1. Helix alignment, chevrons, and edge dislocations in twist-bend ferroelectric nematicsBijaya Basnet, Priyanka Kumari, Sathyanarayana Paladugu, Damian Pociecha, Jakub Karcz, Przemysław Kula, Nataša Vaupotič, Ewa Górecka, Oleg D. Lavrentovich, 2025, original scientific article Keywords: chevron, edge dislocation, heliconical ferroelectric nematic, surface alignment, twist-bend ferroelectric nematic
Published in DKUM: 26.11.2025; Views: 0; Downloads: 2
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2. 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, original scientific article Keywords: chirality, liquid crystals, proper ferroelectrics, twist-grain-boundary
Published in DKUM: 19.08.2025; Views: 0; Downloads: 9
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3. Ferroelectric nematics: materials with high permittivity or low resistivity?Nataša Vaupotič, Tine Krajnc, Ewa Górecka, Damian Pociecha, Vojko Matko, 2025, original scientific article Abstract: 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.
Keywords: ferroelectric nematics, dielectric measurements, high-permittivity model, electrode resistance
Published in DKUM: 14.08.2025; Views: 0; Downloads: 2
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4. Interpretation of dielectric spectroscopy measurements of ferroelectric nematic liquid crystalsVojko Matko, Ewa Górecka, Damian Pociecha, Joanna Matraszek, Nataša Vaupotič, 2024, original scientific article Abstract: 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.
Keywords: liquid crystals, statistical physics
Published in DKUM: 27.11.2024; Views: 0; Downloads: 21
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5. Prepovedani pasovi v fotonskih kristalihAnica Ficko, 2024, master's thesis Abstract: V magistrski nalogi obravnavamo fotonske kristale in se osredotočimo na izračun prepovedanih pasov. Fotonski kristali so materiali, v katerih se dielektrična konstanta s krajem periodično spreminja. V takšnih materialih lahko obstajajo prepovedani pasovi oziroma območje frekvenc, pri katerih se elektromagnetno valovanje ne more razširjati skozi material, ampak se odbije. Prepovedani pas, ki obstaja pri vseh možnih smereh razširjanja elektromagnetnega valovanja skozi material, se imenuje popolni prepovedani pas. Za izračun prepovedanih pasov v dvodimenzionalnih in tridimenzionalnih fotonskih kristalih uporabimo metodo prenosne matrike v realnem prostoru. Izpeljemo disperzijsko zvezo, ki opisuje odvisnost frekvence od valovnega števila. Zvezo uporabimo v numeričnih izračunih, ki jih izvedemo z računalniškim programom Wolfram Mathematica. Pri določeni smeri razširjanja elektromagnetnega valovanja skozi material prikažemo rezultate numeričnih izračunov na grafu frekvence v odvisnosti od valovnega števila, iz katerega so razvidni prepovedani pasovi. Ugotovimo, da metoda na osebnem računalniku v programu Wolfram Mathematica omogoča izračun prepovedanih pasov samo za dvodimenzionalne fotonske kristale. Za izračun prepovedanih pasov v tridimenzionalnih fotonskih kristalih uporabimo programska paketa MEEP in MPB. Programski paket MEEP omogoča shranjevanje geometrijskih lastnosti struktur, numerično reševanje Maxwellovih enačb in različne simulacije elektromagnetnih pojavov. Programski paket MPB je integriran v paket MEEP in je posebej zasnovan za izračun prepovedanih pasov v fotonskih kristalih za različne geometrijske lastnosti. Osredotočimo se na modri tekočekristalni fazi BPI in BPII. V osnovni celici modrih faz BPI in BPII se molekule uredijo v dvojno zvite cilindre. Predpostavimo, da dielektrična konstanta znotraj cilindrov ni odvisna od smeri, zato namesto dielektričnega tenzorja upoštevamo dielektrično konstanto. Ugotovimo, da v modrih fazah BPI in BPII nastanejo prepovedani pasovi, ki pa niso popolni. Za modro fazo BPI upoštevamo, da je velikost osnovne celice enaka 240 nm in izračunamo, da v vidnem območju elektromagnetnega spektra obstajajo prepovedani pasovi. Za modro fazo BPII pa upoštevamo, da je velikost osnovne celice enaka 150 nm in izračunamo, da v vidnem območju elektromagnetnega spektra prav tako obstajajo prepovedani pasovi.
Keywords: fotonski kristali, fotonski prepovedani pasovi, dielektrična konstanta, disperzijska zveza, metoda prenosne matrike v realnem prostoru, modre faze, MEEP in MPB
Published in DKUM: 18.10.2024; Views: 0; Downloads: 26
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6. 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, original scientific article Abstract: 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.
Keywords: chirality, helical structures, liquid crystals, photonic bandgap, twist–bend phases
Published in DKUM: 10.10.2024; Views: 0; Downloads: 11
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7. Modeling of the resonant X-ray response of a chiral cubic phaseTimon Grabovac, Ewa Górecka, Damian Pociecha, Nataša Vaupotič, 2021, original scientific article Abstract: 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.
Keywords: chiral cubic phase, resonant X-ray scattering, theoretical modeling, tensor form factor
Published in DKUM: 14.08.2024; Views: 100; Downloads: 13
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8. 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, original scientific article Abstract: 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.
Keywords: resonant X-ray scattering, chiral cubic thermotropic liquid crystal phase, structural models, carbon absorption edge, sulphur absorption edge
Published in DKUM: 13.06.2024; Views: 140; Downloads: 15
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9. Fizikalni eksperimenti 3 : zbirka laboratorijskih vajRobert Repnik, Robert Hauko, 2022, higher education exercise book Abstract: Fizikalni eksperimenti 3 – zbirka laboratorijskih vaj – so namenjeni študentom 1. stopnje študija v programih Fizika in Predmetni učitelj na Fakulteti za naravoslovje in matematiko v Mariboru. Zbirka zajema snov s področja nihanja in valovanja. Vaje se izvajajo pri predmetu Fizikalni eksperimenti 3 v letnem semestru 2. letnika. Naslovi vaj: Goriščne razdalje leč, Uklon in interferenca, Stefanov zakon, Sklopljeno nihanje, Hitrost ultrazvoka, Nihanje s trenjem, Resonanca, Stoječe valovanje, Frekvenca in hitrost zvoka, Prekinjeno nihalo, Spektrometer. Navodila so namenjena uspešni izvedbi meritev, analizi, interpretaciji in vrednotenju merskih rezultatov. Dodani so naslovi samostojnih projektov, ki so bili v okviru predmeta izvedeni v preteklih letih.
Keywords: fizika, nihanje, valovanje, laboratorijske vaje, fizikalne meritve, projekti
Published in DKUM: 12.07.2022; Views: 902; Downloads: 142
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