arXiv subjects

Georg H. Mehl

Publications and source records attributed to Georg H. Mehl.

At least 19 recordsLinked to original sources

arXiv2025

Ferroelectric and Hyper Dielectric modes in Ferronematic Liquid Crystals

Binary mixtures of the ferronematic compound DIO with recently reported non-ferroelectric material WJ-16 which shows Colossal Permittivity (CP) ~5000 and superparaelectricity (SPE) were studied by POM, electrical switching studies, and dielectric spectroscopy. Three mixtures with different contents of WJ-16 as 10, 25 and 50% in DIO as host were prepared. Our original expectation was the development of new nematic materials with both ferroelectric nematic (NF) and non-ferroelectric CP phases. The non-ferroelectric phase in mixtures exhibits a CP mode originally observed in pure WJ-16 and was termed as superparaelectric. However, the dielectric spectroscopy of mixtures shows two distinct relaxation processes: the typical paraelectric response and the CP mode. Therefore, this CP mode cannot be called superparaelectric and is redefined it as Hyper Dielectric mode. This is the first direct demonstration of materials with both ferroelectric and Hyper-dielectric phases in liquid crystalline materials. The hyper dielectric phase has a good potential as a working media for supercapacitors industry.

cond-mat.mtrl-sci↗

arXiv2025

Some Peculiarities of Dielectric Spectroscopy in Ferroelectric Nematics

Dielectric spectroscopy is known as one of the most powerful techniques for studying ferroelectric and other polar materials. Since the discovery of ferroelectricity in Liquid Crystals, it has been successfully employed for the characterization of ferro-, antiferro- and ferri-electric liquid crystalline phases. However, recently the Boulder group raised the question of the applicability of dielectric spectroscopy for characterizing ferroelectric nematics due to parasitic effects from the insulating alignment layers. This affects the apparent/measured values of the dielectric permittivity. In this paper, we study this effect in greater detail. The following issues will receive special attention: Are the real values of dielectric permittivity lower or higher than the measured ones? Can the real values of dielectric permittivity be recovered in the cell with alignment layers? We also provide an example of the effect of insulating alignment layers in a non-ferroelectric nematic phase.

cond-mat.mtrl-sci↗

arXiv2024

Colossal Dielectric Permittivity and Superparaelectricity in phenyl pyrimidine based liquid crystals

A set of polar rod-shaped liquid crystalline molecules with large dipole moments (mu > 10.4-14.8 D), their molecular structures based on the ferroelectric nematic prototype DIO, are designed, synthesized, and investigated. When the penultimate fluoro-phenyl ring is replaced by phenylpyrimidine moiety, the molecular dipole moment increases from 9.4 D for DIO to 10.4 D for the new molecule and when the terminal fluoro-group is additionally replaced by the nitrile group, the dipole moment rises to 14.8 D. Such a replacement enhances not only the net dipole moment of the molecule, but it also reduces the steric hindrance to rotations of the moieties within the molecule. The superparaelectric nematic (N) and smectic A (SmA) phases of these compounds are found to exhibit colossal dielectric permittivity, obtained both from dielectric spectroscopy, and capacitance measurements using a simple capacitor divider circuit. The electric polarization is measured vs. the field (E). However, no hysteresis in P vs. E is found in the nematic and smectic A phases. The colossal dielectric permittivity persists over the entire fluidic range. The experimental results lead us to conclude that these materials belong to the class of superparaelectrics (SPE) rather than to ferroelectrics due to the absence hysteresis and linear P vs E dependence. The synthesized organic materials are the first fluids for which superparaelectricity is discovered and furthermore these show great potential for the applications in supercapacitors used in storing energy.

cond-mat.soft↗

arXiv2024

Calorimetric evidence for the existence of an intermediate phase between the ferroelectric nematic phase and the nematic phase in the liquid crystal RM734

The idea that rod-like molecules possessing an electric dipole moment could exhibit a ferroelectric nematic phase was suggested more than a century ago. However, only recently such a phase has been reported for two quite different liquid crystals: RM734 (4-[(4-nitrophenoxy)carbonyl)]phenyl 2,4-dimethoxybenzoate) and DIO (2.3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2.6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate). For RM734 a direct ferroelectric nematic (NF) to classical nematic N transition was reported, whereas for DIO an intermediate phase Nx was discovered between the NF and the N phases. Here we present high-resolution calorimetric evidence that an intermediate Nx phase also exists in RM734 along a narrow temperature range between the NF and the N phases.

cond-mat.soft↗

arXiv2023

Dipole-dipole correlations in the nematic phases of symmetric cyanobiphenyl dimers and their binary mixtures with 5CB

We report on the temperature dependence of birefringence and of the static dielectric permittivity tensor in a series of binary mixtures between the symmetric, bent-shaped, 1",9"-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) dimer and the monomeric nematogen 5CB. In the studied composition range the mixtures exhibit two nematic phases with distinct birefringence and dielectric features. Birefringence measurements are used to estimate the temperature dependence of the tilt between the axis defining the nanoscale helical modulation of the low temperature nematic phase with the (local) direction of the maximal alignment of the cyanobiphenyl units. Planar as well as magnetically and/or electrically aligned samples are used to measure the perpendicular and parallel components of the dielectric permittivity in both nematic phases. A self-consistent molecular field theory that takes into account flexibility and symmetry of the constituent mesogens is introduced for the calculation of order parameters and intra-molecular orientational dipolar correlations of the flexible dimers as a function of temperature/concentration. Utilising the tilt angle, as calculated from the birefringence measurements, and the predictions of the molecular theory, dielectric permittivity is modelled in the framework of the anisotropic version of the Kirkwood-Frohlich theory. Using the inter-molecular Kirkwood correlation factors as adjustable parameters, excellent agreement between theory and permittivity measurements across the whole temperature range and composition of the mixtures is obtained. The importance of the orientational, intra- and inter-molecular, dipolar correlations, their relative impact on the static dielectric properties, as well as their connection with the local structure of the nematic phases of bent-shaped bimesogens, is discussed.

cond-mat.soft↗

arXiv2022

Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry

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 the intermediate phase was present in all mixtures over the complete composition range, albeit with strongly decreasing temperature width with decreasing mole fraction of DIO (xDIO). The xDIO dependence of the two transitions could be well described by a quadratic function and both transitions were weakly first order. The true latent heat of the Nx-N transition of DIO was as low as 0.0075 +/- 0.0005 J/g and 0.23 +/- 0.03 J/g for the NF-Nx transition, about twice the previously reported value of 0.115 J/g for the NF-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing xDIO. At all the Nx-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the NF to the higher temperature phase exhibits substantial pretransitional behaviour. Power law analysis of cp(T) resulted in an effective critical exponent of 0.88 +/- 0.10 for DIO and this value decreased in mixtures with decreasing xDIO towards 0.50 +/- 0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior for the total transition enthalpy change

cond-mat.soft↗

arXiv2022

Increased intermolecular interactions and cluster formation at the onset of the twist-bend nematic phase in thioether cyanobiphenyl-based liquid crystal dimers

Infrared spectroscopy (IR) and quantum chemistry calculations, based on density functional theory (DFT) were used to study the structure and the molecular interactions in the nematic (N) and twist-bend (NTB) phases of thioether-linked dimers. Infrared absorbance measurements were conducted in a polarized beam for homogeneously aligned samples to measure and understand the orientation of the vibrational transition dipole moments in the liquid crystal states. Revealing temperature-dependent changes in the mean IR absorbance in the twist-bend nematic phase were observed. In the transition from the N to the NTB phase, is was found to be associated with a decrease in absorbance for the longitudinal dipoles. This is a result of the antiparallel axial interactions of the dipoles, while the absorbance of the transverse dipoles remained unchanged up to 340 K and then increased and the dipoles correlated in parallel. In order to account for the molecular arrangement in the nematic phase, DFT calculations were conducted for the system nearest to the lateral neighborhoods. Changes in the square of transition dipoles were found to be quite similar to the mean absorbance that was observed in the IR spectra. Interactions of molecules that were dominated by pair formation was observed as well as a shift of the long axes of the molecules relative to each other. The most important observation from the spectroscopic measurements was the sudden increase in the intermolecular interactions as the temperature decreased from the N to the NTB phase. This was evidenced by a significant increase in the correlation of the dipoles that were induced in the dimer cores.

cond-mat.soft↗

arXiv2022

Polar nature of the ferro-electric nematic studied by dielectric spectroscopy

The nematic-nematic transitions in a nematic compound DIO are studied in homogeneously planar and homeotropic aligned cells using dielectric spectroscopy in the frequency range 0.1 Hz to 10 MHz over a wide range of temperatures. Three relaxation processes are identified in this material. All the three relaxation processes show large jumps in the dielectric strength and discontinuity in frequency at the NX to NF transition, indicative of it being a first order phase transition. Unlike a conventional nematic liquid crystalline material that usually shows molecular relaxation processes at higher frequencies, this material shows three processes at relatively lower frequencies. The three processes are found to be collective in nature. The least frequency process is proven to be due to the conductivity of ions in the medium and due to the accumulation of charge at the alignment layers unlike mistakenly reported in the literature otherwise. The mode at the highest frequency is proven to be due to the ferroelectric domains in the NF phase. This is evidenced by its dielectric strength two orders of magnitude higher in a homeotropic aligned cell than planar aligned cell. Its mechanism is soft mode like in the N phase. The intermediate frequency mode is proven to be due to the correlation of molecules of the medium as is normally observed in liquid crystalline cybotactic clusters.

cond-mat.soft↗

arXiv2022

Detection of chiral domains and linear electro-optical response in planar-aligned cells of an achiral rod-like ferro-electric nematic liquid crystal

Chiral domains of opposite chirality are observed surprisingly in a nematic phase of an achiral rod-like liquid crystalline mesogen that shows ferroelectric nematic and antiferroelectric smectic phases. The observations of chiral domains of opposite chirality are enabled by application of a weak electric field across a planar-aligned cell. However once domains of opposite chirality are created, these stay permanently even if the field is removed. The observed phenomenon is due to the symmetry breaking of achiral mesogens of rod-shaped molecules. These domains are similarly observed in the Nx phase. However for temperatures close or in the NF, the size of domains is reduced significantly below the wavelength of visible light. There is either a single domain with dipole moments parallel to the surface or domains antiparallel in the NF phase, not possible to determine in this experiment.

cond-mat.soft↗

arXiv2020

The role of intermolecular interactions in stabilizing the structure of the nematic twist-bend phase

The relationship between the molecular structure and the formation of the NTB phase is still at an early stage of development. This is mainly related to molecular geometry, while the correlation between the NTB phase and the electronic structure is ambiguous. To explore the electronic effect on properties and stabilization of the NTB phase we investigated 2,3-difluoro-4,4-dipentyl-p-terphenyl dimers (DTC5Cn). We used IR polarized spectroscopy, which can at least in principle, bring information about the ordering in NTB phase. All dimers show a significant drop of the average value of the transition dipole moment dμ/dQ for parallel dipoles at the transition to the NTB phase, and an increase for perpendicular dipoles, despite its remaining unchanged for the monomer. These results coincide well with DFT simulations of vibrational dipole derivatives for molecules assembled in pseudo-layers of the NTB phase. The DFT calculations were used to determine the geometric and electronic properties of the hydrogen bonded complexes. We have provided experimental and theoretical evidence of stabilization of the NTB phase by arrays of multiple hydrogen bonds (XF...HX, X-benzene ring).

physics.chem-ph↗

arXiv2020

The interplay between spatial and heliconical bond order in twist-bend nematic materials

The nanostructure of two novel sulfur containing dimer materials has been investigated experimentally by hard and by resonant tender X-ray scattering techniques. On cooling the dimers through the nematic to twist-bend nematic (N-NTB) phase transition, the correlation length associated with short-range positional order drops, while the heliconical orientational order becomes more correlated. The heliconical pitch shows a stronger temperature dependence near the N-NTB transition than observed in previously studied dimers, such as the CBnCB series of compounds. We explain both this strong variation and the dependence of the heliconical pitch on the length of the spacer connecting the monomer units by taking into account a temperature dependent molecular bend and intermolecular overlap. and. The heliconical structure is observed even in the upper 3-4°C range of the smectic phase that forms just below the NTB state. The coexistence of smectic layering and the heliconical order indicates a SmCTB -type phase where the rigid units of the dimers are tilted with respect to the layer normal in order to accommodate the bent conformation of the dimers, but the tilt direction rotates along the heliconical axis. This is potentially similar to the SmCTB phase reported by Abberley et al (Nat. Commun. 2018, 9, 228) below a SmA phase.

cond-mat.soft↗

arXiv2020

Molecular biaxiality determines the helical structure -- Infrared measurements of the molecular order in the NTB phase of difluoro terphenyl dimers

IR polarized spectroscopy was employed, to obtain the three components of IR absorbance for a series DTC5Cn (n=5,7,9,11) of bent-shaped dimers containing double fluorinated terphenyl core. Then, they were used to calculate both uniaxial (S) and biaxial (D) order parameters, for various molecular groups of dimers. The molecule bend was estimated due to observed difference between S parameters for the terphenyl core and central hydrocarbon linker. It is found to be significant in N phase despite almost no macroscopic director bend. The temperature dependence of the order parameter, S, distinctly reverses its monotonic trend of increase to decrease at the transition temperature, TN-NTB to the NTB phase as result of the director tilt in the NTB phase. The biaxial order parameter, D, is negligible in the N phase, and then starts increasing on entering the NTB temperature, similarly to sin2q. The local director curvature is found to be controlled by the molecular biaxiality parameter.

cond-mat.soft↗

arXiv2020

Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

Elastic moduli of liquid crystals, known as Frank constants, are of quintessential importance for understanding fundamental properties of these materials and for the design of their applications. Although there are many methods to measure the Frank constants in the nematic phase, little is known about the elastic constants of the chiral version of the nematic, the so-called cholesteric liquid crystal, since the helicoidal structure of the cholesteric renders these methods inadequate. Here we present a technique to measure the bend modulus $K_{33}$ of cholesterics that is based on the electrically tunable reflection of light at an oblique helicoidal $Ch_{OH}$ cholesteric structure. $K_{33}$ is typically smaller than 0.6 pN, showing a non-monotonous temperature dependence with a slight increase near the transition to the twist-bend phase. $K_{33}$ depends strongly on the molecular composition. In particular, chiral mixtures that contain the flexible dimer 1'',7''-bis(4-cyanobiphenyl-4'-yl) heptane (CB7CB) and rod-like molecules such as pentylcyanobiphenyl (5CB) show a $K_{33}$ value that is 5 times smaller than $K_{33}$ of pure CB7CB or of mixtures of CB7CB with chiral dopants. Furthermore, $K_{33}$ in CB11CB doped with a chiral agent is noticeably smaller than $K_{33}$ in a similarly doped CB7CB which is explained by the longer flexible link in CB11CB. The proposed technique allows a direct in-situ determination of how the molecular composition, molecular structure and molecular chirality affect the elastic properties of chiral liquid crystals.

cond-mat.soft↗

arXiv2020

Identification of New Assembly Mode in the Heliconical Nematic Phase via Tender Resonant X-ray Scattering

Helical structures are exciting and are utilized in numerous applications ranging from biotechnology to displays to medicine. Accurate description and understanding of resonance effects in helical structures provides crucial knowledge on molecular packing beyond positional ordering. We exam-ined the manifestation of resonance effects in a nematic phase with heliconical structure, the so called twist bend nematic (NTB) via tender resonant X-ray scattering (TReXS) at the sulfur K-edge. We demonstrate for the first time quantitatively that the energy dependence of the scattering peak in the NTB phase follows the energy dependence of the complex refractive indices measured by X-ray absorption. This allows us to identify a new self-assembly mode for specific sets of liquid crystal dimers in the NTB phase. We anticipate that new avenues in the exploration of complex orientational structures both in static as well as in dynamic modes induced by external stimuli will be pursued.

cond-mat.soft↗

arXiv2019

Probing Molecular Ordering in the Nematic Phases of para-Linked Bimesogen Dimers through NMR Studies of Flexible Prochiral Solutes

The quadrupolar splittings of perdeuteriated n-decane dissolved in nematic phases formed by mesogenic dimers of the CBnCB series, for n=7,9,10,11 are measured throughout the entire temperature range of these phases. The results of the measurements, are reported together with related measurements using the common nematic phase of 5CB as a solvent for n-decane. The data obtained from the $^{13}$C spectra of the cyanobiphenyl mesogenic units of the monomeric and dimeric solvent molecules yield the order parameter of those units. The information obtained from this set of experiments is used to elucidate the structure of the low temperature (N$_X$) and the high temperature (N) nematic phases of CBnCB dimers with n=7,9,11. The polar twisted nematic (N$_{PT}$) model is found to provide a consistent description not only of these measurements, but also of NMR measurements previously reported in the literature for these phases. These findings suggest that the high temperature nematic (N) is not a common, locally uniaxial and apolar nematic, but rather a nematic phase consisting of N$_{PT}$ clusters. The twist-bend (N$_{TB}$) model, often identified with the N$_X$ phase, is shown to be inadequate to account even qualitatively for crucial features of the experimental findings.

cond-mat.mtrl-sci↗

arXiv2018

Tuning selective reflection of light by surface anchoring in cholesteric cells with oblique helicoidal structures

Selective reflection of light by oblique helicoidal cholesteric (ChOH) can be tuned in a very broad spectral range by an applied electric field. In this work, we demonstrate that the peak wavelength of the selective reflection can be controlled by surface alignment of the director in sandwich cells. The peak wavelength is blue-shifted when the surface alignment is perpendicular to the bounding plates and red-shifted when it is planar. The effect is explained by the electric field redistribution within the cell caused by spatially varying heliconical ChOH structure. The observed phenomenon can be used in sensing applications.

cond-mat.soft↗

arXiv2017

Characterization of the sub-micrometer hierarchy levels in the twist-bend nematic phase with nanometric helices via photopolymerization. Explanation for the sign reversal in the polar response

Photo-polymerization of a reactive mesogen mixed with a mesogenic dimer, shown to exhibit the twist-bend nematic phase ($N_{TB}$), reveals the complex structure of the self-deformation patterns observed in planar cells. The polymerized reactive mesogen retains the structure formed by liquid crystalline molecules in the twist bend phase, thus enabling observation by Scanning Electron Microscope (SEM). Hierarchical ordering scales from tens of nanometers to micrometers are imaged in detail. Submicron features, anticipated from earlier X-ray experiments, are visualized directly. In the self-deformation stripes formed in the $N_{TB}$ phase, the average director field is found tilted in the cell plane by an angle of up to 45$^{\circ}$ from the cell rubbing direction. This tilting explains the sign inversion being observed in the electro-optical studies.

cond-mat.soft↗

arXiv2015

Hierarchy of Periodic patterns in the twist-bend nematic phase of mesogenic dimers

Several attractive properties of hydrocarbon-linked mesogenic dimers have been discovered, a major feature of these has been the ability of dimers under confinement to form periodic patterns. At least six different types of stripe-like periodic patterns can be observed in these materials under various experimental conditions. Our most recent finding is the discovery of micrometre-scale periodic patterns at the N-Ntb phase transition interface, and sub-micrometre scale patterns in the Ntb phase. Layer-like sub-micrometre patterns were observed using three-photon excitation fluorescence polarizing microscopy, and these are being reported here.

cond-mat.soft↗