Isotropic Distribution Research Articles

The Effect of Hydrogenation on the Contest between Aromaticity and Antiaromaticity in Norcorrole.

Magnetic shielding studies demonstrate that successive hydrogenation of NiII norcorrole (NiNc), a stable molecule combining aromatic and antiaromatic features, first weakens and then eliminates the central antiaromatic region, even though the NiNc antiaromatic "core", a 14-membered conjugated cycle with 16 π electrons, is formally preserved throughout the H2 NiNc-H8 NiNc series. The differences between aromatic and non-aromatic isotropic shielding distributions and nucleus-independent chemical shift (NICS) values in these hydrogenated porphyrin analogues are highlighted by comparing the results for the members of the H2 NiNc-H8 NiNc series to those for the aromatic NiII porphyrin complex. The results strongly support the unexpected and counterintuitive conclusion that H8 NiNc will be nonaromatic, without even a trace of antiaromaticity. Based on these findings, H8 NiNc is predicted to be the most stable member of the H2 NiNc-H8 NiNc series.

A numerical model for fibril remodeling in articular cartilage

BackgroundCollagen fibrils of articular cartilage have a distinct organization in mature human knee joints. It seems that a mechanobiological process drives the remodeling of newborn collagen fibrils with maturation. Therefore, the goal of the present study was to develop a collagen fibril remodeling algorithm that describes the unique collagen fibril organization in a 3D knee model. MethodA fibril-reinforced, biphasic cartilage model was used with a cuboid and a 3D human knee joint geometries. An isotropic collagen fibril distribution was assigned to the cartilage at the start of the analysis. Each fibril was rotated towards the direction that resulted in a maximum stretch at each time increment of the loading cycle. ResultsThe resulting pattern for the collagen fibrils was compared with split line patterns of porcine knee joint cartilage and also data published in the literature. Fibrils on the articular surface had a radial pattern towards the geometrical centroid of the tibial and femoral cartilage. In the tibiofemoral contact regions of superficial zone, fibrils were oriented circumferentially and randomly. In the porcine samples, the split-line patterns were similar to those obtained theoretically. Depth-wise organization of fibril network was characterized by fibrils perpendicular to the subchondral bone in the deeper layers, and fibrils parallel to the surface of cartilage in the superficial zone. ConclusionsThe maximum stretch criterion, coupled with a biphasic constitutive model, successfully predicted the collagen fibril organization observed in the articular cartilage throughout the depth and on the articular surface.

Photodissociation dynamics of tetrahydrofuran at 193 nm.

Tetrahydrofuran (THF), a cyclic ether with the chemical formula C4H8O, can be considered the simplest analog of the deoxyribose backbone component of deoxyribonucleic acid (DNA). As such, it provides a useful model for probing the photochemistry of such biomolecular motifs. We present a velocity-map imaging study into the ultraviolet dissociation of THF at a wavelength of 193 nm. Excitation to the S1 state occurs via a 3s ← n transition involving a lone-pair electron on the oxygen atom, and has been shown by other authors to result in rapid ring opening via cleavage of one of the C-O bonds to form a ring-opened C4H8O diradical, followed by C-C bond cleavage over a longer timescale to form either OCH2 + C3H6 products (Channel 1a), HOCH2 + C2H5 products (Channel 1b), or OCH2CH2 + C2H4 products (Channel 2). The C2H4O products formed via Channel 2 are unstable on the timescale of our experiment and dissociate further to form CH3 and CHO. We also observe a number of minor products resulting from H or H2 loss from the primary photofragments. The speed distributions observed for all photofragments are broad, indicating excitation of a range of rotational and vibrational states of the products. The angular distributions of the photofragments show an interesting speed dependence: the slowest products have almost isotropic angular distributions, but the magnitude of the recoil anisotropy increases monotonically with photofragment speed. The fastest products exhibit highly anisotropic angular distributions, with the recoil anisotropy parameter β approaching its limiting value of -1 (-0.75 for Channel 1 and -0.5 for Channel 2). This behaviour is attributed to the range of timescales over which the diradical intermediate dissociates into the observed photofragments. Rapid dissociation leads to fast photofragments which retain the correlation between the transition dipole moment for the S1 ← S0 excitation (which lies perpendicular to the ring) and the photofragment velocities (which lie predominantly in the plane of the ring). Slow dissociation results in a high degree of energy redistribution into internal modes, slower photofragments, and loss of correlation between the photofragment velocities and the transition dipole. The higher barrier associated with dissociation via Channel 2 suggests somewhat longer lifetimes for the diradical intermediate and is consistent with a corresponding reduction in the maximum observed value for β.

An Energy-Based Prior for Generative Saliency.

We propose a novel generative saliency prediction framework that adopts an informative energy-based model as a prior distribution. The energy-based prior model is defined on the latent space of a saliency generator network that generates the saliency map based on a continuous latent variables and an observed image. Both the parameters of saliency generator and the energy-based prior are jointly trained via Markov chain Monte Carlo-based maximum likelihood estimation, in which the sampling from the intractable posterior and prior distributions of the latent variables are performed by Langevin dynamics. With the generative saliency model, we can obtain a pixel-wise uncertainty map from an image, indicating model confidence in the saliency prediction. Different from existing generative models, which define the prior distribution of the latent variables as a simple isotropic Gaussian distribution, our model uses an energy-based informative prior which can be more expressive in capturing the latent space of the data. With the informative energy-based prior, we extend the Gaussian distribution assumption of generative models to achieve a more representative distribution of the latent space, leading to more reliable uncertainty estimation. We apply the proposed frameworks to both RGB and RGB-D salient object detection tasks with both transformer and convolutional neural network backbones. We further propose an adversarial learning algorithm and a variational inference algorithm as alternatives to train the proposed generative framework. Experimental results show that our generative saliency model with an energy-based prior can achieve not only accurate saliency predictions but also reliable uncertainty maps that are consistent with human perception.

Ultraviolet photodissociation of 2-methylallyl radical

Ultraviolet photodissociation dynamics of 2-methylallyl radical from the 3p Rydberg state were investigated in the wavelength region of 226–244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The 2-methylallyl radicals were generated by 193 nm photolysis of 3-chloro-2-methyl-1-propene precursors. The photofragment yield spectrum of H-atom products increases in intensity with decreasing wavelengths in 226–244 nm. The TOF spectra of H-atom products show a bimodal structure. The predominant product channel (with ∼98% branching ratio) has a kinetic energy release peaking at ∼7 kcal/mol, with an average ratio of ET in the total available energy, (fT), of ∼0.18 in 226–244 nm and an isotropic product angular distribution. At the low ET, isotropic component is from statistical unimolecular decomposition of highly vibrationally excited hot 2-methylallyl to the methylenecyclopropane+H products, following internal conversion from the excited electronic state. The minor product channel (with ∼2% branching ratio) has a large kinetic energy peaking at ∼50 kcal/mol, with (fT)≈0.63 and an anisotropic angular distribution (β≈−0.2). At the high ET, anisotropic component is non-statistical and is postulated to be from direct loss of H atom via the 3p Rydberg state or repulsive part of the ground state to the 1,3-butadiene+H products.

Voltage waveform tailoring for high aspect ratio plasma etching of SiO2 using Ar/CF4/O2 mixtures: Consequences of ion and electron distributions on etch profiles

The quality of high aspect ratio (HAR) features etched into dielectrics for microelectronics fabrication using halogen containing low temperature plasmas strongly depends on the energy and angular distribution of the incident ions (IEAD) onto the wafer, as well as potentially that of the electrons (EEAD). Positive ions, accelerated to high energies by the sheath electric field, have narrow angular spreads and can penetrate deeply into HAR features. Electrons typically arrive at the wafer with nearly thermal energy and isotropic angular distributions and so do not directly penetrate deeply into features. These differences can lead to positive charging of the insides of the features that can slow etching rates and produce geometric defects such as twisting. In this work, we computationally investigated the plasma etching of HAR features into SiO2 using tailored voltage waveforms in a geometrically asymmetric capacitively coupled plasma sustained in an Ar/CF4/O2 mixture at 40 mTorr. The tailored waveform consisted of a sinusoidal wave and its higher harmonics with a fundamental frequency of 1 MHz. We found that some degree of control of the IEADs and EEADs is possible by adjusting the phase of higher harmonics φ through the resulting generation of electrical asymmetry and electric field reversal. However, the IEADs and EEADs cannot easily be separately controlled. The control of IEADs and EEADs is inherently linked. The highest quality feature was obtained with a phase angle φ = 0° as this value generated the largest (most negative) DC self-bias and largest electric field reversal for accelerating electrons into the feature. That said, the consequences of voltage waveform tailoring (VWT) on etched features are dominated by the change in the IEADs. Although VWT does produce EEADs with higher energy and narrower angular spread, the effect of these electrons on the feature compared to thermal electrons is not large. This smaller impact of VWT produced EEADs is attributed to thermal electrons being accelerated into the feature by electric fields produced by the positive in-feature charging.

Role of substrate clamping on anisotropy and domain structure in the canted antiferromagnet α−Fe2O3

Antiferromagnets have recently been propelled to the forefront of spintronics by their high potential for revolutionizing memory technologies. For this, understanding the formation and driving mechanisms of the domain structure is paramount. In this work, we investigate the domain structure in a thin-film canted antiferromagnet $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. We find that the internal destressing fields driving the formation of domains do not follow the crystal symmetry of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, but fluctuate due to substrate clamping. This leads to an overall isotropic distribution of the N\'eel order with locally varying effective anisotropy in antiferromagnetic thin films. Furthermore, we show that the weak ferromagnetic nature of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ leads to a qualitatively different dependence on the magnetic field compared to collinear antiferromagnets such as NiO. The insights gained from our work serve as a foundation for further studies of electrical and optical manipulation of the domain structure of antiferromagnetic thin films.

High-performance bulk Bi0.4Sb1.6Te3.0 thermoelectrics prepared from nanocrystal precursor synthesized via chemical precipitation

Bi0.4Sb1.6Te3.0 is a known p-type thermoelectric material with high thermoelectric performance near room temperature. In this study, bulk Bi0.4Sb1.6Te3.0 materials are synthesized via spark plasma sintering using single nanometer-sized nanocrystals as precursors, which are prepared in the solution phase by chemical precipitation from metallic ions. The obtained bulk material consists of submicron-to-micrometer-sized disk-shaped grains with isotropic distribution. Additionally, structural distortion such as strain and/or defects can be directly introduced into grains upon sintering, presumably due to the incomplete coalescence of the small nanocrystal precursors to form larger grains during the sintering process. The resultant bulk material exhibits high electrical conductivity and considerably low lattice thermal conductivity, due to the utilization of impurity-free nanocrystal precursors and local lattice distortions included inside grains acting as scattering nodes of phonons, respectively. The maximum dimensionless figure of merit (ZT) is achieved to be 1.42 ​at 355 ​K, and, therefore, this work provides effective methodology to fabricate high-performance chalcogenide-based thermoelectric materials.

A new particle shifting technique for SPH methods based on Voronoi diagram and volume compensation

Particle shifting technique (PST) in smoothed particle hydrodynamics (SPH) is a useful strategy to render particle distribution isotropic for higher numerical accuracy and stability. However, the non-isotropic particle distribution near the free surfaces and volume non-conservation still often occur, which will degenerate accuracy in the long-time simulation with violent flows. In this study, the two major concerns are solved by proposing a new PST. The Voronoi cell is constructed for the particles with incomplete kernel support, and the difference between the Voronoi cell centroid and the corresponding particle location is used to determine the shifting vector, which ensures a very isotropic particle distribution and that no clustering or nonphysical gap occurs in these areas. For the free-surface particles, the shifting vector is determined according to the volume change and thus the volume conservation is ensured, where the volume is calculated conveniently with the merit of Voronoi diagram. The present method is compared with the other two advanced PST methods, and a set of challenging cases with violent flows is simulated to validate its superior properties in terms of maintaining isotropic particle distribution and volume conservation. Moreover, the efficiency of the present method is almost the same as those of other advanced PSTs. Overall, the new PST method presents a strong potential in the long-time simulation of violent flows in coastal engineering.

Importance of experimental evaluation of structural parameters for constitutive modelling of aorta

The study compares stresses and strains in the aortic wall derived using different constitutive models for various stress-strain conditions. Structure-based constitutive models with two fibre families with (GOH) and without (HGO) dispersion of collagen fibres are compared. The constitutive models were fitted to data from equibiaxial tension tests of two separated layers of the porcine aortic wall. The initial fit was evaluated with unrestricted parameters and subsequently, the mean angles of the fibre families and the angular dispersion were fixed to the values obtained from histology. Surprisingly, none of the tested models was capable to provide a good quality fit with histologically obtained structural parameters. Fitting the HGO model to experimental data resulted in two fibre families under angles close to ±45°, while the GOH model resulted in a nearly isotropic fibre distribution. These results indicate that both of these models suffer from the absence of isotropic strain stiffening. After having modified both models with corresponding additional members based on the Yeoh model of matrix, we obtained a perfect fit to the measured data while keeping the structural histology-based parameters. Finally, significant differences in compliance and resulting stresses and strains between different models are shown by means of simulations of uniaxial tension test, equibiaxial tension tests and inflation of the aorta.

Charged compact objects by e–MGD approach

In this work, we analyze the incidence of gravitational decoupling through the extended minimal geometric deformation (e–MGD) approach in the framework of f(R, T) gravity theory, applying it on a spherically symmetric and static charged isotropic matter distribution. Specifically, the well–known Krori–Barua toy model is translated to an anisotropic domain by deforming the complete space–time. To do so, the so–called θ–sector has been solved by using the mimic constraint for the radial pressure and a general equation of state relating the components of the θ μ ν source. A thoroughly study on the main salient features of the output such as density, radial pressure, transverse pressure and anisotropy factor is performed to check the feasibility of the model, in order to determine whether this structure can represent real celestial bodies such as neutron stars. Furthermore, the consequences of e–MGD on some relevant astrophysical parameters, that is, the total mass M, gravitational redshift z and time dilation d τ around the object are explored. It is found that the maximum mass provided by this toy model is M = 2.506M ⊙, corresponding to the massive neutron stars.

Constraining anisotropic cosmological model in f(R,ℒm) Gravity

The observational evidence regarding the present cosmological aspects tells us about the presence of very little anisotropy in the universe on a large scale. Here, in this paper, we attempt to study locally rotationally symmetric (LRS) homogeneous Bianchi-I spacetime with the isotropic matter distribution. This is done within the framework of $f(\mathcal{R},\mathscr{L}_m)$ gravity. Particularly, we consider a non-linear $f(\mathcal{R},\mathscr{L}_m)$ model, $f(\mathcal{R},\mathscr{L}_m)=\dfrac{1}{2}\mathcal{R}+\mathscr{L}_m^{\,{\alpha}}$. Furthermore, $\omega$, the equation of state parameter, which is vital stuff in determining the present phase of the universe is constrained. To constrain the model parameters and the equation of state parameter, we use 57 Hubble data points and 1048 Pantheon supernovae type Ia data sample. And, for our statistical analysis, we use Markoc Chain Monte Carlo (MCMC) simulation. Moreover, with the help of obtained values of parameters, we measure the anisotropy parameter for our model.

Angular and energy dispersion in neutron-rich nuclei by neutron evaporation: A Monte Carlo simulation


 In this paper we present the results of a Monte Carlo simulation of the effects of neutronevaporation on the angular and energy dispersions in neutron-rich nuclei as a function of the massnumber A, the kinetic energy E and the number of emitted neutrons. We based our simulation on theassumption of an isotropic random distribution for the neutron emission angles in space and on aMaxwell-Boltzmann distribution for the kinetic energy of the evaporated neutrons. Our resultsconfirm large angular and energy dispersions for light, low-energy nuclei and for high neutronchannels.

Deep-learned generators of porosity distributions produced during metal Additive Manufacturing

Laser Powder Bed Fusion has become a widely adopted method for metal Additive Manufacturing (AM) due to its ability to mass produce complex parts with increased local control. However, AM produced parts can be subject to undesirable porosity, negatively influencing the properties of printed components. Thus, controlling porosity is integral for creating effective parts. A precise understanding of the porosity distribution is crucial for accurately simulating potential fatigue and failure zones. Previous research on generating synthetic porous microstructures have succeeded in generating parts with high density, isotropic porosity distributions but are often inapplicable to cases with sparser, boundary-dependent pore distributions. Our work bridges this gap by providing a method that considers these constraints by deconstructing the generation problem into its constitutive parts. A framework is introduced that combines Generative Adversarial Networks with Mallat Scattering Transform-based autocorrelation methods to construct novel realizations of the individual pore geometries and surface roughness, then stochastically reconstruct them to form realizations of a porous printed part. The generated parts are compared to the existing experimental porosity distributions based on statistical and dimensional metrics, such as nearest neighbor distances, pore volumes, pore anisotropies and scattering transform based auto-correlations.

A correlation between accreted stellar kinematics and dark-matter halo spin in the artemis simulations

ABSTRACT We report a correlation between the presence of a Gaia-Sausage-Enceladus (GSE) analogue and dark-matter (DM) halo spin in the artemis simulations of Milky Way-like galaxies. The haloes which contain a large population of accreted stars on highly radial orbits (like the GSE) have lower spin on average than their counterparts with more isotropic stellar velocity distributions. The median modified spin parameters λ′ differ by a factor of ∼1.7 at the present day, with a similar value when the haloes far from virial equilibrium are removed. We also show that accreted stars make up a smaller proportion of the stellar populations in haloes containing a GSE analogue, and are stripped from satellites with stellar masses typically ∼4 times smaller. Our findings suggest that the higher spin of DM haloes without a GSE-like feature is due to mergers with large satellites of stellar mass ∼1010 M⊙, which do not result in prominent radially anisotropic features like the GSE.

Effective magnetic susceptibility in magnetoactive composites

Describing macroscopic samples made of composite material requires the development of an effective macro-continuum model where properties and microscopic arrangement of distinct phases are adequately taken into account. Here, we derive the effective magnetization behavior for compounds of magnetizable inclusions embedded in non-magnetizable medium. The homogenization is based on the dipole approximation and allows for a general formulation of the effective magnetic behavior, spanning from linear to saturation regime, for isotropic and anisotropic arrangements. For isotropic distributions presuming linear magnetization we reproduce an expression known from previous works, which proves to be very accurate in comparison to refined full-field calculations. Additionally, compelling agreement to existing experimental data is found. Consequently, we believe that our approach is of high practical relevance also in case of anisotropic distributions and beyond the linear magnetization. With a given particle arrangement and magnetization model on microscale, the expressions formulated here are obtained with low computational effort compared to homogenization based on micro-continuum approaches. The formulation can be directly implemented in a macroscopic material model for the composite of specified sample shape and solved by applying a macro-continuum approach.

Multi-level adaptive particle refinement method with large refinement scale ratio and new free-surface detection algorithm for complex fluid-structure interaction problems

Fluid-Structure Interaction (FSI) is a crucial problem in ocean engineering. The smoothed particle hydrodynamics (SPH) method has been employed recently for FSI problems in light of its Lagrangian nature and its advantage in handling multi-physics problems. The efficiency of SPH can be greatly improved with the Adaptive Particle Refinement (APR) method, which refines particles in the regions of interest while deploying coarse particles in the left areas. In this study, the APR method is further improved by developing several new algorithms. Firstly, a new particle refinement strategy with the refinement scale ratio of 4 is employed for multi-level resolutions, and this dramatically decreases the computational costs compared to the standard APR method. Secondly, the regularized transition sub-zone is deployed to render an isotropic particle distribution, which makes the solutions between the refinement zone and the non-refinement zone smoother and consequently results in a more accurate prediction. Thirdly, for complex FSI problems with free surface, a new free-surface detection method based on the Voronoi diagram is proposed, and the performance is validated in comparison to the conventional method. The improved APR method is then applied to a set of challenging FSI cases. Numerical simulations demonstrate that the results from the refinement with scale ratio 4 are consistent with other studies and experimental data, and also agree well with those employing the refinement scale ratio 2. A significant reduction in the computational time is observed for all the considered cases. Overall, the improved APR method with a large refinement scale ratio and the new free-surface detection strategy shows great potential in simulating complex FSI problems efficiently and accurately.

Gyrokinetic modelling of the Alfvén mode activity in ASDEX Upgrade with an isotropic slowing-down fast-particle distribution

In the present paper, the evolution of the Alfvén modes (AMs) is studied in a realistic ASDEX Upgrade equilibrium by analyzing the results of simulations with the global, electromagnetic, gyrokinetic particle-in-cell code ORB5. The energetic particles (EPs) are modelled both via the newly implemented isotropic slowing-down and with Maxwellian distribution functions. The comparison of the numerical results shows that modelling the EPs with the equivalent Maxwellian rather than with the slowing-down, does not significantly affect the frequency of the driven AM, while its growth rate appears to be underestimated with a quantitative difference as large as almost 30%. Additionally the choice of the isotropic slowing-down allows a better description of the nonlinear modification of the dominant AM frequency, while an equivalent Maxwellian underestimates it. A good comparison with the experimental spectrogram is found.

The Einasto model for dark matter haloes

Context. The Einasto model has become one of the most popular models for describing the density profile of dark matter haloes. There have been relatively few comprehensive studies on the dynamical structure of the Einasto model, mainly because only a limited number of properties can be calculated analytically. Aims. We want to systematically investigate the photometric and dynamical structure of the family of Einasto models over the entire model parameter space. Methods. We used the SpheCow code to explore the properties of the Einasto model. We systematically investigated how the most important properties change as a function of the Einasto index n. We considered both isotropic models and radially anisotropic models with an Osipkov-Merritt orbital structure. Results. We find that all Einasto models with n < 1/2 have a formal isotropic or Osipkov-Merritt distribution function that is negative in parts of phase space, and hence cannot be supported by such orbital structures. On the other hand, all models with larger values of n can be supported by an isotropic orbital structure, or by an Osipkov-Merritt anisotropy, as long as the anisotropy radius is larger than a critical value. This critical anisotropy radius is a decreasing function of n, indicating that less centrally concentrated models allow for a larger degree of radial anisotropy. Conclusions. Studies of the structure and dynamics of models for galaxies and dark matter haloes should not be restricted to completely analytical models. Numerical codes such as SpheCow can help open up the range of models that are systematically investigated. This applies to the Einasto model discussed here, but also to other proposed models for dark matter haloes, including different extensions to the Einasto model.

Ion kinetic effects and instabilities in the plasma flow in the magnetic mirror

Kinetic effects in plasma flow due to a finite ion temperature and ion reflections in a converging–diverging magnetic nozzle are investigated with collisionless quasineutral hybrid simulations with kinetic ions and isothermal Boltzmann electrons. It is shown that in the cold ions limit, the velocity profile of the particles agrees well with the analytical theory, predicting the formation of the global accelerating potential due to the magnetic mirror with the maximum of the magnetic field and resulting in the transonic ion velocity profile. The global transonic ion velocity profile is also obtained for warm ions with isotropic and anisotropic distributions. Partial ion reflections are observed due to a combined effect of the magnetic mirror and time-dependent fluctuations of the potential as a result of the wave breaking and instabilities in the regions when the fluid solutions become multi-valued. Despite partial reflections, the flow of the passing ions still follows the global accelerating profile defined by the magnetic field profile. In simulations with reflecting boundary condition imitating the plasma source and allowing the transitions between trapped and passing ions, the global nature of the transonic accelerating solution is revealed as a constrain on the plasma exhaust velocity that ultimately defines plasma density in the source region.