Structural Correlation Length Research Articles

Fluctuation Electron Microscopy on Amorphous Silicon and Amorphous Germanium.

Variable resolution fluctuation electron microscopy experiments were performed on self-ion implanted amorphous silicon and amorphous germanium to analyze the medium-range order. The results highlight that the commonly used pair-persistence analysis is influenced by the experimental conditions. Precisely, the structural correlation length Λ, a metric for the medium-range order length scale in the material, obtained from this particular evaluation varies depending on whether energy filtering is used to acquire the data. In addition, Λ depends on the sample thickness. Both observations can be explained by the fact that the pair-persistence analysis utilizes the experimentally susceptible absolute value of the normalized variance obtained from fluctuation electron microscopy data. Instead, plotting the normalized variance peak magnitude over the electron beam size offers more robust results. This evaluation yields medium-range order with an extent of approximately (1.50 ± 0.50) nm for the analyzed amorphous germanium and around (1.10 ± 0.20) nm for amorphous silicon.

Evidence for growing structural correlation length in colloidal supercooled liquids.

Using video microscopy, we measure the long-time diffusion coefficients of colloidal particles at different concentrations. The measured diffusion coefficients start to deviate from theoretical predictions based on random collision models upon entering the supercooled regime. The theoretical diffusion relation is recovered by assigning an effective mass proportional to the size of structurally correlated clusters to the diffusing particles, providing an indirect method to probe the growth of static correlation length scales approaching the glass transition. This method is tested and validated in the crystallization of mono-disperse colloids in quasi-two-dimensional experiments. The correlation length obtained for a binary colloidal liquid increases by a power law toward a critical packing fraction of ∼0.79. The system relaxation time exhibits a power-law dependence on the correlation length in agreement with dynamical facilitation theories.

Connecting Packing Efficiency of Binary Hard Sphere Systems to Their Intermediate Range Structure.

Using computed x-ray tomography we determine the three dimensional (3D) structure of binary hard sphere mixtures as a function of composition and size ratio of the particles q. Using a recently introduced four-point correlation function we reveal that this 3D structure has on intermediate and large length scales a surprisingly regular order, the symmetry of which depends on q. The related structural correlation length has a minimum at the composition at which the packing fraction is highest. At this composition also the number of different local particle arrangements has a maximum, indicating that efficient packing of particles is associated with a structure that is locally maximally disordered.

Structural correlation of GeTeSeGa system by XRD and far-infrared spectroscopy

The structural properties of quaternary Ge10Te80Se10-xGax (x = 0 to 10) glassy alloy have been studied with XRD (X-ray diffraction) and FTIR (Fourier transform infrared) spectroscopy. The position of FSDP (first sharp diffraction peak) (2θ) and its FWHM (full width half maxima) have been utilized to estimate the local structure parameters of FSDP like repeating distance y and structural correlation length (L). The far-infrared (IR) transmission spectra of Ge10Te80Se10-xGax (x = 0, 2, 4, 6, 8, 10) have been analysed in the range 30–300 cm−1 to study the formation of bonds using chain crossing model, random covalent network model and chemical bond approach. The theoretical calculations have been executed for bond energies, force constants, wave number, etc., for probable bonds, and the results justify the experimental values. The present study contributes to the understanding of the composition-dependent structural property relationship of chalcogenide glasses.

The role of minor alloying in the plasticity of bulk metallic glasses

Micro- or minor alloying of metallic glasses is of technological interest. An originally ductile Pd-based monolithic bulk metallic glass (Pd40Ni40P20) was selectively manipulated by additions of Fe or Co. The alloying effects were extreme, showing either exceptional ductility upon Co addition or immediate catastrophic failure upon Fe addition when tested under uniaxial compression or 3-point bending. The amorphous structure was characterized prior to deformation with respect to its medium-range order (MRO) using variable resolution fluctuation electron microscopy (VR-FEM). We observe striking differences in the MRO between the ductile and brittle metallic glasses, with the ductile glasses exhibiting a rich structural diversity and MRO correlation lengths up to 6 nm. The MRO diversity seems to enable easier shear banding and hence enhance the deformability.

The local and average structure of Ba(Ti, Ce)O3 perovskite solid solution: effect of cerium concentration and particle size.

The amazing properties of ferroelectric perovskite BaTiO3 (BT) and its solid solutions make them indispensable for many technological applications (e.g. multilayer capacitors). Unfortunately, the so-called `size effect' limits their use. Indeed, under a certain critical particle size, these materials show a suppression of the spontaneous polarization and thus of the ferroelectric properties. In pure nanometric BaTiO3, this is related to a certain local structural disorder. However, only a few studies have explored BT solid solutions, where the doping effect, coupled to the reduced particle size, can play an important role. Therefore, in this work, the structure of BaCexTi1-xO3 (x = 0.02-0.20) was explored by traditional Rietveld method and Pair Distribution Function. Samples present a particle size from 80-160 nm to 400-1000 nm depending on increasing x. The carbox approach was applied, investigating the evolution of the local structure, its modifications and the structural coherent correlation length, as a function of cerium amount. Results demonstrate a cooperative effect of composition and reduced size in the ferroelectricity loss. The two, in fact, contribute to intensify the local structural disorder, decreasing the structural coherent correlation length. The local structural disorder is thus confirmed to be a relevant factor in the ferroelectric properties degradation.

Research progress and application prospect of Fe-based soft magnetic amorphous/nanocrystalline alloys

Amorphous alloy is a kind of metallic materials prepared by rapidly cooling the alloy melt through hindering crystallization in cooling process. Due to the unique structure of atomic random packing, Fe-based amorphous alloys exhibit not only structural and property isotropy, but also small structural correlation length, small magnetic anisotropic constant, and then small coercivity Hc. Like crystalline Fe-based alloys, Fe-based amorphous alloys also possess high saturation induction Bs. As a result, research on engineering applications of Fe-based amorphous alloys has been promoted by their excellent soft magnetic properties. Now Fe-based soft magnetic amorphous/nanocrystalline alloys have been produced and applied to various areas on a large scale. Here in this paper, the processes of discovery, development and application of Fe-based soft magnetic amorphous alloys are reviewed, and the effects of chemical composition, structure and preparation technology on the soft magnetic properties are introduced and discussed. The obtained theoretic results and the technological innovation show that the great contributions have been made to the development and application of Fe-based soft magnetic amorphous/crystalline alloys. Based on the progress of structure and soft magnetic property and our understanding, the development process of the fundamental research and the application progress of Fe-based soft magnetic amorphous alloys could be divided into three periods. In addition, the present challenge topics in their researches and applications are proposed.

Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing

A comprehensive understanding of the characteristics of graphene, with regard to its unique electrical and structural attributes, would be incomplete unless the inevitable presence of defects have been considered. While such imperfections may limit the realization of theoretically predicted characteristics, they may also be vital for uncovering new fundamental phenomena and related applications. Here we show that the charged defect generation, through argon ion based plasma processing, in few layer graphene (FLG) could be integral to the substantial enhancement of the electrical capacitance, and be of potential use in electrochemical (EC) energy storage. By combining EC characterization techniques with detailed Raman spectroscopic analysis, we elucidate the contributions of plasma-induced defects to electrostatic and quantum capacitance. We then show that charged defect generation, through argon ion based plasma processing, in FLG could substantially enhance the electrical capacitance, and could be used for electrochemical energy storage. Detailed consideration of the constituent space charge and quantum capacitances were used to delineate a new length scale, correlated to electrically active defects contributing to the capacitance, and which was found to be smaller than a structural correlation length determined through Raman spectroscopy. The study offers insights into an industrially viable method (i.e., plasma processing) for modifying and enhancing the energy density of graphene-based electrochemical capacitors.

Correlation between temperature variations of static and dynamic properties in glass-forming liquids.

Detailed analysis of the static structure factor S(Q) in several glass-forming liquids reveals that the temperature variations of the width of the main diffraction peak ΔQ(T) correlate with the fragility of these liquids. This observation suggests a direct connection between rather subtle structural changes and sharp slowing down of structural relaxation in glass-forming liquids. We show that this observation can be rationalized using the Adam-Gibbs approach, through a connection between temperature variations of structural correlation length, l_{c}∼2π/ΔQ, and the size of cooperatively rearranging regions.

Methane adsorption on specially designed TiC and Mo2C derived carbons with different pore size and surface morphology

The effect of pore size and surface morphology of carbon materials on the adsorption of methane was studied selecting three microstructurally different carbide-derived carbons (CDC), synthesized from titanium carbide (TiC-CDC 950 °C and TiC-CDC 1100 °C HCl) and molybdenum carbide (Mo2C-CDC 1000 °C). Nitrogen sorption and Raman spectroscopy methods were used to obtain the specific surface area, ratio of micro- and mesopores, the pore size distribution and disorder in structure, respectively. Studied CDCs had high surface area (>800 m2 g−1), but the pore size distribution was remarkably different. TiC-CDC 950 °C contains mainly micropores (from 0.5 to 1 nm), TiC-CDC 1100 °C HCl both micro- and mesopores (from 1.5 to 5 nm) and Mo2C-CDC 1000 °C mainly mesopores (from 2.5 to 10 nm). Structural correlation lengths calculated from Raman spectra showed that CDC with the smallest pores (TiC-CDC 950 °C) was the most disordered of carbon materials studied. Excess isotherms (EI) of methane adsorption were measured at different temperatures (from −100 to 40 °C) and pressures (from 0.03 to 1.35 MPa) and modelled with modified Langmuir equation to obtain the absolute adsorption isotherms, enthalpies and entropies of methane adsorption. It was concluded that the change in entropy is the key factor determining the amount of gas adsorbed per unit of surface area of CDC and up to 55% more methane can be adsorbed if the structure of carbon material is optimized.

Finite-temperature superconducting correlations of the Hubbard model

We utilize numerical linked-cluster expansions (NLCEs) and the determinantal quantum Monte Carlo algorithm to study pairing correlations in the square lattice Hubbard model. To benchmark the NLCE, we first locate the finite-temperature phase transition of the attractive model to a superconducting state away from half filling. We then explore the superconducting properties of the repulsive model for the d-wave and extended s-wave pairing symmetries. The pairing structure factor shows a strong tendency to d-wave pairing and peaks at an interaction strength comparable to the bandwidth. The extended s-wave structure factor and correlation length are larger at higher temperatures but clearly saturate as temperature is lowered, whereas the d-wave counterparts, which start off lower at high temperatures, continue to rise near half filling. This rise is even more dramatic in the d-wave susceptibility. The convergence of NLCEs breaks down as the susceptibilities and correlation lengths become large, so we are unable to determine the onset of long-range order. However, our results extend the conclusion, previously restricted to only magnetic and charge correlations, that NLCEs offer unique window into pairing in the Hubbard model at strong coupling.

Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing.

It is shown that charged defect generation, through argon ion-based plasma processing, in few layer graphene, could substantially enhance the electrical capacitance for electrochemical energy storage. Detailed consideration of the constituent space charge and quantum capacitances were used to delineate a new length scale, correlated to electrically active defects contributing to the capacitance, and was found to be smaller than a structural correlation length determined through Raman spectroscopy. The study offers insights into an industrially viable method (i.e., plasma processing) for modifying and enhancing the energy density of graphene-based electrochemical capacitors.

Small-world networks of optical fiber lattices

We use a simple dynamical model and explore coherent dynamics of wavepackets in complex networks of optical fibers. We start from a symmetric lattice and through the application of a Monte–Carlo criterion we introduce structural disorder and deform the lattice into a small-world network regime. We investigate in the latter both structural (correlation length) as well as dynamical (diffusion exponent) properties and find that both exhibit a rapid crossover from the ordered to the fully random regime. For a critical value of the structural disorder parameter transport changes from ballistic to sub-diffusive due to the creation strongly connected local clusters and channels of preferential transport in the small world regime.

Covalent bond connectivity, medium range order and physical properties of GexIn8Se92−x glasses

The GexIn8Se92−x glasses (14 ≤ x ≤ 23.5) were extensively characterized by X-ray diffraction (XRD). The chemical composition is determined using energy dispersive spectrometers (EDS). The atomic structure of these glasses is investigated by using (XRD) technique. The first sharp diffraction peak (FSDP) in the structural factor indicates the existence of medium range order (MRO) in these glasses. The position of the FSDP and its full width at half maximum (FWHM), obtained after fitting the XRD of the glasses. The repetitive characteristic distance, R and structural correlation length, L, which characterizing the intermediate structure are deduced. The effect of composition and average coordination number <CN>, on the atomic volume, the density and compactness are studied. The connectivity of the system is discussed in terms of the total number of constraints which also influence the mean bond energy of the system. The intensity of the FSDP, the atomic volume and compactness of the structure, have anomalous change at <CN> = 2.67 indicating, a topological threshold occurs. The occurrence of this dependence leads to the Ge content has a marked effect on the MRO and physical properties parameters.

Identification of long-lived clusters and their link to slow dynamics in a model glass former

We study the relationship between local structural ordering and dynamical heterogeneities in a model glass-forming liquid, the Wahnström mixture. A novel cluster-based approach is used to detect local energy minimum polyhedral clusters and local crystalline environments. A structure-specific time correlation function is then devised to determine their temporal stability. For our system, the lifetime correlation function for icosahedral clusters decays far slower than for those of similarly sized but topologically distinct clusters. Upon cooling, the icosahedra form domains of increasing size and their lifetime increases with the size of the domains. Furthermore, these long-lived domains lower the mobility of neighboring particles. These structured domains show correlations with the slow regions of the dynamical heterogeneities that form on cooling towards the glass transition. Although icosahedral clusters with a particular composition and arrangement of large and small particles are structural elements of the crystal, we find that most icosahedral clusters lack such order in composition and arrangement and thus local crystalline ordering makes only a limited contribution to this process. Finally, we characterize the spatial correlation of the domains of icosahedra by two structural correlation lengths and compare them with the four-point dynamic correlation length. All the length scales increase upon cooling, but in different ways.

Examination of a Polycrystalline Thin-Film Model to Explore the Relation between Probe Size and Structural Correlation Length in Fluctuation Electron Microscopy

We examine simulated electron microdiffraction patterns from models of thin polycrystalline silicon. The models are made by a Voronoi tessellation of random points in a box. The Voronoi domains are randomly selected to contain either a randomly-oriented cubic crystalline grain or a region of continuous random network material. The microdiffraction simulations from coherent probes of different widths are computed at the ideal kinematical limit, ignoring inelastic and multiple scattering. By examining the normalized intensity variance that is obtained in fluctuation electron microscopy experiments, we confirm that intensity fluctuations increase monotonically with the percentage of crystalline grains in the material. However, anomalously high variance is observed for models that have 100% crystalline grains with no imperfections. We confirm that the reduced normalized variance, V(k,R) - 1, that is associated with four-body correlations at scattering vector k, varies inversely with specimen thickness. Further, for probe sizes R larger than the mean grain size, we confirm that the reduced normalized variance obeys the predicted form given by Gibson et al. [Ultramicroscopy, 83, 169-178 (2000)] for the kinematical coherent scattering limit.

Insights from Chiral Polyfluorene on the Unification of Molecular Exciton and Cholesteric Liquid Crystal Theories for Chiroptical Phenomena

The Maugin-Oseen-DeVries theory accounts for chiroptical properties of cholesteric films with long-range order. For molecular systems with short-range structural correlation lengths, molecular exciton theory is used. A consistent description for systems with intermediate correlation lengths is lacking. Films of chiral polyfluorene behave according to Maugin-Oseen-DeVries theory when the film thickness exceeds 300 nm. Properties of thin films are consistent with molecular exciton theory. We describe the crossover in the optical properties of the film in a phenomenological way using a dielectric tensor that contains terms from Maugin-Oseen-DeVries and molecular exciton theory. Guided by the experimental findings, we explore the possibility of a unification of the Maugin-Oseen-DeVries and molecular exciton theory involving a coupled oscillator model.

X-ray photon correlation spectroscopy in systems without long-range order: existence of an intermediate-field regime

Successful X-ray photon correlation spectroscopy studies often require that signals be optimized while minimizing power density in the sample to decrease radiation damage and, at free-electron laser sources, thermal impact. This suggests exploration of scattering outside the Fraunhofer far-field diffraction limit d(2)/λ << R, where d is the incident beam size, λ is the photon wavelength and R is the sample-to-detector distance. Here it is shown that, in an intermediate regime d(2)/λ > R >> dξ/λ, where ξ is the structural correlation length in the material, the ensemble averages of the scattered intensity and of the structure factor are equal. Similarly, in the regime d(2)/λ > R >> dξ(τ)/λ, where ξ(τ) is a time-dependent dynamics length scale of interest, the ensemble-averaged correlation functions g(1)(τ) and g(2)(τ) of the scattered electric field are also equal to their values in the far-field limit. This broadens the parameter space for X-ray photon correlation spectroscopy experiments, but detectors with smaller pixel size and variable focusing are required to more fully exploit the potential for such studies.

Two-Dimensional Superconducting Properties of the Organic Superconductor κL-(DMEDO-TSeF)2[Au(CN)4](THF) with Domain Structures

The anisotropy of superconducting coherence lengths of the layered organic superconductor κ L -(DMEDO-TSeF) 2 [Au(CN) 4 ](THF) with domain structures has been investigated, where DMEDO-TSeF is dimethyl(ethylenedioxy)tetraselenafulvalene and THF is tetrahydrofuran. The field angle dependence of the upper critical field and the short interlayer coherence length indicate that the present compound is a two-dimensional superconductor. Although this compound has domain walls parallel to the conducting layer depending on the orientation of the THF molecules, the interlayer coherence length shorter than the structural correlation length shows that the domain walls parallel to the conducting layers do not affect the superconductivity.

Interpretation of the two-components observed in high resolution X-ray diffraction ω scan peaks for mosaic ZnO thin films grown on c-sapphire substrates using pulsed laser deposition

X-ray scattering methods were applied to the study of thin mosaic ZnO layers deposited on c-Al 2O 3 substrates using Pulsed Laser Deposition. High Resolution (HR) studies revealed two components in the ω scans (transverse scans) which were not resolved in conventional “open-detector” ω rocking curves: a narrow, resolution-limited, peak, characteristic of long-range correlation, and a broad peak, attributed to defect-related diffuse-scattering inducing a limited transverse structural correlation length. Thus, for such mosaic films, the conventional ω rocking curve Full Width at Half Maximum linewidth was found to be ill-adapted as an overall figure-of-merit for the structural quality, in that the different contributions were not meaningfully represented. A “Williamson-Hall like” integral breadth (IB) metric for the HR (00.l) transverse-scans was thus developed as a reliable, fast, accurate and robust alternative to the rocking curve linewidth for routine non-destructive testing of such mosaic thin films. For a typical ZnO/c-Al 2O 3 film, the IB method gave a limited structural correlation length of 110 nm ± 9 nm. The results are coherent with a thin film containing misfit dislocations at the film-substrate interface.