phase with or without the formation of an intermediate normal nematic phase, depending on the relative intensity of t…
We have structurally characterized the liquid crystal phase that appears as an intermediate state when a dielectric nematic, having polar disorder of its molecular dipoles, transitions to the almost perfectly polar-ordered ferroelectric nematic. This intermediate phase, which fills a 100-year-old void in the taxonomy of smectics and which we term the "smectic $Z_A$", is antiferroelectric, with the nematic director and polarization oriented parallel to smectic layer planes, and the polarization alternating in sign from layer to layer. The period of this polarization wave ($\sim$180 A) is mesosc…
Adiabatic scanning calorimetry (ASC) is capable of providing simultaneously the specific enthalpy h(T) and the specific heat capacity cp(T), and is an important tool to determine the order of transitions and to render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound DIO and on mixtures with RM734. Both compounds exhibit a low-temperature ferroelectric nematic phase (NF) and a high-temperature paraelectric nematic (N). In DIO these two phases are separated by an intermediate phase (Nx). Detailed data of h(T) and cp(T), indicated that…
Elasticity-Driven Periodic Polarization Patterns in Confined Chiral Ferroelectric Nematic Fluid
Jožef Stefan Institute（约瑟夫·斯特凡研究所）；Faculty of Mathematics and Physics, University of Ljubljana（卢布尔雅那大学数学与物理学院）；CENN Nanocenter（CENN纳米中心）；Department of Physics, Faculty of Science and Technology, University of the Basque Country UPV/EHU（巴斯克国立大学理学院物理系）；School of Chemistry, University of Leeds（利兹大学化学学院）；School of Physics and Astronomy, University of Leeds（利兹大学物理与天文学院）
The recent discovery of the ferroelectric splay nematic phase has opened the door to experimental investigation of one of the most searched liquid crystal phases in decades, with high expectations for future applications. However, at this moment, there are more questions than answers. In this work, we examine the formation and structure of large polar nematic domains of the ferroelectric splay nematic material RM734 in planar liquid crystals cells with different aligning agents and specifications. We observe that confining surfaces have a strong influence over the formation of different types …
Direct visualization of local electric fields in a layer of a ferroelectric nematic liquid
Jožef Stefan Institute（约瑟夫·斯特凡研究所）；Faculty of Mathematics and Physics, University of Ljubljana（卢布尔雅那大学数学与物理学院）；School of Chemistry, University of Leeds（利兹大学化学学院）；School of Physics and Astronomy, University of Leeds（利兹大学物理与天文学学院）
Realignment of ferroelectric nematic by photoinduced electric field
Kent State University（肯特州立大学）；Institute of Physics, National Academy of Sciences of Ukraine（乌克兰国家科学院物理研究所）；Institute of Physical Chemistry, Polish Academy of Sciences（波兰国家科学院物理化学研究所）
The electroviscous effect deals with the change in the viscosity of fluids due to an external electric field. Here, we report experimental studies on the electroviscous effects in a ferroelectric nematic liquid crystal. It was synthesised accomplishing a new synthetic route which provides higher yield than conventional one. We measure electric field-dependent viscosity under a steady shear at different temperatures. In the low field range, the increase in viscosity ($Δη=η(E)-η_0$) is proportional to $E^2$ and the corresponding viscoelectric coefficient ($f\approx10^{-9}$m\textsuperscript{2}/V\…
Calorimetric evidence for the existence of an intermediate phase between the ferroelectric nematic phase and the nematic phase in the liquid crystal RM734
本研究利用高分辨率量热测量证明，液晶 RM734 在铁电向列相 NF 与经典向列相 N 之间的狭窄温区内也存在中间相 Nx。
Nematic liquid crystals have been known for more than a century, but it was not until the 60s-70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular s…
We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxia…
Spontaneous electric polarization of solid ferroelectrics follows aligning directions of crystallographic axes. Domains of differently oriented polarization are separated by domain walls (DWs), which are predominantly flat and run along directions dictated by the bulk translational order and the sample surfaces. Here we explore DWs in a ferroelectric nematic (NF) liquid crystal, which is a fluid with polar long-range orientational order but no crystallographic axes nor facets. We demonstrate that DWs in the absence of bulk and surface aligning axes are shaped as conic sections. The conics bise…
Topological spin and polar textures have fascinated people in different areas of physics and technologies. However, the observations are limited in magnetic and solid-state ferroelectric systems. Ferroelectric nematic is the first liquid-state ferroelectric that would carry many possibilities of spatially distributed polarization fields. Contrary to traditional magnetic or crystalline systems, anisotropic liquid crystal interactions can compete with the polarization counterparts, thereby setting a challenge in understating their interplays and the resultant topologies. Here, we discover chiral…
Shear-mode Direct Piezoelectric Response of Ferroelectric Nematic Liquid Crystals
Institute for Solid State Physics and Optics, HUN-REN Wigner Research Centre for Physics（固态物理与光学研究所，HUN-REN维格纳物理研究中心）；RIKEN Center for Emergent Matter Science（理化学研究所新兴物质科学中心）；Department of Physics, Kent State University（肯特州立大学物理系）；Materials Sciences Graduate Program and Advanced Materials and Liquid Crystal Institute, Kent State University（肯特州立大学材料科学研究生项目和先进材料与液晶研究所）
