Helicoidal Structure Research Articles

Influence of flexoelectric effect on the Fréedericksz transition in chiral nematic liquid crystals.

Electric field driven Frèedericksz transition in chiral nematic (cholesteric) planar liquid crystal cell is studied in the presence of flexoelectric effect. An inhomogeneity of electric field and finiteness of anchoring energy are considered. The extra contribution to the electric field induced by flexoelectricity is taken into account as well as the common contribution arising from the applied voltage U. Equilibrium director distribution is obtained in dependence on voltage and flexoelectric coefficients by numerical minimization of the free energy. The threshold voltage was found to decrease with the increase of the flexoelectric coefficients within the range of high flexoelectric coefficients. Thus flexoelectricity promotes the transition. The orientational structure becomes asymmetric about the center of the cell due to flexoelectricity, even in the case of symmetric boundary conditions. The equilibrium structure was also shown to be different for U>0 and U<0. It was found, that for sufficiently high flexoelectric coefficients the director distribution can be described by a simple function. In this case the planar helicoidal structure transfers into a hybrid-aligned one as the voltage increases. This transformation may form the basis for new flexoelectricity-based switching devices. The nonmonotonic behavior of threshold voltage can be observed in the case of small flexoelectric coefficients. An analytical form of the stability conditions was obtained for the planar helicoidal configuration. We have shown that the Frèedericksz transition can be either continuous or discontinuous depending on the material constants and thickness of the liquid crystal cell.

Helicenes as Molecular Tweezers in the Formation of Cation-π Complexes. Bonding and Circular Dichroism Properties from Relativistic DFT Calculations.

Helicene ligands possess a unique helicoidal π-conjugated framework providing high optical rotation values. This has stimulated a growing interest in helicene derivatives as building blocks, which has triggered the development of simple strategies to tune their properties. In this context, we evaluated the formation of cation-π complexes derived from [6]- and [7]helicene, involving Sn2+ , Cd2+ , and In+ in addition of Ag+ , which appears as a plausible modification of such helicoidal structure. The nature of the cation-π interaction in the studied helicene complexes exhibits a covalent character, accounting for ligand π-donation to 5 s and 5p empty orbitals of the involved cations. Furthermore, the evaluation of their optical activity exhibits notorious modification patterns in the circular dichroism spectrum, suggesting that the modifications of the optical activity are dependent on the nature of the cation and its related charge state. Thus, the plausible formation of new cation-π complexes derived from helicene ligands, as discussed here, may lead to the characterization of novel species expanding the chemistry of helicene metal complexes to even to larger helicene units. We believe that the present study may open new avenues in the formation of cation-π helicene complexes.

Electrically tuned optical reflection band for an artificial helicoidal structure

ABSTRACTWe analysed the control of optical band gaps for axially propagating electromagnetic waves throughout a nanocomposite structurally chiral medium under the influence of a low-frequency (dc) electric field aligned along the same axis as the periodic structure. This medium is made of metallic nanoballs (silver) randomly dispersed in a structurally chiral material whose dielectric properties can be represented by a resonant effective uniaxial tensor. Structurally chiral material is taken to possess locally a point group symmetry and the Pockels effect is assumed. By establishing the Maxwell equations in a matrix representation, we have computed the eigenvalues and eigenvectors of the corresponding matrix in the system rotating along with the helical structure as function of the filling factor and the electric field. We found that the band gap properties of the periodic system depend strongly on the applied low-frequency electric field which is able to increase the bandwidths of the two sub-band gaps generated by the presence of the metallic inclusions obtained above a given filling factor. The applied electric field is even able to open the mentioned band gaps when they are initially closed. We note that by increasing the filling factor, also by keeping the inclination angle and the electric field fixed, the bands are opened and closed. Then when changing the angle of inclination and the electric field the bands shift, they break and new sub-band gaps appear.

Magnetic Reconnection at a Thin Current Sheet Separating Two Interlaced Flux Tubes at the Earth's Magnetopause

AbstractThe occurrence of spatially and temporally variable reconnection at the Earth's magnetopause leads to the complex interaction of magnetic fields from the magnetosphere and magnetosheath. Flux transfer events (FTEs) constitute one such type of interaction. Their main characteristics are (1) an enhanced core magnetic field magnitude and (2) a bipolar magnetic field signature in the component normal to the magnetopause, reminiscent of a large‐scale helicoidal flux tube magnetic configuration. However, other geometrical configurations which do not fit this classical picture have also been observed. Using high‐resolution measurements from the Magnetospheric Multiscale mission, we investigate an event in the vicinity of the Earth's magnetopause on 7 November 2015. Despite signatures that, at first glance, appear consistent with a classic FTE, based on detailed geometrical and dynamical analyses as well as on topological signatures revealed by suprathermal electron properties, we demonstrate that this event is not consistent with a single, homogenous helicoidal structure. Our analysis rather suggests that it consists of the interaction of two separate sets of magnetic field lines with different connectivities. This complex three‐dimensional interaction constructively conspires to produce signatures partially consistent with that of an FTE. We also show that, at the interface between the two sets of field lines, where the observed magnetic pileup occurs, a thin and strong current sheet forms with a large ion jet, which may be consistent with magnetic flux dissipation through magnetic reconnection in the interaction region.

Simulation of Geometrically Accurate, Multibillion Atom Cellular Membrane Structures

Membranes are fundamental to the function and structure of cells and organelles, and molecular dynamics (MD) simulations have the potential to reveal deep insights about the basic physics and biochemistry that produce their behavior. However, MD simulations of cellular membranes have been extremely limited up to this point due to the geometric complexity and sheer size of such membranes. To overcome these limitations, we have developed xMAS (Experimentally-Derived Membranes of Arbitrary Shape) Builder, software designed to turn low-resolution 3D meshes derived from experimental techniques into atomistic membrane models that can be simulated using MD. In the first application of this software, we have used an experimentally-derived 3D mesh and lipid composition to develop a realistic atomistic model of a Terasaki ramp, a helicoidal membrane structure found at the junction between sheets of the endoplasmic reticulum. The model measures approximately 1.97μm by 1.59μm by 0.61μm and is composed of ∼36.6 million lipids (∼4.5 billion atoms), making it one of the largest atomistic biological models to ever be built. Building this model with xMAS Builder involved several methodologically innovative steps, including using MD to simulate ∼36.6 million Lennard-Jones particles while attracted to grid-based potentials to optimize the packing of the membrane lipids, running billion atom MD simulations to fix ring piercings and other complex lipid clashes using a newly developed energy minimization technique, and utilizing grid-based potentials to equilibrate the model while maintaining its experimentally-derived shape. Simulations of the cellular membrane models built using xMAS Builder will allow us to computationally probe the complex interactions that give rise to structural stability and function at an unprecedented atomistic level of resolution, and they will allow us to build better coarse models of the systems for simulation of longer timescale phenomena.

Polymer stabilization of cholesteric liquid crystals in the oblique helicoidal state.

Electrical control of the pitch has been reported in a variant of the cholesteric liquid crystal phase composed of chiral dopants and liquid crystal dimers with a bent conformation, such as CB7CB. For a finite range of applied electric field, the dimeric mesogens assume an oblique helicoidal structure, in which the helical axis is aligned along the electric field and the local director is tilted towards the helical axis (rather than being perpendicular to it). An electric field can directly regulate the periodicity (pitch), allowing reconfiguration of the optical response from a scattering or transparent state to a reflective state. Here, we employ po stabilization to retain the oblique helicoidal state absent an applied field. The polymer stabilized oblique helicoidal structures were investigated under various conditions and material compositions. With polymer stabilization, the magnitude of the selective reflection is found to be dependent on the strength of the applied field. Comparison of the electro-optical response of samples with and without a polymer network elucidates the relative role of boundary conditions, anchoring strength, and elastic energy on the stability of the oblique helicoidal state.

Modulated phases as variational solutions in liquid-crystalline systems

GRAPHICAL ABSTRACTRepresentation of the oblique helicoidal structure with a nanometric pitch of typical of the twist-bend nematic phase.

Influence of alkyl chain length and fluorine atoms substitution on infrared spectra of chiral liquid crystalline esters

The infrared spectra were registered for series of three-ring liquid crystalline esters, differing in the structure of alkyl chain, in the substitution of benzene ring by fluorine atoms and in the type of helicoidal structure in the chiral smectic CA phase with antiferroelectric properties. The influence of molecular structure on the shift of signals coming from carbonyl group, located in the rigid core, was observed. Keywords: liquid crystals, helicoidal structure, chiral smectic CA phase, antiferroelectric crystalline phase, infrared spectroscopy

Influence of Cr doping on magnetocaloric effect and physical properties of slowly cooled NiMn1−xCrxGe

Slowly cooled NiMn1−xCrxGe (x = 0.04, 0.11, 0.18 and 0.25) has been studied by X-ray (90–480 K) and neutron diffraction (1.6–400 K), differential scanning calorimetry (300–480 K) and magnetic (2.0–400 K, magnetic field up to 90 kOe) measurements. The compounds undergo a martensitic phase transition changing their crystal structure between the orthorhombic low temperature phase (space group Pnma) and the hexagonal high temperature one (space group P63/mmc). The temperature of the structural phase transition significantly decreases with increasing x (TS = 464 K for x = 0.04 and 373 K for 0.25 on heating and similarly TS = 445 K for x = 0.04 and 355 K for 0.25 on cooling). Antiferromagnetic helicoidal ordering with the propagation vector k = (kx, 0, 0) for x = 0.04 and 0.11 (TN = 357 and 353 K, respectively) and ferromagnetic one for x = 0.25 (TC = 360 K) has been found. The sample with x = 0.18 shows a coexistence of a helicoidal antiferromagnetic structure and the ferromagnetic one below ∼170 K while at higher temperatures the ferromagnetic ordering remains stable up to 362 K. For all investigated samples the magnetocaloric effect is observed. Maximum entropy change (−ΔS) increases with increasing Cr concentration from about 8 J/(kg K) at 90 kOe, found for x = 0.04 and 0.11 at the Néel temperature, up to 29 J/(kg K) observed for x = 0.25 in cooling regime at the magnetostructural phase transition temperature.

Influence of molecular structure on helicoidal structure and NMR spectra of chiral liquid crystalline esters

Three-ring chiral liquid crystalline compounds, forming antiferroelectric phase, differing in the structure of alkyl chain and in the substitution of benzene ring by fluorine atoms were studied. The measurements of helical pitch, based on the selective light reflection phenomenon, were performed as well as 1H and 13C NMR spectra at room temperature were registered. The influence of molecular structure as well as the type of helicoidal structure on the chemical shift of signals coming from chiral center was observed. Keywords: liquid crystals, helicoidal structure, nuclear magnetic resonance, antiferroelectric phase

Mössbauer study of the modulated magnetic structure of FeVO4

Mossbauer spectroscopy is used to study the FeVO4 multiferroic, which undergoes two magnetic phase transitions at T N1 ≈ 22 K and T N2 ≈ 15 K. The first transition (T N1) is related to transformation from a paramagnetic state into a magnetically ordered state of a spin density wave, and the second transition (T N2) is associated with a change in the type of the spatial magnetic structure of the vanadate. The electric field gradient tensor at 57Fe nuclei is calculated to perform a crystal-chemical identification of the partial Mossbauer spectra corresponding to various crystallographic positions of Fe3+ cations. The spectra measured in the range T N2 < T < T N1 are analyzed on the assumption about amplitude modulation of the magnetic moments of iron atoms μFe. The results of model intersection of the spectra recorded at T < T N2 point to a high degree of anharmonicity of the helicoidal magnetic structure of the vanadate and to elliptic polarization of μFe. These features are characteristic of type-II multiferroics. The temperature dependences of the hyperfine interaction parameters of 57Fe nuclei that were obtained in this work are analyzed in terms of the Weiss molecular field model on the assumption of orbital contribution to the magnetic moments of iron cations.

Modeling of light interaction with exoskeletons of scarab beetles.

Some beetles of the family Scarabaeidae produce brilliant metallic-looking colors by their pure dielectric exoskeletons and reflect light with a high degree of circular polarization. In the present work, we discuss three models for simultaneously describing scattering, spectral, and polarization characteristics of scarab beetles. Each model consists of three slabs: an outer thin epicuticle, an exocuticle having a helicoidal structure, and a thick uniform slightly absorbing endocuticle. Scattering features are defined by rough interfaces of the epicuticle and/or nonuniformities of the exocuticle. As an example, a slightly modified model of an earlier study of Chrysina aurata is considered. The modification is aimed at including surface and volume nonuniformities that affect not only spectral and polarization properties but also scattering. Another example of using the proposed models is based on the analysis of image formations of a specimen of the species Mimela chinensis, which was studied in a polarizing microscope at different magnifications. The results show that the proposed models can be applied for explanation of light interaction with the exoskeletons of scarab beetles.

Enhancement of the optical absorption in cholesteric liquid crystals due to photonic effects: an experimental study

ABSTRACTAn anomalous strong optical absorption was measured in a cholesteric liquid crystal (CLC) at both edges of its photonic band gap. The experiment was carried out by studying the luminescence generated by the CLC sample doped with a small amount of fluorescent dye. The material was excited with monochromatic light at different angles of incidence and polarisations. Clear peaks were found in the luminescence response at angles for which the pumping wavelength coincides with the positions of the gap edges. The effect is especially noticeable for excitation under circularly polarised light of the same handedness as that of the CLC helix, and it is the highest at the long-wavelength edge. The modification of the absorption is originated by the helicoidal (photonic) structure of the material, which drastically influences the propagation of electromagnetic waves at certain frequencies and polarisations. The results were analysed numerically using an extension of the Berreman method that incorporates absorption effects. Good agreement with the experiment was found.

Exposing different in-depth pitches in the cuticle of the scarab beetle Cotinis mutabilis

The cuticle of the scarab beetle Cotinis mutabilis reflects left-handed polarized light indicating the presence of a helicoidal structure. Different in-depth pitches in the cuticle are corroborated by optical microscopy images of the cuticle which originally is yellowish or reddish but becomes greenish after gently scratching its top side. Using the Mueller-matrix formalism the degree of polarization (total and circular) of reflected light is determined for unpolarized incident light. The effects of the finite thickness of the cuticle on the broadening and strength of the selective Bragg reflection are discussed on the basis of dispersion relations for optical modes in helicoidal structures and simulated spectra.

AFM Identification of Beetle Exocuticle: Bouligand Structure and Nanofiber Anisotropic Elastic Properties

One of the common architectures in natural materials is the helicoidal (Bouligand) structure, where fiber layers twist around a helical screw. Despite the many studies that have shown the existence of Bouligand structures, methods for nanoscale structural characterization and identification of fiber mechanical properties remain to be developed. In this study, we used the exocuticle of Cotinis mutabilis (a beetle in the Cetoniinae) as a model material to develop a new experimental‐theoretical methodology that combines atomic force microscopy‐based nanoindentation and anisotropic contact mechanics analysis. Using such methodology, we studied both the helicoidal structure and the mechanical properties of its constituent fibers. The twist angle between the layers was found to be in the range of 12°–18° with a pitch size of 220 nm for the helicoidal pattern. In addition, the constituent fiber diameter was measured to be approximately 20 nm, which is consistent with the fiber diameters found in helicoids of other arthropod species. The longitudinal, transverse, and shear modulus of the nanofiber were determined to be 710 MPa, 70 MPa, and 90 MPa, respectively. The established experimental‐theoretical methodology promises to be a useful tool for nanoscale characterization of helicoidal and other structures found in biological materials.

Synthetic, Optical and Theoretical Study of Alternating Ethylenedioxythiophene-Pyridine Oligomers: Evolution from Planar Conjugated to Helicoidal Structure towards a Chiral Configuration.

A series of alternating 3,4-ethylenedioxythiophene-alkynylpyridine oligomers (DA)n with increased solubility are synthesized and their photophysical properties and nonlinear optical properties are investigated. Their quadratic polarizabilities are determined from hyper-Rayleigh scattering experiments to obtain information on their conformations in solution. These chromophores, based on the alternation of electron-rich (D) and electron-deficient (A) moieties, exhibit optical properties that arise from the combination of dipolar and helicoidal features in the (DA)n homologue series where n=1-4. The transition from dipolar conjugated planar structures (n=1, 2) to helicoidal structures (n=3, 4) is clearly evidenced by results from symmetry-sensitive second-order nonlinear optical experiments. This suggests an approach towards highly efficient chiral chromophores for second-order nonlinear optics. Interestingly, this structural evolution also has significant impact on the photophysical properties: both absorption and fluorescence emission show bathochromic and hyperchromic shifts with increasing number of repeating units in the dipolar planar derivatives (n=1-2) but show saturation effects in the helicoidal structures (n=2-4). In addition, the helicoidal structures show sizeable two-photon absorption at 700-750 nm (40-100 GM) for compounds lacking either electron-donating or electron-withdrawing substituents.

Structural colour from helicoidal cell-wall architecture in fruits of Margaritaria nobilis.

The bright and intense blue-green coloration of the fruits of Margaritaria nobilis (Phyllanthaceae) was investigated using polarization-resolved spectroscopy and transmission electron microscopy. Optical measurements of freshly collected fruits revealed a strong circularly polarized reflection of the fruit that originates from a cellulose helicoidal cell wall structure in the pericarp cells. Hyperspectral microscopy was used to capture the iridescent effect at the single-cell level.

Magnetic-field induced orientational transition in a helicoidal liquid-crystalline antiferromagnet

The magnetic-field induced orientational transition in helicoidal liquid-crystalline antiferromagnets representing compensated suspensions of magnetic nanoparticles in cholesteric liquid crystals is theoretically studied. The untwisting of a helicoidal structure and the behavior of mean magnetization as a function of the field strength and material parameters are investigated. It is shown that the magnetic subsystems in the field-untwisted ferronematic phase are not completely compensated, and the ferronematic phase is ferrimagnetic.

Significance of Helicoidal Structures Found by EUS in the Diagnosis of Intraductal Papillary Mucinous Neoplasms(IPMN)

Introduction: Aim: To explore whether the helicoidal structures found by EUS could be used as a specific structure for diagnosis of IPMN, and thus the diagnostic rate of IPMN can be improved by EUS examination. Methods: From April 2015 to may 2016,120 patients of small pancreatic cystic neoplasms were enrolled to perform EUS,among whom 54 patients diagnosed with benign neoplasms by post-surgery pathology were included. We compared the incidence of helicoidal structures in different types of cystic neoplasms. Results: Among the 54 patients, 22 patients were serous cystadenoma(SCN), 20 patients were mucinous cystadenoma(MCN), 12 patients were IPMN, and 45 of them underwent surgical resection of the tumor. Of the 12 patients with IPMN, 11patients were found helicoidal structures by EUS. Only 1 of the 20 patients with MCN was found helicoidal structures by EUS. Helicoidal structure was found in none of the 22 patients with SCN. The sensitivity, specificity and consistency of helicoidal structures found by EUS in the diagnosis of IPMN were 91.6%, 95.2% and 94.4%, respectively. Conclusion: Helicoidal structures found by EUS is a very simple to recognize and pormising diagnostic sign for IPMN,the validity of which still needs further study with large sample.Figure 1Figure 2Figure 3

Doping-induced temperature evolution of a helicoidal spin structure in the MnGe compound

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn0.75Fe0.25Ge, the magnetic-ordering temperature TN decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than TN. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K.