arXiv subjects

Ewa Gorecka

Publications and source records attributed to Ewa Gorecka.

At least 19 recordsLinked to original sources

arXiv2026

Mirror vs. inversion symmetry breaking in mesogenic dimers: NTB vs. NF phase

The recently discovered twist-bend nematic NTB and ferroelectric nematic NF phases are distinct examples of spontaneous symmetry breaking in liquid crystals. Here, we report the occurrence of both type nematic phases within the same homologous series of dimers consisting of two strongly dipolar mesogenic units linked by a flexible spacer. The NF phase, observed for dimers having spacer with even number of atoms, exhibits the strong polar order; in the NTB phase, observed for dimers with odd number of atoms in the spacer, short-pitch heliconical director structure develops.

cond-mat.soft↗

arXiv2025

Interplay of polar order and positional order in liquid crystals -- observation of re-entrant ferroelectric nematic phase

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 renormalized by spontaneous electric polarization.

cond-mat.soft↗

arXiv2025

Twist Grain Boundary phases in proper ferroelectric liquid crystals realm

The twist-grain-boundary (TGB) phases, characterized by a periodic, helical arrangement of blocks made of polar smectic phases, SmAF and SmCF, have been discovered. They have been 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 SmAAF 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 light selective reflection in the visible range, while the value of electric polarization confirms an almost perfectly ordered dipole alignment.

cond-mat.soft↗

arXiv2025

Spontaneous helix formation in polar smectic phase

In soft ferroelectric crystals, the depolarization field can be reduced by periodic distortion of the polarization direction. In the polar nematic and tilted smectic phases, this process is energetically favorured , as it only requires changes in the director orientation. We demonstrate the spontaneous formation of a helical structure in the proper ferroelectric tilted smectic (SmCTBF) phase, the phase is formed below the heliconical polar nematic (NTBF) phase. The helical pitch in the smectic phase is approximately 600 nm and remains nearly constant across the entire temperature range of the phase. Under weak electric fields, the helix reorients while its structure remains largely intact; however, in stronger fields, the helix is destroyed as the electric polarization aligns along the electric field.

cond-mat.soft↗

arXiv2024

Ferroelectric nematics: Materials with high permittivity or low resistivity?

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.

cond-mat.mtrl-sci↗

arXiv2024

Interpretation of dielectric spectroscopy measurements of ferroelectric nematic liquid crystals

The magnitude of the relative permittivity of the ferroelectric nematic phase (NF) is under a lively scientific discussion since the phase was recently discovered. Dielectric spectroscopy measurements (DSM) 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 NF layer, type of electrodes and presence/absence of nanoscale-thick surface layers. To model the DSM results, cells are presented by an equivalent electric circuit that includes a capacitor due to the NF layer with frequency dependent complex relative permittivity, capacitors due to surface layers, and a resistor describing limited conductivity of electrodes. DSM results for different cells with the same liquid crystal in the NF phase, are semi-quantitatively reproduced by the same set of physical parameters if a huge relative permittivity of the NF, which is even orders of magnitude larger than the measured apparent values, is assumed. We show that the capacitance of surface layers should be considered also in cells with no polymer alignment layer on electrodes.

cond-mat.soft↗

arXiv2024

Ferroelectric nematic -- isotropic critical end point

A critical end point above which an isotropic phase continuously evolves into a polar (ferroelectric) nematic phase with an increasing electric field is found in a ferroelectric nematic liquid crystalline material. The critical end point is approximately 30 K above the zero-field transition temperature from the isotropic to nematic phase and at an electric field of the order of 10 V/micron. Such systems are interesting from the application point of view because a strong birefringence can be induced in a broad temperature range in an optically isotropic phase.

cond-mat.soft↗

arXiv2024

Tilt or twist-competing synclinic and anticlinic interactions in SmC phases of bent-core mesogens

Recent liquid-crystalline (LC) research is focused on structurally new molecular systems distinct from simple nematic or smectic phases. Sophisticated molecular shape may reveal structural complexity, combining helicity and polarity. Achiral symmetry-breaking in bent-core molecules leads to propensity for synclinic and anticlinic molecular structures within consecutive smectic layers. Moreover, despite their achiral character, dimers readily adopt helical phases. In our study, we investigated a hybrid molecular structure incorporating both characteristics, namely a rigid-bent core and an attached bulky polar group via a flexible spacer. To perform phase identification, we enriched the standard experimental methods with the sophisticated resonant soft x-ray scattering. Notably, we have observed a distinct preference for specific phase types depending on the length of the homologue. Longer homologues exhibit a predisposition towards the formation of tilted smectic phases, characterized by complex sequences of synclinic and anticlinic interfaces. Conversely, shorter homologues manifest a propensity for helical smectic structures. For intermediate homologues, the frustration is alleviated through the formation of several modulated smectic phases. Based on the presented research, we describe the preconditions for high level structures in relation with conflicting constraints.

cond-mat.soft↗

arXiv2023

To be or not to be polar: the ferroelectric and antiferroelectric nematic phases

We report the properties of two new series of compounds that show the ferroelectric nematic phase in which the length of a terminal chain is varied. The longer the terminal chain, the weaker the dipole-dipole interactions of the molecules are along the director, and thus the lower the temperature at which the axially ferroelectric nematic phase is formed. For homologues of intermediate chain length, between the non-polar and ferroelectric nematic phases, there is a wide temperature range nematic phase with antiferroelectric character. The size of the antiparallel ferroelectric domains critically increases upon transition to the ferroelectric phase. In dielectric studies, both collective ("ferroelectric" and non-collective fluctuations are present, the "ferroelectric" mode softens weakly at the N-NX phase transition because the polar order in this phase is weak. The transition to the NF phase is characterized by a much stronger lowering of the mode relaxation frequency and an increase in its strength, typical critical behavior is observed.

cond-mat.soft↗

arXiv2022

Dielectric response of a ferroelectric nematic liquid crystalline phase in thin cells

We studied dielectric properties of a polar nematic phase (NF) sandwiched between two gold or ITO electrodes, serving as a cell surfaces. In bulk, NF is expected to exhibit a Goldstone mode (phason), because polarization can uniformly rotate with no energy cost. However, because the coupling between the direction of nematic director and polarization is finite, and the confinement, even in the absence of the aligning surface layer, induces some energy cost for a reorientation of polarization, the phason dielectric relaxation frequency is measured in a kHz regime. The phason mode is easily quenched by a bias electric field, which enables fluctuations in the magnitude of polarization to be followed in both, the ferronematic and nematic phases. This amplitude (soft) mode is also influenced by boundary conditions. A theory describing the phase and amplitude fluctuations in the NF phase shows that the free energy of the system and, consequently, the dielectric response are dominated by polarization-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 polarization fluctuations.

cond-mat.soft↗

arXiv2021

Complex chiral columns made of achiral quinoxaline derivatives with semi-flexible cores

Mesogenic materials, quinoxaline derivatives with semi-flexible cores, are reported to form new type of 3D columnar structure with large crystallographic unit cell and Fddd symmetry below columnar hexagonal phase. The 3D columnar structure is a result of frustration imposed by arrangement of helical columns of opposite chirality into triangular lattice. The studied materials exhibit fluorescent properties that could be easily tuned by modification of molecular structure, compounds with the extended π electron conjugated systems form aggregates and fluorescence is quenched. For molecules with flexible structure the fluorescence quantum yield reaches 25%. On the other hand, compounds with more rigid mesogenic core, for which fluorescence is suppressed show strong hole photocurrent. For some materials also bi-polar: hole and electron transfer was observed.

cond-mat.mtrl-sci↗

arXiv2019

Organic nanotubes created from mesogenic derivatives

A facile route to prepare the nanotubes from rod-like mesogens dissolved in typical organic solvents is reported. For selected types of chiral rod-like molecules, both enantiomers as well as the racemic mixtures formed the nanotubes by slow evaporation from a solution, regardless of the solvent, concentration or deposition type. Obtained supramolecular assemblies were studied using AFM and SEM techniques. Additional methods (IR, UV-Vis spectroscopy and x-ray diffraction) were applied to characterize the observed nanotubes. The difference in the surface tension at opposite crystallite surfaces is suggested as a possible mechanism for the nanotube nucleation.

cond-mat.mes-hall↗

arXiv2019

Multi-level Chirality in Liquid Crystals Formed by Achiral Molecules

In many biological materials with a hierarchical structure there is an intriguing and unique mechanism responsible for the 'propagation' of order from the molecular to the nano- or micro-scale level. Here we present a much simpler molecular system built of achiral mesogenic dimeric molecules that shows a similar complexity with four levels of structural chirality (i) layer chirality, (ii) helicity of a basic 4-layer repeating unit, (iii) a helix with a pitch of several layers and (iv) mesoscopic helical filaments. As seen in many biological systems, there is a coupling between chirality at different levels. The structures were identified by a combination of hard and soft x-ray diffraction measurements, optical studies and theoretical modelling.

cond-mat.soft↗

arXiv2018

Nature of nematic to twist bend nematic phase transition

The critical behavior at the transition from uniform nematic to twist-bend modulated nematic phase is revealed and shown to be well explained by the mean field approximation. The study was performed on a group of materials that exhibit an unusually broad temperature range of the nematic phase above the twist-bend modulated nematic phase, so the critical range in which the order parameter fluctuations are strong and can be experimentally observed is wide. The formation of instantaneous helices is observed already several degrees above the transition temperature, strongly influencing the birefringence of the system. The analysis of a critical part of the birefringence changes provides a set of critical exponents that are consistent with the mean field approximation, indicating a large correlation length of helical fluctuations in the nematic phase.

cond-mat.soft↗

arXiv2017

Heliconical smectic phases formed by achiral molecules

A series of asymmetric dimers with an odd number of atoms in the spacer were found to form different types of twisted structures despite being achiral. The formation of a variety of helical structures is accompanied by a gradual freezing of molecular rotation. The tight pitch heliconical nematic (NTB) phase and heliconical tilted smectic C (SmCTB) phase are formed. In the lowest temperature smectic phase, HexI, the twist is expressed through the formation of hierarchical structure: nano-scale helices and mesoscopic helical filaments. The short pitch helical structure in smectic phases is confirmed by resonant x-ray measurements.

cond-mat.soft↗

arXiv2017

Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic phase resolved by resonant soft X-ray scattering

Periodic structures of phases with orientational order of molecules, but homogenous electron density distribution: a short pitch cholesteric, blue phase and twist-bend nematic phase, were probed by a resonant soft x-ray scattering (RSoXS) at the carbon K-edge. The theoretical model shows that in case of a simple heliconical nematic structure two resonant signals corresponding to the full and half pitch band should be present, while only the full pitch band is observed in experiment. This suggests that the twist-bend nematic phase has complex structure with a double-helix, built of two interlocked, shifted helices. We confirm that the helical pitch in the twist-bend nematic phase is in a 10 nm range, for both, the chiral and achiral materials. We also show that the symmetry of a blue phase can unambiguously be determined through a resonant enhancement of x-ray diffraction signals, by including polarization effects, which are found to be an important indicator in phase structure determination.

cond-mat.soft↗

arXiv2016

Linkage-length dependent structuring behaviour of bent-core molecules in helical nanostructures

We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction were used to elucidate the linkage length-dependent formation of HNFs.

cond-mat.soft↗

arXiv2014

Do the short helices exist in the nematic TB phase?

Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical texture related to the formation of arrays of focal conic defects. Some of the focal conics show submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, suggesting surface freezing.

cond-mat.soft↗