1. Twist grain boundary phases in proper ferroelectric liquid crystals realmDamian Pociecha, Jadwiga Szydłowska, Nataša Vaupotič, Katarzyna Kwiatkowska, Marijus Juodka, Julian Spiess, John M. D. Storey, Corrie T. Imrie, Rebecca Walker, Ewa Górecka, 2025, izvirni znanstveni članek Ključne besede: chirality, liquid crystals, proper ferroelectrics, twist-grain-boundary
Objavljeno v DKUM: 19.08.2025; Ogledov: 0; Prenosov: 0
 Celotno besedilo (2,52 MB)
Gradivo ima več datotek! Več... |
2. Ferroelectric nematics: materials with high permittivity or low resistivity?Nataša Vaupotič, Tine Krajnc, Ewa Górecka, Damian Pociecha, Vojko Matko, 2025, izvirni znanstveni članek Opis: Two models have recently been proposed to describe dielectric spectroscopy measurements of ferroelectric nematics (NF) in thin planar capacitors. The polarisation-external capacitance Goldstone reorientation mode (PCG model) considers the NF layer between the electrodes as an effective low resistivity material; the resistivity is 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. We study implications of both models and show, why both models describe the 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-� model. The high-� model predicts that the frequency range within which capacitors filled with NF material effectively store energy can be increased by using thicker cells, while within the PCG model this frequency range reduces with increasing capacitor thickness. Within both models, a crucial parameter that determines the behaviour of the capacitors filled with an 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.
Ključne besede: ferroelectric nematics, dielectric measurements, high-permittivity model, electrode resistance
Objavljeno v DKUM: 14.08.2025; Ogledov: 0; Prenosov: 1
Celotno besedilo (5,83 MB) |
3. Interpretation of dielectric spectroscopy measurements of ferroelectric nematic liquid crystalsVojko Matko, Ewa Górecka, Damian Pociecha, Joanna Matraszek, Nataša Vaupotič, 2024, izvirni znanstveni članek Opis: The magnitude of the relative permittivity of the ferroelectric nematic phase (NF ) 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 NF 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 NF 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 NF phase
are semiquantitatively 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 also be considered in cells with no polymer alignment layer on the
electrodes.
Ključne besede: liquid crystals, statistical physics
Objavljeno v DKUM: 27.11.2024; Ogledov: 0; Prenosov: 10
Celotno besedilo (669,38 KB)
Gradivo ima več datotek! Več... |
4. Photonic bandgap in achiral liquid crystals - a twist on a twistDamian Pociecha, Nataša Vaupotič, Magdalena Majewska, Ewan Cruickshank, Rebecca Walker, John M. D. Storey, Corrie T. Imrie, Cheng Wang, Ewa Górecka, 2021, izvirni znanstveni članek Opis: Achiral mesogenic molecules are shown to be able to spontaneously assemble into liquid crystalline smectic phases having either simple or double-helical structures. At the transition between these phases, the double-helical structure unwinds. As a consequence, in some temperature range, the pitch of the helix becomes comparable to the wavelength of visible light and the selective reflection of light in the visible range is observed. The photonic bandgap phenomenon is reported for achiral liquid crystals.
Ključne besede: chirality, helical structures, liquid crystals, photonic bandgap, twist–bend phases
Objavljeno v DKUM: 10.10.2024; Ogledov: 0; Prenosov: 10
Celotno besedilo (3,53 MB)
Gradivo ima več datotek! Več... |
5. Modeling of the resonant X-ray response of a chiral cubic phaseTimon Grabovac, Ewa Górecka, Damian Pociecha, Nataša Vaupotič, 2021, izvirni znanstveni članek Opis: The structure of a continuous-grid chiral cubic phase made of achiral constituent molecules is a hot topic in the field of thermotropic liquid crystals. Several structural models have been proposed so far. Resonant X-ray scattering (RXS), which gives information on the molecular orientation in the unit cell, could be applied to select the most appropriate model. We modeled the RXS response for the recently proposed chiral cubic phase structure with an all-hexagon chiral continuous grid. A tensor form factor of a unit cell is constructed, which enables calculation of intensities of peaks for all Miller indices. We find that all the symmetry allowed peaks are resonantly enhanced, and their intensity is much stronger than the intensity of the symmetry forbidden (resonant) peaks. In particular, we predict that a strong resonant enhancement of the symmetry allowed peaks (011) and (002), not observed in a nonresonant scattering, could be observed by RXS at the carbon absorption edge. By RXS at the sulfur absorption edge, one might observe a resonant peak (113) and resonantly enhanced peak (233), and resonant enhancement of all the peaks that are observed in a nonresonant scattering, which probably hide the rest of the predicted resonant peaks.
Ključne besede: chiral cubic phase, resonant X-ray scattering, theoretical modeling, tensor form factor
Objavljeno v DKUM: 14.08.2024; Ogledov: 100; Prenosov: 11
Celotno besedilo (2,18 MB)
Gradivo ima več datotek! Več... |
6. Unmasking the structure of a chiral cubic thermotropic liquid crystal phase by a combination of soft and tender resonant X-ray scatteringTimon Grabovac, Ewa Górecka, Chenhui Zhu, Damian Pociecha, Nataša Vaupotič, 2022, izvirni znanstveni članek Opis: A resonant X-ray scattering response for two structural models of a chiral cubic phase with a giant unit cell, one composed of a continuous grid and micelles and the other with three continuous grids, is studied theoretically and compared to experimental measurements. For both structural models resonant enhancement of all the symmetry-allowed diffraction peaks is predicted, as well as the existance of several symmetry forbidden peaks (pure resonant peaks). Experimental measurements were performed at the carbon and sulphur absorption edge. Only one pure resonant peak was observed, which is predicted by both models. Two low-angle symmetry allowed peaks, not observed in non-resonant scattering, were resonantly enhanced and their intensity angular dependence can distinguish between the two structural models.
Ključne besede: resonant X-ray scattering, chiral cubic thermotropic liquid crystal phase, structural models, carbon absorption edge, sulphur absorption edge
Objavljeno v DKUM: 13.06.2024; Ogledov: 140; Prenosov: 14
Celotno besedilo (2,00 MB)
Gradivo ima več datotek! Več... |
