Lattice Order Research Articles

Magnetic structures andZ2vortices in a non-Abelian gauge model

The magnetic order of the triangular lattice with antiferromagnetic interactions is described by an SO(3) field and allows for the presence of Z2 magnetic vortices as defects. In this work we show how these Z2 vortices can be fitted into a local SU(2) gauge theory. We propose simple Ansatzes for vortex configurations and calculate their energies using well-known results of the Abelian gauge model. We comment on how Dzyaloshinskii-Moriya interactions could be derived from a non-Abelian gauge theory and speculate on their effect on non trivial configurations.

Symmetry‐Driven Atomic Rearrangement at a Brownmillerite–Perovskite Interface

Many of the recent advancements in oxide heterostructures have been attributed to modification of spin, charge, lattice, and orbital order parameters at atomically well-defined interfaces. However, the details on the structural, chemical, and electrostatic evolution of interfaces comprised of materials with different crystallographic symmetries remain to be understood. In this work, we have mapped out the interfacial connectivity of atoms of two dissimilar materials, the perovskite SrTiO3 and the brownmillerite SrCoO2.5, using high resolution scanning transmission electron microscopy and geometric phase analysis. We observed unique symmetry-mismatch driven atomic displacements restricted to only the first few atomic layers, which can critically modify the properties of the system. Provided that SrCoO2.5 is a promising energy material due to its open framework structure, the improved understanding of the interfacial structure on the atomic level can lead to the rational design of novel oxide heterostructures.

Structural and chemical modifications of typical South African biomasses during torrefaction

Torrefaction experiments were carried out for three typical South African biomass samples (softwood chips, hardwood chips and sweet sorghum bagasse) to a weight loss of 30wt.%. During torrefaction, moisture, non-structural carbohydrates and hemicelluloses were reduced, resulting in a structurally modified torrefaction product. There was a reduction in the average crystalline diameter (La) (XRD), an increase in the aromatic fraction and a reduction in aliphatics (substituted and unsubstituted) (CPMAS 13C NMR). The decrease in the aliphatic components of the lignocellulosic material under the torrefaction conditions also resulted in a slight ordering of the carbon lattice. The degradation of hemicelluloses and non-structural carbohydrates increased the inclusive surface area of sweet sorghum bagasse, while it did not change significantly for the woody biomasses.

Chain Architecture as an Orthogonal Parameter To Influence Block Copolymer Morphology. Synthesis and Characterization of Hyperbranched Block Copolymers: HyperBlocks

For block copolymers there is usually a strong correlation between the copolymer composition (volume fraction of each block) and the resulting solid state morphology. However, for a variety of potential applications, e.g., semipermeable membranes or templates, it might be desirable to vary the microphase morphology independently of copolymer composition. The use of chain branching is an additional and orthogonal parameter to influence morphology, independently of composition, and we explore for the first time the impact of a long-chain, hyperbranched architecture on the microphase-separated, solid state morphology of branched block copolymers. To this end, a series of functionalized linear ABA (polystyrene–polyisoprene–polystyrene) triblock copolymers (macromonomers), hyperbranched ABA triblock copolymers (HyperBlocks), and blends of HyperBlocks with a commercially available linear ABA triblock copolymeric thermoplastic elastomer were prepared. Moreover, the “macromonomer” approach is the only feasible route to prepare hyperbranched block copolymers. The solid state morphology of the resulting materials was investigated by a combination of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) which showed a dramatic impact of the chain architecture on the resulting morphology. While the linear ABA triblock copolymers showed the expected microphase-separated morphology with long-range order dependent upon composition, no long-range order was observed in the HyperBlocks. Instead, the HyperBlocks revealed a microphase-separated morphology without long-range lattice order, irrespective of macromonomer composition or molecular weight. Furthermore, when HyperBlocks were subsequently blended with a commercially available linear ABA triblock copolymer (Kraton D1160), the HyperBlock appeared to impose a microphase-separated morphology without long-range lattice order upon the linear copolymer even when the HyperBlock is present as the minor component in the blend at levels as low as 10 wt %.

Strain‐Mediated Inverse Photoresistivity in SrRuO3/La0.7Sr0.3MnO3 Superlattices

In the pursuit of novel functionalities by utilizing the lattice degree of freedom in complex oxide heterostructure, the control mechanism through direct strain manipulation across the interfaces is still under development, especially with various stimuli, such as electric field, magnetic field, light, etc. In this study, the superlattices consisting of colossal‐magnetoresistive manganites La0.7Sr0.3MnO3 (LSMO) and photostrictive SrRuO3 (SRO) have been designed to investigate the light‐dependent controllability of lattice order in the corresponding functionalities and rich interface physics. Two substrates, SrTiO3 (STO) and LaAlO3 (LAO), have been employed to provide the different strain environments to the superlattice system, in which the LSMO sublayers exhibit different orbital occupations. Subsequently, by introducing light, we can modulate the strain state and orbital preference of LSMO sublayers through light‐induced expansion of SRO sublayers, leading to surprisingly opposite changes in photoresistivity. The observed photoresistivity decreases in the superlattice grown on STO substrate while increases in the superlattice grown on LAO substrate under light illumination. This work has presented a model system that demonstrates the manipulation of orbital–lattice coupling and the resultant functionalities in artificial oxide superlattices via light stimulus.

Effects of phase stability, lattice ordering, and electron density on plastic deformation in cubic TiWN pseudobinary transition-metal nitride alloys

We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5N, as estimated by stress–strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [111]-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along <110> directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of <111>-ordered Ti0.5W0.5N with respect to mixing of cubic-B1 TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5Mo0.5N epitaxial layers.

An efficient algorithm for increasing the granularity levels of attributes in formal concept analysis

In the basic setting of formal concept analysis, a many-valued attribute needs to be replaced with several one-valued attributes. These one-valued attributes can be interpreted as a certain level of granularity of the corresponding many-valued attribute. In this paper, we explore theoretical relationships between concepts before and after increasing the granularity level of one attribute, based on which we introduce an efficient method of concept classification. Moreover, a new preprocessing routine is proposed to help generate new concepts and restore lattice order relation. These two procedures can considerably reduce the comparisons between sets, compared to the original Zoom-In algorithm. By employing these two procedures, we introduce an efficient algorithm, referred to as Unfold, to increase the granularity levels of attributes. The algorithm can perform a Zoom-In operation on a concept lattice associated with a coarser data granularity to obtain a new one that consists of finer formal concepts without building the new lattice from scratch. We describe the algorithm and present an experimental evaluation of its performance and comparison with another Zoom-In algorithm. Empirical analyses demonstrate that our algorithm is superior when applied to various types of datasets.

A lattice-free concept lattice update algorithm

Upon a change of input data, one usually wants an update of output computed from the data rather than recomputing the whole output over again. In Formal Concept Analysis, update of concept lattice of input data when introducing new objects to the data can be done by any of the so-called incremental algorithms for computing concept lattice. The algorithms use and update the lattice while introducing new objects to input data one by one. The present concept lattice of input data without the new objects is thus required by the computation. However, the lattice can be large and may not fit into memory. In this paper, we propose an efficient algorithm for updating the lattice from the present and new objects only, not requiring the possibly large concept lattice of present objects. The algorithm results as a modification of the Close-by-One algorithm for computing the set of all formal concepts, or its modifications like Fast Close-by-One, Parallel Close-by-One or Parallel Fast Close-by-One, to compute new and modified formal concepts and the changes of the lattice order relation only. The algorithm can be used not only for updating the lattice when new objects are introduced but also when some existing objects are removed from the input data or attributes of the objects are changed. We describe the algorithm, discuss efficiency issues and present an experimental evaluation of its performance and a comparison with the AddIntent incremental algorithm for computing concept lattice.

Magnesium from bioresorbable implants: Distribution and impact on the nano- and mineral structure of bone

Biocompatibility is a key issue in the development of new implant materials. In this context, a novel class of biodegrading Mg implants exhibits promising properties with regard to inflammatory response and mechanical properties. The interaction between Mg degradation products and the nanoscale structure and mineralization of bone, however, is not yet sufficiently understood. Investigations by synchrotron microbeam x-ray fluorescence (μXRF), small angle x-ray scattering (μSAXS) and x-ray diffraction (μXRD) have shown the impact of degradation speed on the sites of Mg accumulation in the bone, which are around blood vessels, lacunae and the bone marrow. Only at the highest degradation rates was Mg found at the implant–bone interface. The Mg inclusion into the bone matrix appeared to be non-permanent as the Mg-level decreased after completed implant degradation. μSAXS and μXRD showed that Mg influences the hydroxyl apatite (HAP) crystallite structure, because markedly shorter and thinner HAP crystallites were found in zones of high Mg concentration. These zones also exhibited a contraction of the HAP lattice and lower crystalline order.

Self-consistent approach for magnetic ordering and excited site occupation processes in a two-level system

Ferromagnetic ordering in a two-level partially excited system is studied in detail. Magnitudes of magnetization (magnetic order parameter) and lattice ordering (excited level occupation number) are calculated self-consistently. The influence of an external magnetic field and excited level gap on the ferromagnetic phase transition is discussed.

Tuning the antiferromagnetic to ferromagnetic phase transition in FeRh thin films by means of low-energy/low fluence ion irradiation

Ion irradiation induced modifications of the thermomagnetic properties of equiatomic FeRh thin films have been investigated. The application of 20keV Ne+ ions at different fluencies leads to broadening of the antiferromagnetic to ferromagnetic phase transition as well as a shift of the transition temperature towards lower temperatures with increasing ion fluence. Moreover, the ferromagnetic background at low temperatures generated by the ion irradiation leads to pronounced saturation magnetisation at 5K. Complete erasure of the transition, i.e. ferromagnetic ordering through the whole temperature regime was achieved at a Ne+ fluence of 3×1014ions/cm2. It does not coincide with the complete randomization of the chemical ordering of the crystal lattice.

Structural Organization in Magnetic Fluids with Magnetized Aggregates in Rotating Magnetic Field

The results of the study of structures forming in magnetic colloids with magnetized aggregates under the action of an elliptically polarized rotating magnetic field are discussed. When a magnetic colloid layer with magnetized aggregates is affected by an elliptically polarized rotating magnetic field, some ordering of the structural lattice is observed, which is determined by the frequency and orientation of the rotating magnetic field polarization ellipse as well as by the value of an additionally imposed steady magnetic field.

Asymmetric norms, cones and partial orders

If X is a real vector space and p an asymmetric norm on X, the set Cp={x∈X:p(−x)=0} is a proper cone in X which induces a partial order on X compatible with the linear structure of X. Using the norm ps(x)=max⁡{p(x),p(−x)}, a second asymmetric norm can be defined by qp(x)=inf⁡{ps(x+y):y∈Cp}. In the case where the partial order induced by Cp is a lattice order, it is possible to define a third asymmetric norm by p+(x)=p(x+), where x+ is the positive part of x. The paper investigates the relationships between these three asymmetric norms, with special attention to the case where X is finite-dimensional.

Matrix ℓ-algebras over ℓ-fields

It is shown that if a matrix l-algebra Mn(K) over certain l-fields K contains a positive n-cycle e such that I+e+⋯+en-1 is a d-element on K then it is isomorphic to the l-algebra Mn(K) over K with the entrywise lattice order.

Reversible ratchet effects for vortices in conformal pinning arrays

A conformal transformation of a uniform triangular pinning array produces a structure called a conformal crystal which preserves the six-fold ordering of the original lattice but contains a gradient in the pinning density. Here we use numerical simulations to show that vortices in type-II superconductors driven with an ac drive over gradient pinning arrays produce the most pronounced ratchet effect over a wide range of parameters for a conformal array, while square gradient or random gradient arrays with equivalent pinning densities give reduced ratchet effects. In the conformal array, the larger spacing of the pinning sites in the direction transverse to the ac drive permits easy funneling of interstitial vortices for one driving direction, producing the enhanced ratchet effect. In the square array, the transverse spacing between pinning sites is uniform, giving no asymmetry in the funneling of the vortices as the driving direction switches, while in the random array, there are numerous easy-flow channels present for either direction of drive. We find multiple ratchet reversals in the conformal arrays as a function of vortex density and ac amplitude, and correlate the features with a reversal in the vortex ordering, which is greater for motion in the ratchet direction. The enhanced conformal pinning ratchet effect can also be realized for colloidal particles moving over a conformal array, indicating the general usefulness of conformal structures for controlling the motion of particles.

Magnetic, electrical and structural properties of the Re-doped ruthenocuprate Ru1−xRexSr2GdCu2Oy

Despite the discovery of new superconductors classes, high-Tc oxides continue to be a current topic, because of their complex phase diagrams and doping-dependant effects (allowing one to investigate the interaction between orbitals), as well as structural properties such as lattice distortion and charge ordering, among many others. Ruthenocuprates are magnetic superconductors in which the magnetic transition temperature is much higher than the critical superconducting temperature, making them unique compounds. With the aim of investigating the dilution of the magnetic Ru sub-lattice, we proposed the synthesis of the Ru1−xRexSr2GdCu2Oy ruthenocuprate-type family, adapting the known two-step process (double perovskite+CuO) by directly doping the double perovskite, thus obtaining the perovskite compound Sr2GdRu1−xRexOy, which represents a new synthesis process to the best of our knowledge. Our samples were structurally characterized through X-ray diffraction, and the patterns were analysed via Rietveld refinement. A complete magnetic characterization as a function of temperature and applied field, as well as transport measurements were carried out. We discuss our results in the light of the two-lattice model for ruthenocuprates, and a relation between RuO2 (magnetic) and CuO2 (superconductor) sub-lattices can clearly be observed.

Biodegradation of Coal Minerals by Gluconic Acid and its Effect on the Stacking Structure of Carbon: An Investigation

Leaching of minerals (Silicates, Calcium, Aluminum) from bituminous coal by gluconic acid of varying concentration was carried out and the chemical mechanism was studied. Stacking structural parameters of the coal like aromaticity, rank, number of aromatic layers, carbon atoms per aromatic lamellae, stacking height and lattice spacing were determined using X-ray analysis. An ordering of lattice was noticed with gluconic acid leaching and is maximum for 20% and 40% concentration. The number of lyers and average number of carbon atoms per lamellae are found to be varying from 6-7 and 11-17 respectively. The mineral content (Al and Si) shows systematic decrease with increase in concentration of leachant. The optimum demineralization is noticed for the successively leached coal sample with glucoinc acid and mild Hydrofluoric acid.

Epitaxially Textured Pr0.6Ca0.4MnO3 Thin Films Under Considerably Low Substrate Temperature

We report the growth of well-crystallized and epitaxially textured Pr0.6Ca0.4MnO3 thin films on SrTiO3 substrates by pulsed laser deposition at considerably low substrate temperatures, as low as 450 ̊C, without high-temperature post-annealing treatments. Although a strong ferromagnetic interaction as well as a large irreversible metamagnetic transition with a training effect have been observed for films grown at 450 ̊C, the in-plane and out-of-plane lattice ordering is slightly improved with increasing substrate temperature. Therefore, a lowest magnetic field of 2 T for melting the insulating charge-ordering state at 70K has been observed for films grown with the substrate temperature between 550 ̊C and 600 ̊C. The formation and growth of Pr0.6Ca0.4MnO3 on SrTiO3 substrate at exceptionally low substrate temperature is qualitatively modelled by the combination of the kinetic energies and redox potentials of the components of the ablation plasma, while the heat flow from the substrate is assumed to be less important.

Interlaced crystals having a perfect Bravais lattice and complex chemical order revealed by real-space crystallography.

The search for optimal thermoelectric materials aims for structures in which the crystalline order is disrupted to lower the thermal conductivity without degradation of the electron conductivity. Here we report the synthesis and characterisation of ternary nanoparticles (two cations and one anion) that exhibit a new form of crystalline order: an uninterrupted, perfect, global Bravais lattice, in which the two cations exhibit a wide array of distinct ordering patterns within the cation sublattice, forming interlaced domains and phases. Partitioning into domains and phases is not unique; the corresponding boundaries have no structural defects or strain and entail no energy cost. We call this form of crystalline order 'interlaced crystals' and present the example of hexagonal CuInS2. Interlacing is possible in multi-cation tetrahedrally bonded compound with an average of two electrons per bond. Interlacing has minimal effect on electronic properties, but should strongly reduce phonon transport, making interlaced crystals attractive for thermoelectric applications.

Controllable 3D morphology and growth mechanism of quasicrystalline phase in directionally solidified Al–Mn–Be alloy

Abstract