Measure Of Local Structure Research Articles

Species diversity of bats along an altitudinal gradient on Mount Mulanje, southern Malawi

Abstract:A climate model, based on effects of water availability and temperature, was recently proposed to explain global variation in bat species richness along altitudinal gradients. Yet such studies are sparse in the tropics and near-absent in Africa. Here we present results from an altitudinal study of bat diversity from Mount Mulanje, Malawi. Using ground nets, canopy nets and harp traps, we sampled eight sites across three habitat zones from 630 m to 2010 m asl. We assessed the influence of climatic, geographic and biotic variables on measures of estimated species richness, Fisher's α, and an unbiased index of compositional turnover. We recorded 723 individuals and 30 species along the gradient, revealing a ‘low plateau’ pattern in estimated species richness, peaking at 1220 m, which is congruent with the global climate model. Measures of local habitat structure significantly explained a large degree of variation in species richness and compositional turnover between sites. Fisher's α was further significantly correlated to mean annual relative humidity, suggesting a background climatic influence.

Local structure of the superconductor K0.8Fe1.6+xSe2: Evidence of large structural disorder

The local structure of superconducting single crystals of K0.8Fe1.6+xSe2 with Tc = 32.6 K was studied by x-ray absorption spectroscopy. Near-edge spectra reveal that the average valence of Fe is 2+. The room temperature structure about the Fe, K and Se sites was examined by iron, selenium and potassium K-edge measurements. The structure about the Se and Fe sites shows a high degree of order in the nearest neighbor Fe-Se bonds. On the other hand, the combined Se and K local structure measurements reveal a very high level of structural disorder in the K layers. Temperature dependent measurements at the Fe sites show that the Fe-Se atomic correlation follows that of the Fe-As correlation in the superconductor LaFeAsO0.89F0.11 - having the same effective Einstein temperature (stiffness). In K0.8Fe1.6+xSe2, the nearest neighbor Fe-Fe bonds has a lower Einstein temperature and higher structural disorder than in LaFeAsO0.89F0.11. The moderate Fe site and high K site structural disorder is consistent with the high normal state resistivity seen in this class of materials. For higher shells, an enhancement of the second nearest neighbor Fe-Fe interaction is found just below Tc and suggests that correlations between Fe magnetic ion pairs beyond the first neighbor are important in models of magnetic order and superconductivity in these materials.

Mountain hazards: reducing vulnerability by adapted building design

Despite the long tradition of technical mitigation on a catchment scale in European mountain regions, losses due to mountain hazards are still considerably high in number and monetary loss. Therefore, the concept of technical mitigation had been supplemented by land-use planning and, more recently, local structural protection. Local structural protection includes measures directly implemented at or adjacent to endangered objects, and has proven to be particularly cost-effective with respect to integral risk management strategies. However, the effect of local structural protection in reducing the vulnerability of elements at risk, and the associated consequences with respect to a reduction of structural vulnerability have not been quantified so far. Moreover, there is a particular gap in quantifying the expenditures necessary for local structural protection measures. Therefore, a prototype of residential building adapted to mountain hazards is presented in this study. This prototype is equipped with various constructional elements to resist the incurring impact forces, i.e., of fluvial sediment transport and of snow avalanches. According to possible design loads emerging from these hazard processes, the constructive design necessary is presented, and the amount of additional costs required for such an adaptation is presented. By comparing these costs with quantitative loss data it is shown that adapted building design is particularly effective to reduce the consequences of low-magnitude, high-frequency events in mountain regions.

Measurements and ab initio molecular dynamics simulations of the high temperature ferroelectric transition in hexagonal RMnO3

Measurements of the structure of hexagonal RMnO3 [R = rare earths (Ho) and Y] for temperatures significantly above the ferroelectric transition temperature (TFE) were conducted to determine the nature of the transition. The local and long range structural measurements were complemented by ab initio molecular dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find no large atomic (bond distances or thermal factors), electronic structure changes, or rehybridization on crossing TFE from local structural methods. The local symmetry about the Mn sites is preserved. With respect to the local structure about the Ho sites, a reduction of the average Ho–O bond with increased temperature is found. Ab initio molecular dynamics calculations on HoMnO3 reveal the detailed motions of all ions. Above ∼900 K there are large displacements of the Ho, O3, and O4 ions along the z axis which reduce the buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to toward central points between pairs of Ho ions on the z axis. These structural changes make the coordination of Ho sites more symmetric thus extinguishing the electric polarization. At significantly higher temperatures, rotation of the MnO5 polyhedra occurs without a significant change in electric polarization. The Born effective charge tensor is found to be highly anisotropic at the O sites but does not change appreciably at high temperatures.

Image-based algorithm for analysis of transient trapping in single-particle trajectories

Particle tracking has become an increasingly useful tool in microfluidics and biophysics, allowing measurement of microrheology, local structure, and flow. We introduce a novel, automated approach to analyze single-particle trajectories with transient elements, based on image-processing approaches and physical analysis of probe motion. In many physical and active biological systems, such as living cells, probe particles experience thermally mediated Brownian motion combined with active transport processes that can lead to transient-trajectories of local diffusion and trapping, punctuated by segments of active transport. Analyzing such a trajectory as a single unit masks the intermittent nature of the motion. Moreover, directly applying the generalized Stokes–Einstein relation in out-of-equilibrium systems is incorrect and returns inaccurate rheological parameters. We present an automated image-processing-based method to identify and segment transient trap-escape trajectories, allowing quantitative analysis of each segment. We define and discuss effects of controlling parameters, such as particle size and camera frame rate. Our algorithm provides a general and automated method to segment and analyze transient elements in trajectories of single particles, which can be applied to many different experiments. Our image-based approach allows identification of trapping segments, unbiased by specific step sizes within those traps or the mechanism driving those steps. As an example, we successfully apply this method to experiments of laser tweezers trapped particles and show that trajectory segmentation allows us to calculate both trap and fluid parameters. We accurately identify a round trap, calculate the trap stiffness at 3.1 pN/μm, and find that significant local heating reduces fluid viscosity.

Investigation of the Electrochemical Behavior of Nano LiNi0.5Mn1.5O4-δ by TEM,EELS and XAS

not Available.

Order and disorder in the local and long-range structure of the spin-glass pyrochlore,Tb2Mo2O7

To understand the origin of the spin-glass state in molybdate pyrochlores, the structure ofTb2Mo2O7 is investigated using two techniques: the long-range lattice structure was measured usingneutron powder diffraction, and local structure information was obtained from theextended x-ray absorption fine structure technique. While the long-range structureappears generally well ordered, enhanced mean-squared site displacements on theO(1) site and the lack of temperature dependence of the strongly anisotropicdisplacement parameters for both the Mo and O(1) sites indicate some disorder exists.Likewise, the local structure measurements indicate some Mo–Mo and Tb–O(1)nearest-neighbor disorder exists, similar to that found in the related spin-glass pyrochlore,Y2Mo2O7. Although the freezingtemperature in Tb2Mo2O7, 25 K, isslightly higher than in Y2Mo2O7, 22 K, the degree of local pair distance disorder is actually less inTb2Mo2O7. This apparent contradiction is considered in light of the interactions involved in thefreezing process.

High-pressure x-ray diffraction studies on the structure of liquid silicate using a Paris–Edinburgh type large volume press

An experimental setup for high-pressure liquid structure studies with synchrotron x-ray diffraction using the Paris-Edinburgh press has been installed at station 16-BM-B (HPCAT) of the Advanced Photon Source, Argonne National Laboratory. By collecting energy-dispersive data with a synchrotron white beam at various 2θ angles, the present device allows us to obtain the structure factor, S(Q), over a wide range of Q ( = 4πsinθ∕λ) owing to the excellent angular accessibility up to 37° in 2θ and high energy photons well beyond 100 keV. We have successfully collected XRD data on silicate (albite, NaAlSi(3)O(8)) liquids with Q up to ∼22 Å(-1) and pressure up to 5.3 GPa and temperature 1873 K, and obtained the radial distribution function, G(r), with a reasonable resolution. The T-O bond length (where T = Al, Si), which is a fundamental measure of local structure for aluminous silicate consisting of SiO(n) and AlO(n) polyhedra (tetrahedra at 1 atm condition), was found to be slightly shortened to 1.626 Å compared to that of glass at 1 atm. The T-O-T bound angle, which is the linkage of the above polyhedra, is the most responsible for densification. The T-O-T peak in G(r) splits into two peaks, suggesting a differentiation of the bond angle at high-pressure. The present technical development demonstrates that the Paris-Edinburgh press is suitable for studies of silicate liquids under high-pressure conditions.

High temperature emissivity, reflectivity, and x-ray absorption of BiFeO3

We report on the lattice evolution of BiFeO3 as function of temperature using far infrared emissivity, reflectivity, and x-ray absorption local structure. A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below TN∼640 K it ceases softening as consequence of BiFeO3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ∼700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above TC∼1090 K. Temperature dependent x-ray absorption near edge structure (XANES) at the Fe K edge shows that lower temperature Fe3+ turns into Fe2+. While this matches the FeO wüstite XANES profile, the Bi LIII-edge downshift suggests a high temperature very complex bond configuration at the distorted A perovskite site. Overall, our local structural measurements reveal high temperature defect-induced irreversible lattice changes, below, and above the ferroelectric transition, in an environment lacking of long-range coherence. We did not find an insulator to metal transition prior to melting.

Neither retinal nor brain atrophy can be shown in patients with isolated unilateral optic neuritis at the time of presentation

Background:Acute monosymptomatic optic neuritis (ON) may be the earliest manifestation of multiple sclerosis (MS). Atrophy has been shown to be a prominent feature of MS with great impact on disability. Objectives:The objectives of this study were to evaluate retinal and brain atrophy and possible associations at the earliest possible stages of MS. Methods:In a prospective observational cohort study we included 60 untreated patients with monosymptomatic ON and 19 healthy volunteers. Unaffected fellow eyes were examined with optical coherence tomography (OCT) and normalized brain volumes were calculated based on MRI. Additionally, visual evoked potentials (VEPs) were recorded. Results:Neither OCT measurements nor brain volume measures revealed signs of localized or generalized atrophy in patients compared with healthy volunteers. Stratification of patients into high risk based on the presence of white matter lesions did not reveal differences. The association between OCT measures and brain volumes previously found could not be confirmed at the time of the first clinical event. VEP latency was significantly prolonged in patients with white matter lesions compared to those without lesions. A trend towards a relationship between VEP amplitude of fellow eyes and brain volumes was noted. Conclusions:In this cohort we were not able to show atrophic features in the retina or the brain, and the association between structural measures of the retina and the brain as indicated in the later stages of MS could not be reproduced. These findings suggest that atrophy does require time to evolve and indicate the complexity of the relationship between local and general structural measures.

Empirical extraction of mechanisms underlying real world network generation

The generation mechanisms of real world networks have been described using multiple models. The mathematical features of these models are usually extrapolated from statistical properties of a snapshot of these networks. We here propose an alternative method based on direct measurement of a sequence of consecutive snapshots to uncover the dynamics underlying real world generation. We assume that the probability of adding a node or an edge depends only on local features surrounding the newly added node/edge, and directly measure the contribution of these features to the node/edge addition probability. These measurements are performed using newly defined N-node local structures. Each N-node local structure represents the configuration of edges surrounding a newly added edge. The N-node local structure measurements reproduce for some networks the now classical addition of edges between high degree node mechanisms. It also provides quantitative estimates of more complex mechanisms driving other networks’ evolution, such as the effect of common first and second neighbors. This new methodology reveals the relative importance of different generation mechanisms. We show, for example, that the main mechanism driving hyperlink addition between two websites is the existence of a third website linking to both the source and the target of the new hyperlink.

Detailed relationship between local structure, polarons, and magnetization forLa1−xCaxMnO3(0.21≤x≤0.45)

We present detailed local structure measurements (using the extended x-ray absorption fine structure technique) for the colossal magnetoresistive material ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ $(0.21<x<0.45)$ as a function of temperature and magnetic field. The local distortions of the Mn-O bonds are parameterized using $\ensuremath{\sigma}$, the width of the Mn-O pair-distribution function (PDF). After subtracting thermal phonon contributions, we show that the contributions to ${\ensuremath{\sigma}}^{2}$ from polaron and Jahn-Teller (JT) distortions, ${\ensuremath{\sigma}}_{\text{JT}/polaron}^{2}$, are a universal function of the magnetization, independent of how the magnetization is achieved via changes in temperature or magnetic field. However this universal behavior is only observed for $B\text{ }\text{fields}\ensuremath{\ge}2\text{ }\text{T}$, likely as a result of domain canting in low $B$ fields. The resulting curve is well described by two straight lines with significantly different slopes. These regimes represent two distinctly differ distortions of the oxygen octahedra about the Mn. For low magnetizations up to $\ensuremath{\sim}65\mathrm{%}$ of the theoretical maximum magnetization, ${M}_{T}$, the slope is low and the distortion removed as the sample becomes magnetized is small---we argue this arises from polarons which have a low distortion around two (or possibly three) Mn sites. At high magnetizations large distortions per Mn site are removed as these sites become magnetized. The data are also analyzed in terms of a two Mn-O peak distribution using experimental standards for Mn-O. The results agree well with recent neutron PDF results but not with some earlier results. We discuss the limitations of assuming a two peak distribution in view of the two distortions needed to describe the Mn-O distortions as a function of $T$ and $B$ for $B\ensuremath{\ge}2\text{ }\text{T}$. It is likely that there is a distribution of longer bonds. Finally we show that with increasing $B$ field, the Mn-Mn peak also has a small $B$-field-induced change---a measure at the unit cell level of magnetostriction but find that there is no observable $B$-field-induced change in the Mn-La/Ca pair distribution for fields up to $\ensuremath{\sim}10\text{ }\text{T}$.

Local spin-coupled distortions in multiferroic hexagonalHoMnO3

Local structural measurements have been performed on hexagonal HoMnO3 in order to ascertain the specific changes in bond distances which accompany magnetic ordering transitions. The transition from paramagnetic to the antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn (nearest and next nearest) and Ho-Ho correlations. These changes in bond distances reveal strong spin-lattice coupling. The similarity in magnitude of the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The structural changes are interpreted in terms of a model of competing spin order and local structural distortions. Density functional calculations are used to estimate the energies associated with ionic displacements. The calculations also reveal asymmetric polarization of the charge density of Ho, O3 and O4 sites along the z-axis in the ferroelectric phase. This polarization facilitates coupling between Ho atoms on neighboring planes normal to the z-axis.

Measurements of local flow structures of conical spouted beds by optical fibre probes

AbstractA new calibration set‐up was designed and assembled, and a comprehensive sensitivity analysis was conducted to investigate factors that may affect the calibration of the effective distance of an optical fibre particle velocity probe. It is shown that the glass window has the most significant impact on the calibration result.The optical fibre probe was then applied to measure local particle velocities and solid fractions inside a half and a full circular conical spouted bed. It is found that the overall particle velocity profiles, as well as the shapes of the spout and fountain are quite similar in both columns.

Local structure of multiferroic RMn 2O 5: Important role of the R site

The temperature and magnetic field dependent local structure of RMn 2O 5 systems was examined. While no significant displacements of the Mn ions are observed, it is found that the R–O distribution exhibits changes at low temperature which are possibly related to the changes in the electric polarization. Density functional computations are used to explore the system dynamics and to link the local structural measurements with anomalous changes in the infrared absorption spectra. The anomalous R–O distribution and observed coupling to magnetic fields point to the need to properly treat the 4f electrons on the R sites in these systems.

Donor defects enhanced ferromagnetism in Co:ZnO films

We experimentally report the role of donor defects on the origin of room temperature ferromagnetism in Zn 0.985Co 0.015O films. Undoped ZnO, Co:ZnO and (Co, N):ZnO thin films are fabricated by radio-frequency magnetron sputtering. The local structural measurements indicate that Co 2+ substitutes for Zn 2+ in the ZnO wurtzite lattice with the upper limit of ~ 10% Co metal of the total cobalt dopants. The Raman spectra reveal vibrational modes at 273, 470, 639, 691 and 854 cm − 1 in addition to the host phonons of ZnO, indicating the substitutional behavior of N 3− for O 2− and the increase in donor defects. Acceptor doping with nitrogen compensates the electron carriers and hence decreases the electron carrier concentration. Combined with the enhancement of ferromagnetism, it is found that the donor defects other than electron carriers are responsible for the room temperature ferromagnetism in Co:ZnO films.

Assessing the importance of seed immigration on coexistence of plant functional types in a species-rich ecosystem

Modelling and empirical studies have shown that input from the regional seed pool is essential to maintain local species diversity. However, most of these studies have concentrated on simplified, if not neutral, model systems, and focus on a limited subset of species or on aggregated measures of diversity only (e.g., species richness or Shannon diversity). Thus they ignore more complex species interactions and important differences between species. To gain a better understanding of how seed immigration affects community structure at the local scale in real communities we conducted computer simulation experiments based on plant functional types (PFTs) for a species-rich, fire-prone Mediterranean-type shrubland in Western Australia. We developed a spatially explicit simulation model to explore the community dynamics of 38 PFTs, defined by seven traits – regeneration mode, seed production, seed size, maximum crown diameter, drought tolerance, dispersal mode and seed bank type – representing 78 woody species. Model parameterisation is based on published and unpublished data on the population dynamics of shrub species collected over 18 years. Simulation experiments are based on two contrasting seed immigration scenarios: (1) the ‘equal seed input number’ scenario, where the number of immigrant seeds is the same for all PFTs, and (2) the ‘equal seed input mass’ scenario, where the cumulative mass of migrating seeds is the same for all PFTs. Both scenarios were systematically tested and compared for different overall seed input values. Without immigration the local community drifts towards a state with only 13 coexisting PFTs. With increasing immigration rates in terms of overall mass of seeds the simulated number of coexisting PFTs and Shannon diversity quickly approaches values observed in the field. The equal seed mass scenario resulted in a more diverse community than did the seed number scenario. The model successfully approximates the frequency distributions (relative densities) of all individual plant traits except seed size for scenarios associated with equal seed input mass and high immigration rate. However, no scenario satisfactorily approximated the frequency distribution for all traits in combination. Our results show that regional seed input can explain the more aggregated measures of local community structure, and some, but not all, aspects of community composition. This points to the possible importance of other (untested) processes and traits (e.g., dispersal vectors) operating at the local scale. Our modelling framework can readily allow new factors to be systematically investigated, which is a major advantage compared to previous simulation studies, as it allows us to find structurally realistic models, which can address questions pertinent to ecological theory and to conservation management.

Local protection against mountain hazards – state of the art and future needs

Abstract. During the last decades, settlement activities increased in European mountain regions. Due to the scarceness of areas suitable for development, residential estates were extended into areas endangered by natural hazards such as mass movements. These settlements generally show a considerable vulnerability to tangible assets. Integral risk management strategies to reduce the vulnerability to tangible assets are presented for the assessment of such endangered areas. Conventional mitigation and local structural measures are discussed with respect to the necessary delimitation of endangered areas, the preparedness of people and possible financial prevention. According to different natural hazard processes (flash floods with and without bedload transport, debris flows, land slides, rock falls and avalanches) and various structural elements of buildings, a catalogue of local structural measures is presented with respect to occurring process impacts and protection objectives. Thereby, different local structural measures are classified and recommended according to a possible implementation for newly-erected buildings and for upgrading existing buildings, respectively. Based on these recommendations, future needs for a sustainable and comprehensive reduction of risk in settlement areas endangered by mass movements are outlined. Above all, this includes a prescription of building codes and the re-introduction of an obligatory final inspection of buildings.

Biological network comparison using graphlet degree distribution

Analogous to biological sequence comparison, comparing cellular networks is an important problem that could provide insight into biological understanding and therapeutics. For technical reasons, comparing large networks is computationally infeasible, and thus heuristics, such as the degree distribution, clustering coefficient, diameter, and relative graphlet frequency distribution have been sought. It is easy to demonstrate that two networks are different by simply showing a short list of properties in which they differ. It is much harder to show that two networks are similar, as it requires demonstrating their similarity in all of their exponentially many properties. Clearly, it is computationally prohibitive to analyze all network properties, but the larger the number of constraints we impose in determining network similarity, the more likely it is that the networks will truly be similar. We introduce a new systematic measure of a network's local structure that imposes a large number of similarity constraints on networks being compared. In particular, we generalize the degree distribution, which measures the number of nodes 'touching' k edges, into distributions measuring the number of nodes 'touching' k graphlets, where graphlets are small connected non-isomorphic subgraphs of a large network. Our new measure of network local structure consists of 73 graphlet degree distributions of graphlets with 2-5 nodes, but it is easily extendible to a greater number of constraints (i.e. graphlets), if necessary, and the extensions are limited only by the available CPU. Furthermore, we show a way to combine the 73 graphlet degree distributions into a network 'agreement' measure which is a number between 0 and 1, where 1 means that networks have identical distributions and 0 means that they are far apart. Based on this new network agreement measure, we show that almost all of the 14 eukaryotic PPI networks, including human, resulting from various high-throughput experimental techniques, as well as from curated databases, are better modeled by geometric random graphs than by Erdös-Rény, random scale-free, or Barabási-Albert scale-free networks. Software executables are available upon request.

Influence of strain on the atomic and electronic structure of manganite films

A study of the long-range, local and electronic structure of Nd 0.5Sr 0.5MnO 3 films of varying thickness between 500 and 2000 Å has been performed. Local structure measurements at the Sr K-edge reveal a reduction of the Mn–O–Mn bond angles in films below 1000 Å. Spin-polarized measurements reveal splitting of the Mn 3d e g state in the strained region of the films and are consistent with a two-layer model for thick films with a relaxed undistorted layer on top of a strained structurally distorted layer near the substrate.