Dependent Dispersion Research Articles

Ab initio study of the structural response to magnetic disorder and van der Waals interactions in FeSe

The electronic structure in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the $k_z$ dependence of the electronic dispersion, the gap structure and the pairing mechanisms of iron-based superconductivity. Here, we present a detailed investigation of the van der Waals interaction in FeSe and its interplay with magnetic disorder and real space structural properties. Using density functional theory we show that they need to be taken into account upon investigation of the 3-dimensional effects, including non-trivial topology, of FeSe$_{1-x}$Te$_x$ and FeSe$_{1-x}$S$_x$ systems. In addition, the impact of paramagnetic (PM) disorder is considered within the spin-space average approach. Our calculations show that the PM relaxed structure supports the picture of different competing ordered magnetic states in the nematic regime, yielding magnetic frustration.

Image-based multiscale modeling with spatially varying microstructures from experiments: Demonstration with additively manufactured metal in fatigue and fracture

This manuscript presents a novel modeling framework to predict mechanical performance in material with spatially varying microstructure. The framework combines an efficient process model for AM, a database of experimental 3D images of defects in AM metal, and a microstructure-based multiscale modeling method that leverages recent advances in reduced order modeling. Thus the examples presented will explore heterogeneous and processing dependent dispersion of voids in additively manufactured (AM) metals. The method presented here allows for parametric studies with repeated instantiations of different possible configurations of microstructures (images of defects) throughout the simulated part. Two demonstrations of the method are provided using a database of synchrotron x-ray computed tomography images of porosity collected at the Advanced Photon Source for Inconel 718 built with Laser Engineered Net Shaping®: one case is high cycle fatigue crack incubation and the other is fracture initiation. In both, we show that the model can capture the effects on performance of variability within and between builds. Although not all variability is captured or quantified, the method shows promise for application in AM metals because of its unique ability to mechanistically connect part-scale performance with individual microstructures and the distribution of these microstructures throughout the part.

Surface-dependent activity of model CoMoS hydrotreating catalysts

The current environmental and industrial contexts require constant revision and improvement of hydrotreating processes for obtaining cleaner (bio)fuels through the development of supported transition metal sulfide (TMS) catalysts. The present work proposes a surface-science approach to describe the surface-dependent genesis of the cobalt-promoted active phase (CoMoS) supported on α-Al2O3 single crystals used as surrogates of the industrial γ-Al2O3 support. Co-Mo precursor solutions are deposited by aqueous-phase impregnation onto four α-Al2O3 single crystals of C(0001), A(112¯0), M(101¯0), and R(11¯02) orientations, which display specific OH sorption sites. The model catalysts are characterized in the oxide and sulfide states by XPS, AFM, and TEM revealing a surface dependent sulfidation and dispersion as a result of metal-support interactions of diverse nature. The surfaces can be ranked following metal-support interactions of increasing strength C(0001) < A(112¯0), M(101¯0) < R(11¯02) in good agreement with model non-promoted catalysts. However, AFM and TEM imaging also reveal that cobalt incorporation leads to an increase in the stacking of CoMoS particles with respect to non-promoted catalysts. The activity of the model catalysts in thiophene hydrodesulfurization cannot be simply related to the common features of the active phase (sulfidation, dispersion, promotion extent) but a volcano-shape curve is obtained when plotting the activity as a function of metal-support interactions. These results suggest that high activity is obtained on surfaces with intermediate metal-support interactions that lead to an optimal metal-sulfur bond energy in agreement with the Sabatier principle. Transposing these results to the traditional γ-Al2O3 support suggests an enhanced activity of the (100) facet, which can lead to the development of improved supports for hydrotreating catalysts.

Ablative laser processing of titanium surface in n-hexane

The work is devoted to the problem of high-spherical powders synthesis by the method of laser ablation processing of refractory metals. An investigation of interaction of ultrashort laser pulses with titanium in an n-hexane medium has been carried out. The dependence of granules size and dispersion of synthesized micro- and nanoparticles on the parameters and processing conditions, namely, laser radiation energy, scanning speed, working medium and pressure is shown.

Membrane Deformation Modulated by Hydration and Cholesterol Unveiled by Solid-State 2H NMR Spectroscopy

Phospholipid membranes are biological liquid crystals in which intermolecular interactions at mesoscopic length scales play key roles in the emergence of membrane physical properties like structure and rigidity. Such interactions modulated by membrane hydration [1] and cholesterol [2] have been investigated using solid-state 2H NMR spectroscopy. The average structure is manifested by the lineshapes through principal values of static or residual coupling tensors due to quadrupolar interactions. The residual quadrupolar couplings (RQCs) yield segmental order parameters for the individual acyl segments (i), providing average membrane properties such as bilayer thickness, area/lipid, and spontaneous curvature [3]. We show that membrane dehydration and increased cholesterol content result in greater RQCs, indicative of membrane thickening. Elastic membrane deformations are exhibited as collective order-director fluctuations (ODF) contributing to the nuclear magnetic relaxation. At mesoscopic length scales the thermal fluctuations connect to membrane mechanical properties via the fluctuation-dissipation theorem. Model-free interpretation of the functional dependence of spin-lattice relaxation rates, R1Z, on segmental order parameters (square-law) reveals emergent material properties in the liquid-crystalline state. The R1Z relaxation dispersion and temperature dependence of square-law plots indicate a nematic-like membrane hydrocarbon core. Solid-state 2H NMR relaxation studies demonstrate that both dehydration and increased cholesterol content in the membrane lead to decreased slopes of square-law plots, indicating an increase in membrane bending rigidity, consistent with neutron spin-echo (NSE) measurements. Analogous 2H NMR studies with various membrane lipid compositions and peptide-reconstituted membranes have addressed lipid-mediated cell signaling and lipid-protein interactions. Molecular dynamics simulations provide further insights on the correspondence of the solid-state NMR and NSE results. [1] K.J. Mallikarjunaiah et al. (2019) Phys. Chem. Chem. Phys. 21, 18422. [2] S. Chakraborty et al. (2020) Proc. Natl. Acad. Sci. 117, 21896. [3] Mallikarjunaiah et al. (2011) Biophys. J. 100, 98.

The diffusing-velocity random walk: a spatial-Markov formulation of heterogeneous advection and diffusion

Abstract

Unveiling the strong dependence of the α-relaxation dispersion on mixing thermodynamics in binary glass-forming liquids.

Structural α-relaxation dispersion in binary molecular glass forming mixtures with distinct mixing enthalpy ΔHmix was investigated using enthalpic and dielectric relaxation measurements across the entire composition range. This study focused on the dependence of the relaxation dispersion on the mixing thermodynamics by determining the non-exponential exponent β, and its composition dependence. The β values determined by the enthalpic and dielectric relaxations agree well. Remarkably, it is found that the systems with positive enthalpy of mixing (exothermic, ΔHmix >0) have positive deviations in the composition dependence of β from the linear averaging of the two β values of the pure components, while negative deviations are observed for the systems with negative enthalpy of mixing (endothermic, ΔHmix <0). Furthermore, the relation between the non-exponential behaviors and entropy of mixing is discussed, revealing that the positive or negative deviation of β in its composition dependence on mixing is accompanied by the same sign of the excess entropy of mixing relative to the ideal one.

Constructing Dispersion Characteristic of Adjustment Factor When Testing Compact Square-Section Products Using Impact Echo Method

The results of calculating the adjustment factor for the geometric dispersion of sound velocity $$\beta $$ in compact products of square cross section for various ratios of thickness to the side of cross section are presented. Using simulation in the ANSYS software environment, it has been shown that as the ratio increases, the main vibration mode transforms, causing the emergence of intrinsic resonance in the compact product. The transformation is from longitudinal waves in compact products of the “pile” type to Lamb waves in compact products of the “plate” type. At the same time, a nonlinear change in the coefficient $$\beta $$ occurs in compact products that makes it possible to make a correction for the effect of geometric dispersion. The dispersion dependence of the coefficient $$\beta $$ on the ratio of the thickness to the side of the product section has been calculated in compact square cross-section objects.

Оптические свойства и электронные характеристики празеодима и неодима в конденсированном состоянии

The optical properties of unoxidized surfaces of polycrystalline Pr and Nd films in the spectral range 0.4 - 2.6 μm are studied. Optical constants were measured by Beattie ellipsometric method. Dispersion dependences of the optical conductivity σ, the reflectivity R, imaginary and real parts of the dielectric constant ε1 and ε2, and the function of characteristic electron energy losses Im (ε) -1 were calculated using the obtained optical constants. The paper presents the optical properties of both films and liquid metals studied earlier. The electronic characteristics of praseodymium and neodymium in solid thin-film and liquid states are calculated using the results of measurements in the infrared region of the spectrum using the two-band conductivity model.

A Two-Level Factored Crank-Nicolson Method for Two-Dimensional Nonstationary Advection-Diffusion Equation with Time Dependent Dispersion Coefficients and Source Terms

A Two-Level Factored Crank-Nicolson Method for Two-Dimensional Nonstationary Advection-Diffusion Equation with Time Dependent Dispersion Coefficients and Source Terms

An Analytical Solution of Spatially Dependent One-Dimensional Advection-Dispersion Equation for a Varying Pulse Type Input Source

In present study, solution of advection-dispersion equation is obtained to determine concentration distribution of solute introduced from a varying pulse type point source in one-dimensional heterogeneous semi-infinite porous medium. Heterogeneity of the medium gives rise to space dependent groundwater velocity, dispersion coefficient and retardation factor. Groundwater velocity is some exponent ξ to a linear function of space. The dispersion and retardation factor are also exponents of same linear function with exponents (ξ+1) and (ξ-1), respectively, where ξ takes the value 0 or 1. At one end of the domain, a time dependent varying nature source, which involves step-size increasing function of time, acts along the flow up to a certain time the n eliminated while concentration gradient is considered zero at the other end of the domain. Initially, medium is uniformly polluted. Firstly, the advection-dispersion equation is reduced into constant coefficients by using certain transformations and then Laplace Integral Transformation Technique is utilized to get the solution. The obtained result is illustrated with numerical examples to study the effect of various parameters.

A fourth-order two-level factored implicit scheme for solving two-dimensional unsteady transport equation with time-dependent dispersion coefficients

This paper analyzes a two-level factored implicit scheme in a numerical solution of two-dimensional unsteady advection-diffusion equation with time dependent dispersion coefficients subjects to initial and boundary conditions. The proposed approach is fast and efficient: unconditionally stable, second order accurate in time, spatial fourth order convergent and it requires less computing time. In fact, the two-level factored technique reduces to solve a tridiagonal system of linear equations at each calculation step. This reduces the computational cost of the algorithm. The analysis of the stability of the numerical scheme considers the -norm while the error estimates and convergence rate use L 2-norm. A wide set of numerical evidences are presented and discussed.

SLOW SURFACE ELECTROMAGNETIC WAVES AT THE METASURFACE / DISSIPATIVE DIELECTRIC INTERFACE

The possibilities of the slow surface electromagnetic waves propagation along the flat boundary of a metasurface with a dissipative dielectric are studied. The metasurface is a thin flat slab of metamaterial with simultaneously negative permittivity and permeability with "amplification". All media were assumed to be isotropic. Dispersion dependences are obtained for the eigenmodes of such a waveguide structure. The possibility of full compensation of the energy losses of surface waves by the appropriate choice of the “gains” values is demonstrated.

Revision of the freezing concept in relaxor ferroelectrics: the case of Na0.5Bi0.5TiO3-Sr0.7Bi0.2TiO3 solid solutions

Presence of a frequency-independent maximum and a frequency-dependent shoulder in temperature-frequency dependence of dielectric permittivity are characteristic features of NBT-based solid solutions, while interpretation of these features is insufficient. In the present work, dielectric dispersion is studied in a wide frequency range (20 mHz–30 GHz) in (1-x)NBT-xSr0.7Bi0.2TiO3 solid solutions, in which a continuous transfer from NBT-like to pure relaxor-like behaviour is observed upon increasing of Sr0.7Bi0.2TiO3 concentration. Temperature dependence of dielectric dispersion and static dielectric permittivity in different compositions of this solid solutions group is analysed. The observed behaviour of static dielectric permittivity upon increasing of x is interpreted as a diminishing of transition between the low and the high-temperature states, while the character of the dielectric dispersion remains the same in both sides of the transition. Upon increasing of x, the dielectric dispersion transforms from the behaviour characteristic to relaxor ferroelectrics into the behaviour resembling dipolar glass. Problems, arising from introducing of the concept of freezing, described by the Vogel-Fulcher law, are discussed.

Unsteady solute dispersion in two-fluid flowing through narrow tubes: A temperature-dependent viscosity approach

The drug delivery or transportation of nutrients to our body involves the solute dispersion process through physiological systems and hence affected by the varying nature of viscosity, heat transfer and other factors. The majority of the previous works involving the solute dispersion in fluid flow through microvessels assumed the constant blood viscosity but in treatments involving temperature variations, the blood viscosity will be affected by the change in temperature and hence it is interesting to analyze its effect on the diffusion process. The motivation of the present work is to analyze the simultaneous impact of temperature-dependent viscosity and heat transfer on the solute dispersion in a two-fluid model of blood flow through narrow tubes by adopting the solution technique proposed by Sankarasubramanian and Gill (1973). Blood is considered as Herschel-Bulkley fluid with temperature-dependent viscosity filled up in a central region of the blood vessel and an outer cell free layer of plasma encircled over the central region consists of Newtonian fluid with constant viscosity. The desired flow related information using heat transfer aspect are obtained analytically for varying viscosity model and these expressions for velocity are then used to compute the diffusion coefficients and mean concentration. The entire dispersion process described by the three main diffusion coefficients known as exchange, convective and dispersion coefficients. An important observation is that the whole diffusion process (asymptotic convection, asymptotic dispersion coefficients and mean concentration) isaffected by the temperature-dependent viscosity with temperature parameters and same observation persist for three different types of fluids like Newtonian fluid (NF), Bingham-plastic fluid (BP), Power-law fluid (PL) which are specific cases of our model. A noteworthy observation is that an enhancement in viscosity index (and hence decay in viscosity of the core region fluid) affects the fluid flow velocity and hence affects the diffusion coefficients. Further, the dominance of thermal buoyancy forces expedite the diffusion process. The outcome of the results reveal the dependence of the solute dispersion on temperature-dependent viscosity and heat transfer. The study may be applicable to drug delivery through bloodstreams in the treatments involving the temperature variations such as chemotherapy.

On the Loci of Relaxation Time and Magnetic Dispersion Maxima in the Mean-Field Ising Model

Based on the phenomenological approach, loci of relaxation time and magnetic dispersion maxima near the critical regime in a spin-1/2 mean-field Ising model were performed. The shift in temperature (T) of relaxation time (τ) maximum was detected and its behavior near the second-order transition points are presented at different magnetic field values (h) and different lattice coordination numbers (q). An expression for the dynamic (or complex) susceptibility (χ = χ1 - iχ2) is also derived. The temperature dependence of the magnetic dispersion (χ1) and magnetic absorption (χ2) factors have been studied near the critical regime. It is found that the maximum of χ1 as a function of frequency (ω) and kinetic coefficient (L) obeying an approximately exponential increases and decreases in T- ω and T-L planes near the critical region.

Multiangle Spectrophotometric Methods of Reflection for Determining Refractive Coefficients

The features of the development and application of the procedures for measuring refractive coefficients based on multiangle spectrophotometric methods of reflection are considered. The effect of the shape, size, and surface treatment of the samples on their spectral dependences of the reflection is described. The possibility of determining the refraction coefficients by two spectrophotometric methods is demonstrated: using the spectrum of reflection from a single face at a small angle of light incidence close to normal and using the method of reflection of light incident at the Brewster angle. The method of reflection at an incidence angle close to normal can be applied in the case of a nonabsorbing sample characterized by an extinction coefficient not exceeding 10–6 to 10–4. This is a so-called express method, because it allows us to immediately obtain the dispersion dependence of the refraction coefficient. The method allows measuring the dispersion dependences of the refraction coefficients for the samples whose shape excludes multiple reflections: plates with one polished side, plates with a large thickness polished on two sides, and prisms or plates with non-plane-parallel faces. In measurements using the Brewster method, there are no requirements for the value of the extinction coefficient of the sample (absorption) and we can use a sample of any shape, including plates with a small thickness polished on both sides. However, the resulting values of the refraction coefficients are discrete, and a large array of measurement results must be accumulated. The determined measurement accuracy of both methods was Δ = ±0.001 at a confidence probability of P = 0.95. Applicability of the spectrophotometric methods of measurement is demonstrated for the samples of a gadolinium aluminum gallium garnet, which is a crystal of a cubic system and is characterized by a single refraction coefficient. It is shown that the values of the refraction coefficients obtained by these two methods agree well within the measurement accuracy.

Birefringence dispersion of 6CHBT liquid crystal determined in VIS-NIR spectral range

The dispersion dependence of the birefringence of optical materials is important for many practical applications. However, it seems that such measurements are rare for some kinds of liquid crystals. The paper aims to partially fill the gap by investigating the dispersion dependence of the group and phase birefringence of a pure 6CHBT liquid crystal sample at a constant temperature. The inner surfaces of the cell containing the investigated 6CHBT liquid crystal sample were specially treated to have the molecules aligned parallel to the cell’s surfaces with no pretilt. The group and phase birefringence of the liquid crystal was investigated in the wavelength range of (400–800) nm and (900–1800) nm using a house-made planar polariscope. Analysis of the measured spectra yielded dispersion dependencies of group and phase birefringence.

Modeling cross-flow ultrafiltration of permeable particle dispersions.

Cross-flow ultrafiltration is a pressure-driven separation and enrichment process of small colloidal particles where a colloidal feed dispersion is continuously pumped through a membrane pipe permeable to the solvent only. We present a semi-analytic modified boundary layer approximation (mBLA) method for calculating the inhomogeneous concentration-polarization (CP) layer of particles near the membrane and the dispersion flow in a cross-flow filtration setup with a hollow fiber membrane. Conditions are established for which unwarranted axial flow and permeate flow reversal are excluded, and non-monotonic CP profiles are observed. The permeate flux is linked to the particle concentration on the membrane wall using the Darcy-Starling expression invoking axially varying osmotic and trans-membrane pressures. Results are discussed for dispersions of hard spheres serving as a reference system and for solvent-permeable particles mimicking non-ionic microgels. Accurate analytic expressions are employed for the concentration and solvent permeability dependent dispersion viscosity and gradient diffusion coefficient entering into the effective Stokes flow and advection-diffusion equations. We show that the mBLA concentration and flow profiles are in quantitative agreement with results by a finite element method. The mBLA results are compared with predictions by an earlier CP layer similarity solution, showing the higher precision of the former method.

The effect of volume inclusions of the ZnGeP2 single-crystal on the dispersion of the refraction index and the absorption coefficient in mid-IR and terahertz ranges of wavelengths

Abstract Volume filamentary inclusions in ZnGeP2 have been visualized using the digital holography method. The chemical composition of the filamentous volume inclusions Zn3P2 and Ge have been determined using the method of X-ray diffraction analysis. It is shown that the presence of volume inclusions in the ZnGeP2 single-crystal leads to an increase in the reflection coefficient in the region of 12.5 μm. The dispersion dependences of the refractive index and the absorption coefficient of the studied ZnGeP2 samples at wavelengths of 300–1000 μm in the region of non-fundamental absorption have been obtained. It has been shown that the presence of volume inclusions in a single-crystal leads to an increase in the refractive index in the entire THz range under study by the value of Δnmax = 0,0008. The difference between absorption coefficients for the studied samples varies from Δα = 0.1 cm−1 to Δα = 0.15 cm−1 depending on the wavelength.