Kinks In Chains Research Articles

Confinement of Fractional Excitations in a Triangular Lattice Antiferromagnet.

High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of Cs_{2}CoBr_{4}, a distorted-triangular-lattice antiferromagnet with nearly XY-type anisotropy. What was previously thought of as a broad excitation continuum [L. Facheris etal., Phys. Rev. Lett. 129, 087201 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.087201] is shown to be a series of dispersive bound states reminiscent of "Zeeman ladders" in quasi-one-dimensional Ising systems. At wave vectors where interchain interactions cancel at the mean field level, they can indeed be interpreted as bound finite-width kinks in individual chains. Elsewhere in the Brillouin zone their true two-dimensional structure and propagation are revealed.

Structural systematics of SFCA-I type solid-solutions in the system CaO\u2013Fe2O3\u2013FeO\u2013Al2O3

Effects of Fe ↔ Al substitution on triclinic SFCA-I-type compounds with general formula A40O56 (A: Ca, Al, Fe3+, Fe2+) have been studied using single-crystal X-ray diffraction. Crystals of sufficient quality and size were synthesized in the temperature range between 1200 and 1300 °C. Six samples with Al/FeTot ratios of 0.127, 0.173, 0.216, 0.310, 0.349 and 0.459 have been structurally characterized. SFCA-I can be described with a modular approach involving the stacking sequence < PSS > of “P” and “S” modules that can be imagined as being cut from the well-known pyroxene (P) and spinel (S) structure types. Furthermore, SFCA-I is related to the sapphirine supergroup of minerals. Within the present solid-solution series, the contents in calcium show only minor variations (≈ 6.7 a.p.f.u.). The twenty crystallographically independent tetrahedrally (T) and octahedrally (M) coordinated cation sites exhibit considerable differences concerning the Al uptake. Indeed, Al is preferentially incorporated into the tetrahedra belonging to the single-chains located in the pyroxene modules. Ferrous iron, on the other hand, is restricted to one of the T-positions within the spinel blocks. Most structural aspects from unit-cell parameters and cell volumes to site occupancies, tetrahedral chain kinking as well as polyhedral distortions are defined by linear or nearly linear trends when plotted against the Al/FeTot ratio. Analysis of the < T–O > and < M–O > distances showed a complex interplay between the different coordination polyhedra resulting in a contrasting behavior of these values with positive or negative change rates as a function of composition. Evaluation of the average chemical strain tensor derived from the sets of lattice parameters for the two samples of the abovementioned series showing the highest and lowest Al/FeTot ratios indicated, that the major contraction with increasing Al content is perpendicular to the pyroxene- and spinel modules. Furthermore, the pyroxene module seems to be more affected when compared with the spinel block. There is evidence that the SFCA-I-type solid-solution series is limited on both the Al- and Fe-rich sides. The present investigation provides—for the first time—a detailed crystallographic analysis on the impact of chemical variations on a compound that is of relevance to the field of applied mineralogy related to the technologically important process of iron-ore sintering.

Peierls-Nabarro potential for kinks in nonlinear chains

The aim of this work is to review various discrete models supporting topological solitons, in which the Peierls-Nabarro potential can be significantly lowered or even reduced to zero. These theoretical results are discussed in relation to the Peierls stresses for dislocations in a variety of crystals. Derivation of the discrete models free of the Peierls-Nabarro potential has been done by a number of authors with the use of analytical calculations. Peierls stresses for dislocations in crystals described in the literature have been estimated within the framework of molecular dynamics and ab initio simulations. These theoretical results are discussed in connection with the variability of the Peierls stress in different crystals.

Static Kinks in Chains of Interacting Atoms

We theoretically analyse the equation of topological solitons in a chain of particles interacting via a repulsive power-law potential and confined by a periodic lattice. Starting from the discrete model, we perform a gradient expansion and obtain the kink equation in the continuum limit for a power-law exponent n ≥ 1 . The power-law interaction modifies the sine-Gordon equation, giving rise to a rescaling of the coefficient multiplying the second derivative (the kink width) and to an additional integral term. We argue that the integral term does not affect the local properties of the kink, but it governs the behaviour at the asymptotics. The kink behaviour at the center is dominated by a sine-Gordon equation and its width tends to increase with the power law exponent. When the interaction is the Coulomb repulsion, in particular, the kink width depends logarithmically on the chain size. We define an appropriate thermodynamic limit and compare our results with existing studies performed for infinite chains. Our formalism allows one to systematically take into account the finite-size effects and also slowly varying external potentials, such as for instance the curvature in an ion trap.

Lung surfactant monolayer – A good natural barrier against dibenzo-p-dioxins

The present study deals with interaction of two air pollutants: dibenzodioxin, DD, and its’ monochlorinated derivative, 2-chlorodibenzodioxin, 2CLDD, with models of the lung surfactant (LS) system. A monolayer composed of DPPC and POPC in 1:1 molar ratio was used as a model of LS. One component monolayers of DPPC and POPC were also examined, to model the interiors of LC and LE domains in LS, respectively. Molecular dynamics simulations and measurements of surface pressure isotherms, as well as polarization modulation-infrared reflection-absorption spectra were employed to study the influence of dioxins on the monolayers. We demonstrate, that both dioxins adsorb and accumulate in the hydrophobic parts of all three monolayers. DD molecules prefer flat orientation on the surface at large areas. Upon compression, they lift and orient perpendicularly to the monolayer. Flat orientation of DD molecules leads to their large surface area. In consequence they preferentially locate in vicinity of unsaturated chains of POPC – they are small enough to fill void spaces created by kinks in unsaturated chains. 2CLDD orient along monolayer normal already at the largest areas and preference for POPC was not observed for them. In laterally relaxed states, a condensing effect, connected with reduction of surface area available to the lipids was observed for both dioxins. In the case of 2CLDD, additional locally ordering influence of dioxin molecules was detected. In compressed states, the presence of dioxin molecules hinders alignment and uniform ordering of lipid chains.

Electrospinning and post-drawn processing effects on the molecular organization and mechanical properties of polyacrylonitrile (PAN) nanofibers

Abstract

Kinks in chains with on-site bistable nondegenerate potential: Beyond traveling waves.

This paper revisits the well-known transition fronts (kinks) in chains of coupled oscillators with nondegenerate on-site potentials. Usually, such transition fronts are considered in terms of traveling-wave solutions. We explore the loss of stability of such traveling waves. Generically, it corresponds to one of the common scenarios for fixed points of discrete maps. For example, one can encounter the quasiperiodic kink propagation (due to Hopf bifurcation), or the Feigenbaum cascade of period doublings, leading to a chaoticlike propagation pattern. The aforementioned scenarios show up, for instance, for triparabolic and φ^{4} on-site potentials. Numeric evidence suggests that the loss of stability occurs due to resonances between the frequency associated with the kink propagation, and the linear band gaps of the chain. Particular resonance mechanisms are model dependent. For the classical Atkinson-Cabrera model with a biparabolic on-site potential, the stability threshold is estimated by the simple means of linear algebra. The loss of stability in this model occurs through Hopf bifurcation. The results are in good agreement with numerical simulations.

Charged ultradiscrete supersonic kinks and discrete breathers in nonlinear molecular chains with realistic interatomic potentials and electron-phonon interactions

The problem of electron or hole trapping by supersonic lattice kink is revisited. Supersonic kinks in molecular chains with realistic interatomic potential produce local compression of the lattice. Lattice compression enhances electron Fermi energy and therefore produces for the electron a local potential hill, rather than a potential well, through the deformation potential of the proper sign. Here we discuss the possibility of electron trapping above the top of its tight-binding conduction band, where it possesses negative effective mass, by supersonic kink in a molecular chain with realistic interatomic potentials and electron-phonon interactions. The localization length of the electron wave function is much larger than lattice period in the case of adiabatic electron dynamics and decreases with the velocity of the ultradiscrete supersonic kink with the approximately sinusoidal envelope with the “magic” wave number. Such kinks were revealed in lattices with different interatomic potentials with hardening anharmonicity. Electron or hole can also be trapped by discrete breather (intrinsic localized mode) in the lattice with realistic asymmetric anharmonic potential. The local quasi-static strain, produced by the stationary or slowly-moving discrete breather in the lattice, can trap the electron (or hole) with its localization below the lower edge of the conduction (or above the upper edge of the valence) band.

The (Na,Li)FeGe2O6 clinopyroxene-type series: a temperature-dependent single-crystal X-ray diffraction and 57Fe Mössbauer spectroscopic study

Pyroxene-type compounds along the solid solution series LiFeGe2O6 (P21/c space group symmetry) and NaFeGe2O6 (C2/c) have been synthesized at 1273–1373 K and investigated by single-crystal X-ray diffraction at 298 K and between 105 and 298 K for Li0.4Na0.6FeGe2O6 in order to study structural variations associated with the chemical- and temperature-driven P21/c ⇔ C2/c phase transition. 57Fe Mossbauer spectroscopy, differential thermal analysis, and determination of thermal expansion coefficients from high-temperature powder X-ray diffraction data complete the investigations. The C2/c phase is stabilized with increasing Na+ content and temperature, and the phase boundary is found at 789 K in pure LiFeGe2O6, decreasing to 109 K in the compound Li0.3Na0.7FeGe2O6. From the available data, a T–X structural phase diagram is constructed. The P21/c ⇔ C2/c phase transition is accompanied by distinct variations in lattice parameters, rearrangements in M2 site coordination as well as changes in tetrahedral site chain kinking. Also, chemical strain and the thermal expansion tensor change in size and orientation with Li+ by Na+ replacement. The study will provide a detailed description of the observed variations in bond lengths, bond angles, and distortion parameters.

Raman spectroscopy of CaM2+Ge2O6(M2+ = Mg, Mn, Fe, Co, Ni, Zn) clinopyroxenes

The Raman spectra of Ge-clinopyroxenes CaM2+Ge2O6 (M2+ = Mg, Mn, Fe, Co, Ni, Zn), general formula M2M1T2O6, are reported for the first time. Their spectral features are discussed in comparison with corresponding Si-pyroxenes. The vibrational wavenumbers of germanates may be roughly obtained by a scale factor of about ~0.8 by those of the corresponding silicates, due to the Ge-Si mass difference. The main peaks in the germanate Raman spectra at ~850 and ~540 cm−1 may be related to Ge-O tetrahedral stretching and chain bending, respectively; minor peaks between 200 and 400 cm−1 are ascribed to bending and stretching of the non-tetrahedral cations. Within Ge-pyroxenes, possible correlations between crystallographic parameters and the vibrational wavenumbers are investigated. The main stretching mode at ~850 cm−1 shows wavenumber changes with M2+ substitutions, but no simple correlation can be found with M2+ cation mass or size. On the other hand, the chain bending wavenumber linearly decreases with increasing ionic radius of the M2+ cation: the expansion of the M1 polyhedron reduces the chain kinking angle and the Ge-Ge distances correspondingly increase. Copyright © 2015 John Wiley & Sons, Ltd.

Speed selection for coupled wave equations

We discuss models for coupled wave equations describing interacting fields, focusing on the speed of travelling wave solutions. In particular, we propose a general mechanism for selecting and tuning the speed of the corresponding (multi-component) travelling wave solutions under certain physical conditions. A number of physical models (molecular chains, coupled Josephson junctions, propagation of kinks in chains of adsorbed atoms and domain walls) are considered as examples.

Ab initio study of the formation of structural kinks in carbon chains

Our theoretical studies using methods of electron density functional theory show that hydrogen, fluorine, and oxygen impurities must be present in the chamber during the synthesis of linear-chain carbon to form and stabilize experimentally observed kinks in carbon chains. The kink in the carbon chain forms during the adsorption of impurity atoms onto the chain. It is shown that it is energetically advantageous for impurity atoms to form kinks rather than to occupy other adsorption positions. The optimal kink angle of carbon chains is determined. The stability of bent carbon chains is estimated as a function of the length of linear fragments. It is shown that the formation of kinks leads to that the formation of a narrow band gap near the Fermi level: EgH = 0.56 eV, EgF = 0.59 eV, and EgO = 1.04 eV.

High-pressure behavior of NaInSi 2O 6 and the influence of Me 3+ on the compressibility of NaMe 3+Si 2O 6 silicates

The equation of state of a single-crystal synthetic silicate NaInSi 2O 6 was determined from the unit cell parameters measured by X-ray diffraction using a diamond-anvil cell at 12 different pressures up to 7.83(1) GPa in order to provide a definitive model capable of predicting the high-pressure behavior of NaMe 3 +Si 2O 6 silicate compounds. The P – V data were fitted by a third-order Birch–Murnaghan equation of state obtaining the following coefficients: V 0 = 463.42 ( 3 ) Å 3 , K T 0 = 109.0 ( 6 ) GPa , K ′ = 3.3 ( 2 ) . Our results show similar results for NaMe 3 +Si 2O 6 compounds and confirm that for this mineral family, the empirical K T 0 × V 0 = constant relationship proposed in previous investigations is valid for isostructural compounds only if they share the same valence electron character. In addition, we collected intensity data on the same compound at 16 different pressures up to 9.467 GPa using a diamond-anvil cell with diamond backing plates. The main structural compression mechanism is played by the tetrahedral chain kinking, which is related to the shortening of Na–O3 long distances. Comparing our equation of state and high-pressure crystal structural results with previous data on NaMe 3 +Si 2O 6 pyroxenes, we show that their compressibility increases much faster with the size of Me 3 + cation when this one is a 3d transition metal.

Effect of hydrogen on the formation of the atomic structure of linear carbon chains: An ab initio approach

An ab initio study of the features of the formation of the atomic structure of carbon chains, which are structural parts of films of linear carbon chains, has been performed using the electron density functional theory. It has been shown that the formation and stabilization of experimentally observed kinks of carbon chains require the presence of hydrogen impurities in the chamber during the synthesis of linear carbon chains. The kinks of a carbon chain are formed in the process of the adsorption of hydrogen atoms on a chain. The optimal kink angle of carbon chains has been determined. The stability of kinks of carbon chains has been estimated as a function of the length of linear fragments. An additional mechanism has been proposed for the formation of kinks in carbon chains owing to the joining of short carbon chains with hydrogen ends.

Quantum chemistry and molecular dynamics studies of the entropic elasticity of localized molecular kinks in polyisoprene chains

We investigate the thermodynamic consequences of the distribution of rotational conformations of polyisoprene on the elastic response of a network chain. In contrast to the classical theory of rubber elasticity, which associates the elastic force with the distribution of end-to-end distances, we find that the distribution of chain contour lengths provides a simple mechanism for an elastic force. Entropic force constants were determined for small contour length extensions of chains constructed as a series of localized kinks, with each kink containing between one and five cis-1,4-isoprene units. The probability distributions for the kink end-to-end distances were computed by two methods: (1) by constructing a Boltzmann distribution from the lengths corresponding to the minimum energy dihedral rotational conformations, obtained by optimizing isoprene using first principles density functional theory, and (2) by sampling the trajectories of molecular dynamics simulations of an isolated molecule composed of five isoprene units. Analogous to the well-known tube model of elasticity, we make the assumption that, for small strains, the chain is constrained by its surrounding tube, and can only move, by a process of reptation, along the primitive path of the contour. Assuming that the chain entropy is Boltzmann's constant times the logarithm of the contour length distribution, we compute the tensile force constants for chain contour length extension as the change in entropy times the temperature. For a chain length typical of moderately crosslinked rubber networks (78 isoprene units), the force constants range between 0.004 and 0.033 N/m, depending on the kink size. For a cross-linked network, these force constants predict an initial tensile modulus of between 3 and 8 MPa, which is comparable to the experimental value of 1 MPa. This mechanism is also consistent with other thermodynamic phenomenology.

High-pressure behavior of Ca/Na clinopyroxenes: The effect of divalent and trivalent 3d-transition elements

Two C 2/ c pyroxenes, CaMnSi 2 O 6 and CaZnSi 2 O 6 , were investigated at high pressure by single-crystal X-ray diffraction. Their unit-cell parameters were determined at 9 different pressures up to 7.55 and 8.23 GPa, respectively. No evidence of phase transformations was found for either sample throughout the investigated pressure range. The pressure-volume data were fit using a third-order Birch-Murnaghan equation of state and it was possible to refine simultaneously the unit-cell volume V 0 , the bulk modulus K T0 and its first pressure derivative K ′ providing the following coefficients: V 0 = 462.01(3) A 3 , K T0 = 97.3(4) GPa, K ′ = 7.5(2) for CaMnSi 2 O 6 and V 0 = 444.79(2) A 3 , K T0 = 109.6(5) GPa, K ′ = 7.0(2) for CaZnSi 2 O 6 . The substitution of Zn for Mn at the M1 crystallographic site causes a strong decrease of the bulk modulus (by about 11%). Intensity data were collected at several pressures for both samples. The main compression mechanism is related to the tetrahedral chain kinking and the consequent shortening of the Ca-O3 long bond distances, which show a markedly larger decrease for the CaMnSi 2 O 6 sample. Comparing the results obtained in this work with previous data for Ca/Na-bearing C 2/ c pyroxenes containing trivalent and divalent 3 d -transition elements at M1 site, we identified two distinct compressibility trends: for a similar unit-cell volume at room pressure the pyroxenes with Na on M2 and trivalent cations on M1 are more compressible than those having Ca on M2 and divalent cations at the M1 site. The analysis of the structure evolution for all samples indicates that this is strongly correlated with the evolution of kinking of the tetrahedral chains and the shortening of the M2-O3 bonds.

High-pressure phase transformation in LiFeGe2O6 pyroxene

A synthetic pyroxene with composition LiFeGe 2 O 6 and space group P 2 1 / c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a , c , β, and unit-cell volume. The transition is marked by the disappearance of b -type reflections ( h + k = odd) forbidden in a C -centered lattice. The volume bulk modulus of the P 2 1 / c phase is estimated to be 110 GPa as compared to 147 GPa of the C 2/ c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P 2 1 / c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C 2/ c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.

The crystal structure of diopside at pressure to 10 GPa

The crystal structure of diopside has been determined at various pressures to 10.16 GPa. The results to 5 GPa are consistent with Levien and Prewitt (1981). The crystal structures have been analyzed using the geometric pyroxene model of Thompson and Downs (2004), the anion packing algorithm of Thompson and Downs (2001), and a new algorithm that quantifies the distortion of observed pyroxenes from their geometric model equivalents. Diopside is shown to compress via three main mechanisms: isotropic scaling, kinking of the tetrahedral chains, and collapse of the M1 chain toward its axis. The kinking of the chains accounts for most of the anisotropy of compression observed in the a-c plane, and the collapse of the M1 chain explains the anisotropy seen in the b-c plane. Model behavior is shown to reproduce many of the observations of previous workers. Anion-anion interactions are shown to be important in the distortion of observed diopside from its geometric model equivalent.

In silico characterization of antifreeze proteins using computational tools and servers

In this paper, seventeen different fish Antifreeze Proteins (AFPs) retrieved from Swiss-Prot database are analysed and characterized using in silico tools. Primary structure analysis shows that most of the AFPs are hydrophobic in nature due to the high content of non-polar residues. The presence of 11 cysteines in the rainbow smelt fish and sea raven fish AFPs infer that these proteins may form disulphide (SS) bonds, which are regarded as a positive factor for stability. The aliphatic index computed by Ex-Pasy’s ProtParam infers that AFPs may be stable for a wide range of temperature. Secondary structure analysis shows that most of the fish AFPs have predominant α-helical structures and rest of the AFPs have mixed secondary structure. The very high coil structural content of rainbow smelt fish and sea raven fish AFPs are due to the rich content of more flexible glycine and hydrophobic proline amino acids. Proline has a special property of creating kinks in polypetide chains and disrupting ordered secondary structure. SOSUI server predicts one transmembrane region in winter flounder fish and atlantic cod and two transmembrane regions in yellowtail flounder fish AFP. The predicted transmembrane regions were visualized and analysed using helical wheel plots generated by EMBOSS pepwheel tool. The presence of disulphide (SS) bonds in the AFPs Q01758 and P05140 are predicted by CYS_REC tool and also identified from the three-dimensional structure using Rasmol tool. The disulphide bonds identified from the three-dimensional structure using the Rasmol tool might be correct as the evaluation parameters are within the acceptable limits for the modelled 3D structures.

Kinks in dipole chains

It is shown that the topological discrete sine-Gordon system introduced by Speight and Ward models the dynamics of an infinite uniform chain of electric dipoles constrained to rotate in a plane containing the chain. Such a chain admits a novel type of static kink solution which may occupy any position relative to the spatial lattice and experiences no Peierls–Nabarro barrier. Consequently the dynamics of a single kink is highly continuum-like, despite the strongly discrete nature of the model. Static multikinks and kink–antikink pairs are constructed, and it is shown that all such static solutions are unstable. Exact propagating kinks are sought numerically using the pseudo-spectral method, but it is found that none exist, except, perhaps, at very low speed.