Nataša Vaupotič

The formation of chiral structures from achiral building blocks, and the propagation of chirality across length-scales in soft matter are still poorly understood phenomena. Here, we have studied a system revealing an exceptional diversity of spontaneously chiral phases formed by achiral mesogenic dimers linked by spacers with an odd number of atoms. Depending on terminal chain length and temperature, these compounds form a well-known heliconical nematic (N _TB ) phase and distinct helical smectic phases - ranging from the nanoscale-pitch SmC _TB–SH , through the SmC _TB–DH phase with ~ 50 nm periodicity, to the newly identified SmC _TB–C phase featuring a micron-scale helix. In the SmC _TB–C phase heliconical order is preserved even though the interlayer molecular intercalations strongly suppress the azimuthal rotation of the director. Resonant soft X-ray scattering (RSoXS) measurements were performed for the first time across an entire homologous series. Results confirmed the double-helical structure of the SmC _TB–DH phase in which a longer helix is superimposed on the short one. The intensity of the resonant signals revealed an anomaly: the non-monotonic temperature evolution is due to the transient passage of the structure from a four-layer helix, through a nearly perfect three-layer clock-like helix, before decoupling of the short and long helices at lower temperatures.

Surface alignment and edge dislocations are explored in the recently discovered twist‐bend ferroelectric nematic, N _TBF , in which the vector of spontaneous polarization follows an oblique helicoidal trajectory around a polar twist‐bend axis. In a planar cell, the polar axis aligns at some angle to the rubbing direction to mitigate the surface electric charge. It is demonstrated that the pseudolayers in planar cells form chevron defects, a hallmark defect of one‐dimensionally positionally ordered phases, such as smectic A and smectic C. The polar character of the twist‐bend axis prevents the cores of N _TBF edge dislocations from splitting into semi‐integer disclinations, in stark contrast to dislocations in paraelectric and ferroelectric chiral nematics. The tilt of pseudolayers around the defect core allows estimation of the elastic penetration length as being close to the pitch of N _TBF . Compression/dilation stresses around the core modify the heliconical tilt angle of molecules as evidenced by a substantial variation in local birefringence. The climb of dislocations exhibits high mobility, allowing the system to equilibrate the temperature‐dependent pitch. The uncovered properties facilitate the development of N _TBF materials for electro‐optical applications, such as electrically controlled diffraction lattices and structural colors.

We show that development of polar order may spontaneously destroy the lamellar structure of a liquid crystal. This results in an unusual sequence of phases, with the ferroelectric nematic phase appearing below a non‐polar smectic phase. The effect is related to unfavourable dipole interactions within the smectic layers and can be explained by Landau theory, in which the temperature‐dependent term is non‐monotonic as it is renormalised by spontaneous electric polarisation.

We show that development of polar order may spontaneously destroy the lamellar structure of a liquid crystal. This results in an unusual sequence of phases, with the ferroelectric nematic phase appearing below a non‐polar smectic phase. The effect is related to unfavourable dipole interactions within the smectic layers and can be explained by Landau theory, in which the temperature‐dependent term is non‐monotonic as it is renormalised by spontaneous electric polarisation.

The twist‐grain‐boundary (TGB) phases, characterized by a periodic, helical arrangement of blocks made of polar smectic phases, SmA _F and SmC _F , have been discovered. They are observed for rod‐like molecules with a strong longitudinal dipole moment, featuring an (S)‐2‐methylbutyl end group having only weak twisting power, and emerge below the antiferroelectric SmA _AF phase, where the lamellar structure is already well established. It is suggested that the structure is governed by electrostatic interactions amplified by weak chiral forces, in striking contrast to the mechanism of TGB phase formation found in non‐polar materials. The TGB phases exhibit selective light reflection in the visible range, while the value of electric polarization confirms an almost perfectly ordered dipole alignment.

Two models have recently been proposed for a description of dielectric spectroscopy measurements of ferroelectric nematics (NF) in thin planar capacitors. The polarization-external capacitance Goldstone reorientation mode (PCG model) considers the NF layer between the electrodes as an effective low resistivity material, the resistivity being inversely proportional to the square of polarisation magnitude. The high-ε model considers the NF material as having a huge permittivity due to the ease of polarisation rotation. In this paper we study implications of both models and show, why both models describe majority of the observed dielectric spectroscopy results equally well. We point out differences among the models predictions and explain why some observations can be explained only by the high-ε model. The major difference between the models is that the high-ε model predicts that the increase in the cell thickness can lead to an increase in the frequency range within which capacitors filled with NF material can be used for energy storage while within the PCG model this frequency range reduces with increasing capacitor thickness. Within both models a crucial parameter which determines the behaviour of the capacitors filled with a NF material is parasitic resistance, primarily due to the electrode resistance. We present measurements of electrode resistance and find that in ITO cells it is of the order of few hundred ohms.

The magnitude of the relative permittivity of the ferroelectric nematic phase ( N F ) has been the subject of lively scientific discussion since the phase was recently discovered. Dielectric spectroscopy measurements (DSMs) give a huge value of relative permittivity, which depends on the cell thickness, but this is argued to result from a misinterpretation of the DSM results. We have conducted DSM using a set of cells differing in thickness of the N F layer, type of electrodes, and presence/absence of nanoscale-thick surface polymer layers. To model the DSM results, cells are presented by an equivalent electric circuit that includes a capacitor due to the N F layer with frequency dependent complex relative permittivity, capacitors due to surface layers, and a resistor describing the limited conductivity of electrodes. DSM results for different cells with the same liquid crystal in the N F phase are semiquantitatively reproduced by the same set of physical parameters if a huge relative permittivity of the N F , which is even orders of magnitude larger than the measured apparent values, is assumed. We show that the capacitance of surface layers should also be considered in cells with no polymer alignment layer on the electrodes. Published by the American Physical Society 2024

We studied the dielectric properties of a polar nematic phase (N_F) sandwiched between two gold or ITO electrodes, serving as a cell surface. In bulk, N_F is expected to exhibit a Goldstone mode (phason), because polarisation can uniformly rotate with no energy cost. However, because the coupling between the direction of nematic director and polarisation is finite, and the confinement, even in the absence of the aligning surface layer, induces some energy cost for a reorientation of polarisation, the relaxation frequency of the phason mode is measured in a kHz regime. The phason mode is easily quenched by a bias electric field, which enables fluctuations in the magnitude of polarisation to be followed in both the ferroelectric and paraelectric nematic phases. This amplitude (soft) mode is also influenced by boundary conditions. A theory describing the phase and amplitude fluctuations in the N_F phase shows that the free energy of the system and, consequently, the dielectric response are dominated by polarisation-related terms with the flexoelectricity being relevant only at a very weak surface anchoring. Contributions due to the nematic elastic terms are always negligible. The model relates the observed low-frequency mode to the director fluctuations weakly coupled to polarisation fluctuations. GRAPHICAL ABSTRACT

A resonant X-ray scattering for two structural models of a chiral cubic liquid crystalline phase with a giant unit cell is studied theoretically and compared to experimental measurements.