Evolution Of Cascades Research Articles

Molecular dynamics simulation of temperature effects on low energy near-surface cascades and surface damage in Cu

The temperature effects on near-surface cascades and surface damage in Cu(001) surface under 500eV argon ion bombardment were studied using molecular dynamics (MD) method. In present MD model, substrate system was fully relaxed for 1ns and a read-restart scheme was introduced to save total computation time. The temperature dependence of damage production was calculated. The evolution of near-surface cascades and spatial distribution of adatoms at varying temperature were analyzed and compared. It was found that near-surface vacancies increased with temperature, which was mainly due to the fact that more atoms initially located in top two layers became adatoms with the decrease of surface binding energy. Moreover, with the increase of temperature, displacement cascades altered from channeling-like structure to branching structure, and the length of collision sequence decreased gradually, because a larger portion of energy of primary knock-on atom (PKA) was scattered out of focused chain. Furthermore, increasing temperature reduced the anisotropy of distribution of adatoms, which can be ascribed to that regular registry of surface lattice atoms was changed with the increase of thermal vibration amplitude of surface atoms.

Variations on Debris Disks. IV. An Improved Analytical Model for Collisional Cascades

Abstract We derive a new analytical model for the evolution of a collisional cascade in a thin annulus around a single central star. In this model, r max the size of the largest object changes with time, , with 0.1–0.2. Compared to standard models where r max is constant in time, this evolution results in a more rapid decline of M d , the total mass of solids in the annulus, and L d , the luminosity of small particles in the annulus: and . We demonstrate that the analytical model provides an excellent match to a comprehensive suite of numerical coagulation simulations for annuli at 1 au and at 25 au. If the evolution of real debris disks follows the predictions of the analytical or numerical models, the observed luminosities for evolved stars require up to a factor of two more mass than predicted by previous analytical models.

Atomistic simulation of structural damage during ion irradiation of iron single crystals

The evolution of atomic displacement cascades initiated near free surfaces with different crystallographic orientations in bcc iron specimens was studied on the base of the molecular dynamics approach. The craters surrounded by adatom mounds were formed in the case of the (111) surface irradiation. The dislocation loops consisted of vacancies were generated after the (110) surface irradiation. The dislocation Burgers vector was a/2 <111> or a <100>. It was shown that the type of structural damage is determined by the anisotropy of propagation of shock waves generated by atomic displacement cascades. For energies of the atomic displacement cascade lower than 20 keV the number of adatoms and survived point defects was higher for specimen with the (110) free surface due to the different character of surface damage. The increase in the cascade energy up to 20 keV results in formation of almost the equal number of survived point defects for the (110) and (111) free surfaces as displacement cascades were developed on larger distance from the irradiated surfaces.

Tracking Switched Dynamic Network Topologies From Information Cascades

Contagions such as the spread of popular news stories, or infectious diseases, propagate in cascades over dynamic networks with unobservable topologies. However, "social signals" such as product purchase time, or blog entry timestamps are measurable, and implicitly depend on the underlying topology, making it possible to track it over time. Interestingly, network topologies often "jump" between discrete states that may account for sudden changes in the observed signals. The present paper advocates a switched dynamic structural equation model to capture the topology-dependent cascade evolution, as well as the discrete states driving the underlying topologies. Conditions under which the proposed switched model is identifiable are established. Leveraging the edge sparsity inherent to social networks, a recursive $\ell_1$-norm regularized least-squares estimator is put forth to jointly track the states and network topologies. An efficient first-order proximal-gradient algorithm is developed to solve the resulting optimization problem. Numerical experiments on both synthetic data and real cascades measured over the span of one year are conducted, and test results corroborate the efficacy of the advocated approach.

Evolution of displacement cascades in Fe–Cr structures with different [001] tilt grain boundaries

ABSTRACTReduced-activation ferritic/martensitic steels of Cr concentration between 2.25 and 12 wt% are candidate structural materials for next-generation nuclear reactors. In this study, molecular dynamics (MD) simulation is used to generate the displacement cascades in Fe–Cr structures with different Cr concentrations by using different primary knock-on atom (PKA) energies between 2 and 10 keV. A concentration-dependent model potential has been used to describe the interactions between Fe and Cr. Single crystals (SCs) of three different coordinate bases (e.g. [310], [510], and [530]) and bi-crystal (BC) structures with three different [001] tilt grain boundaries (GBs) (e.g. Σ5, Σ13, and Σ17) have been simulated. The Wigner–Seitz cell criterion has been used to identify the produced Frenkel pairs. The results show a marked difference between collisions observed in SCs and those in BC structures. The numbers of vacancies and interstitials are found to be significantly higher in BC structures than those found in SCs. The number of point defects exhibits a power relationship with the PKA energies; however, the Cr concentration does not seem to have any influence on the number of survived point defects. In BC models, a large fraction of the total survived point defects (between 59% and 93%) tends accumulate at the GBs, which seem to trap the generated point defects. The BC structure with Σ17 GB is found to trap more defects than Σ5 and Σ13 GBs. The defect trapping is found to be dictated by the crystallographic parameters of the GBs. For all studied GBs, self-interstitial atoms (SIAs) are easily trapped within the GB region than vacancies. An analysis of defect composition reveals an enrichment of Cr in SIAs, and in BC cases, more than half of the Cr-SIAs are found to be located within the GB region.

Quantifying the evolution of citation cascades

ABSTRACTCitation is an important measurement in science which provides valuable clues to the historical development and trend forecast of science. However, the evolution of citation structure remains poorly understood despite long‐period and frequent use of citation counts for assessment purposes within scientific community. We observe citation structure from a cascade perspective and construct a large and long‐range citation graph based on real data. Two metrics are used to quantify the structural virality and cascade size. Preliminary results show that (1) age plays an important role for the evolution of citation cascades; (2) average depth tends to be influenced by both lifespan and the whole volume of scientific literature and grows much slower than cascade size; (3) there exists a stable positive relationship between average depth and cascade size, and it seems that an ultimate status exists where most papers eventually have large values for both metrics.

Simulation of Single Particle Displacement Damage in Silicon – Part I: Global Approach and Primary Interaction Simulation

A comprehensive approach is developed for the simulation of Single Particle Displacement Damage in silicon, from the incident particle interaction in silicon, to the resulting electrical effect observed experimentally. The different steps of the global approach are described. The paper then focuses on the first step corresponding to Monte Carlo simulation of the primary interaction. The characteristics of the Primary Knock-On Atom (PKA) generated by neutron- or proton-silicon interactions for different energies are explored, analyzing in particular the PKA range in energies and species. This leads to the selection of 1 and 10 keV silicon atoms as good candidates to best represent the displacement cascades generated by all PKA. These PKA characteristics will be used as input in the following Molecular Dynamics simulation step, developed in a separate paper to simulate the displacement cascade generation and evolution. Monte Carlo simulations are also performed in a geometry representative of an image sensor, analyzing the distribution of non-ionizing deposited energy. The obtained distributions appear very similar for incident neutrons from 3 to 18 MeV and incident protons of 200 MeV, in agreement with similarities observed in experimentally measured dark current distributions in image sensors. The effect of geometric parameters on these distributions is finally explored.

Molecular dynamics simulation of primary radiation damage in Fe–Cr alloy

The atomic displacement cascades generated by radiation in the iron-chromium alloy were studied using molecular dynamics simulation. Atomic displacement cascades were generated by the primary knock-on atom, which energy varied from 1 to 20 keV. The simulation of the dynamics of atomic displacement cascades and the calculation of the cascade parameters (durations of the main stages, the size of the radiation-damaged region) in the iron-chromium alloy were made for different energies of primary knock-on atom. Sizes of formed point defect clusters and their distribution in the crystallite volume were calculated. To study radiation damage in iron-chromium alloy caused by generation and evolution of displacement cascades we analyzed spatial and quantitative distribution of the generated point defects and their clusters.

Cascading Boost Effect on the Capacity of Nitrogen-Doped Graphene Sheets for Li- and Na-Ion Batteries.

Specific capacity and cyclic performance are critically important for the electrode materials of rechargeable batteries. Herein, a capacity boost effect of Li- and Na-ion batteries was presented and clarified by nitrogen-doped graphene sheets. The reversible capacities progressively increased from 637.4 to 1050.4 mAh g-1 (164.8% increase) in Li-ion cell tests from 20 to 185 cycles, and from 187.3 to 247.5 mAh g-1 (132.1% increase) in Na-ion cell tests from 50 to 500 cycles. The mechanism of the capacity boost is proposed as an electrochemical induced cascading evolution of graphitic N to pyridinic and/or pyrrolic N, during which only these graphitic N adjacent pyridinic or pyrrolic structures can be taken precedence. The original and new generated pyridinic and pyrrolic N have strengthened binding energies to Li/Na atoms, thus increased the Li/Na coverage and delivered a progressive capacity boost with cycles until the entire favorable graphitic N transform into pyridinic and pyrrolic N.

MD simulation of primary radiation damage in metals with internal structure

A review on simulation of the primary radiation damage (PRD) and formation of radiation defects by atomic displacement cascades is conducted. Results of the simulation by the molecular dynamics method of the atomic displacement cascades and peculiarities of their interaction with the defects of the internal crystal structure (point defect, pores, dislocations, grain boundaries (GBs), and free surfaces) are given. It is shown that the defects exert a significant impact on the evolution of the atomic displacement cascades and formation of the PRD in metals.

RELAXATION PROCESSES WITHIN FLUX ROPES IN SOLAR WIND

Flux ropes are localized structures in space plasma whose tube-like organized magnetic configuration can be well approximated by a force-free field model. Both numerical simulations and simple models suggest that the ideal magnetohydrodynamics (MHD) can relax toward a minimum energy state, where magnetic helicity is conserved, characterized by force-free magnetic fields (Taylor relaxation). In this paper, we evaluate MHD rugged invariants within more than 100 flux ropes identified in the solar wind at 1 AU, showing that the magnetic and cross-helicity content carried out by these structures tend to be attracted toward a particular subphase in the parameter plane. The final configuration of the MHD rugged invariants in the parameter plane suggests indeed that flux ropes represent well-organized structures coming from the dynamical evolution of MHD turbulent cascade. These observational results, along with a simple model based on a truncated set of nonlinear ordinary differential equations for both the velocity and magnetic field Fourier coefficients, thus, support a scenario in which the flux ropes naturally come out from the ideal MHD decay to large-scale magnetic field in space plasmas, probably governed by relaxation processes similar to those observed in laboratory plasmas.

Effects of two-temperature model on cascade evolution in Ni and NiFe

We perform molecular dynamics simulations of Ni ion cascades in Ni and equiatomic NiFe under the following conditions: (a) classical molecular dynamics (MD) simulations without consideration of electronic energy loss, (b) classical MD simulations with the electronic stopping included, and (c) using the coupled two-temperature MD (2T-MD) model that incorporates both the electronic stopping and the electron-phonon interactions. Our results indicate that the electronic effects are more profound in the higher energy cascades and that the 2T-MD model results in a smaller amount of surviving damage and smaller defect clusters, while less damage is produced in NiFe than in Ni.

Spectral evolution of helical electron magnetohydrodynamic turbulence

AbstractProperties of turbulence in space plasmas above and below the proton gyroscale are very different and different descriptions should be used for the large and the small scales. On scales larger than the proton gyroscale, we can treat the plasmas as conducting fluids and use magnetohydrodynamics (MHD). However, on scales smaller than the proton gyroscale, we cannot use MHD due to various kinetic effects. Electron magnetohydrodynamics (EMHD) is a simple fluid‐like model for small‐scale plasmas. In this paper, we discuss properties of EMHD waves and turbulence. In particular, we focus on inverse cascade in EMHD turbulence. Since EMHD waves are helical and magnetic helicity is a conserved quantity, inverse cascade is expected to be a common phenomenon in EMHD. We first focus on inverse cascade and spectral evolution of completely imbalanced EMHD wave packets, in which all waves are moving in the same direction. Then, we discuss inverse cascade and spectral evolution of driven imbalanced EMHD turbulence. We also discuss “pinning” of energy spectra in imbalanced EMHD turbulence.

Integration analysis of MKK and MAPK family members highlights potential MAPK signaling modules in cotton.

Mitogen-activated protein kinase (MAPK) cascades play a crucial role in plant growth and development, as well as their biotic and abiotic stress responses. As a nodal point of the MAPK cascade, the MKK gene family has not been systematically studied in cotton. Here, we identified 11 putative MKK genes in the Gossypium raimondii genome. Phylogenetic analysis showed that the MKKs were supported by architectures of conserved protein motifs. Expression patterns of MKKs under hormone treatments or abiotic stresses revealed their diverse functions in stress responses. Based on a yeast two hybrid, a total of 63 interactive pairs of MKKs and MAPKs were identified in cotton. Among these, 40 interactive pairs were newly identified compared to that reported previously in Arabidopsis. Integration analysis of the interaction network and expression patterns of MKK and MAPK family members revealed 13 potential MAPK signaling modules that are involved in the complicated cross-talk between hormones and abiotic stresses. Taken together, our data enhance the understanding of the evolution and function of MAPK cascades in cotton, and lay the foundation for the improvement of various defense responses that use MAPK signaling modules in the future.

Local cascades induced global contagion: How heterogeneous thresholds, exogenous effects, and unconcerned behaviour govern online adoption spreading.

Adoption of innovations, products or online services is commonly interpreted as a spreading process driven to large extent by social influence and conditioned by the needs and capacities of individuals. To model this process one usually introduces behavioural threshold mechanisms, which can give rise to the evolution of global cascades if the system satisfies a set of conditions. However, these models do not address temporal aspects of the emerging cascades, which in real systems may evolve through various pathways ranging from slow to rapid patterns. Here we fill this gap through the analysis and modelling of product adoption in the world’s largest voice over internet service, the social network of Skype. We provide empirical evidence about the heterogeneous distribution of fractional behavioural thresholds, which appears to be independent of the degree of adopting egos. We show that the structure of real-world adoption clusters is radically different from previous theoretical expectations, since vulnerable adoptions—induced by a single adopting neighbour—appear to be important only locally, while spontaneous adopters arriving at a constant rate and the involvement of unconcerned individuals govern the global emergence of social spreading.

Free-stream turbulence effects on the boundary layer of a high-lift low-pressure-turbine blade

The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally investigated at low and high free-stream turbulence intensity conditions. Measurements have been carried out in order to analyze the boundary layer transition and separation processes at a low Reynolds number, under both steady and unsteady inflows. Static pressure distributions along the blade surfaces as well as total pressure distributions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions. Particle Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields. The flow field has been surveyed in two orthogonal planes (a blade-to-blade plane and a wall-parallel one). These measurements allow the identification of the Kelvin-Helmholtz large scale coherent structures shed as a consequence of the boundary layer laminar separation under steady inflow, as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks. A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state, thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.

Evolution of Vertebrate Phototransduction: Cascade Activation.

We applied high-throughput sequencing to eye tissue from several species of basal vertebrates (a hagfish, two species of lamprey, and five species of gnathostome fish), and we analyzed the mRNA sequences for the proteins underlying activation of the phototransduction cascade. The molecular phylogenies that we constructed from these sequences are consistent with the 2R WGD model of two rounds of whole genome duplication. Our analysis suggests that agnathans retain an additional representative (that has been lost in gnathostomes) in each of the gene families we studied; the evidence is strong for the G-protein α subunit (GNAT) and the cGMP phosphodiesterase (PDE6), and indicative for the cyclic nucleotide-gated channels (CNGA and CNGB). Two of the species (the hagfish Eptatretus cirrhatus and the lamprey Mordacia mordax) possess only a single class of photoreceptor, simplifying deductions about the composition of cascade protein isoforms utilized in their photoreceptors. For the other lamprey, Geotria australis, analysis of the ratios of transcript levels in downstream and upstream migrant animals permits tentative conclusions to be drawn about the isoforms used in four of the five spectral classes of photoreceptor. Overall, our results suggest that agnathan rod-like photoreceptors utilize the same GNAT1 as gnathostomes, together with a homodimeric PDE6 that may be agnathan-specific, whereas agnathan cone-like photoreceptors utilize a GNAT that may be agnathan-specific, together with the same PDE6C as gnathostomes. These findings help elucidate the evolution of the vertebrate phototransduction cascade from an ancestral chordate phototransduction cascade that existed prior to the vertebrate radiation.

Electron–phonon coupling in Ni-based binary alloys with application to displacement cascade modeling

Energy transfer between lattice atoms and electrons is an important channel of energy dissipation during displacement cascade evolution in irradiated materials. On the assumption of small atomic displacements, the intensity of this transfer is controlled by the strength of electron–phonon (el–ph) coupling. The el–ph coupling in concentrated Ni-based alloys was calculated using electronic structure results obtained within the coherent potential approximation. It was found that Ni0.5Fe0.5, Ni0.5Co0.5 and Ni0.5Pd0.5 are ordered ferromagnetically, whereas Ni0.5Cr0.5 is nonmagnetic. Since the magnetism in these alloys has a Stoner-type origin, the magnetic ordering is accompanied by a decrease of electronic density of states at the Fermi level, which in turn reduces the el–ph coupling. Thus, the el–ph coupling values for all alloys are approximately 50% smaller in the magnetic state than for the same alloy in a nonmagnetic state. As the temperature increases, the calculated coupling initially increases. After passing the Curie temperature, the coupling decreases. The rate of decrease is controlled by the shape of the density of states above the Fermi level. Introducing a two-temperature model based on these parameters in 10 keV molecular dynamics cascade simulation increases defect production by 10–20% in the alloys under consideration.

Kinetic approach to a relativistic Bose-Einstein condensate.

We apply a Boltzmann approach to the kinetic regime of a relativistic Bose-Einstein condensate of scalar bosons by decomposing the one-particle distribution function in a condensate part and a nonzero momentum part of excited modes, leading to a coupled set of evolution equations which are then solved efficiently with an adaptive higher order Runge-Kutta scheme. We compare our results to the partonic cascade Monte Carlo simulation BAMPS for a critical but far from equilibrium case of massless bosons. Motivated by the color glass condensate initial conditions in QCD with a strongly overpopulated initial glasma state, we also discuss the time evolution starting from an overpopulated initial distribution function of massive scalar bosons. In this system a self-similar evolution of the particle cascade with a nonrelativistic turbulent scaling in the infrared sector is observed as well as a relativistic exponent for the direct energy cascade, confirming a weak wave turbulence in the ultraviolet region.

Is cascade reinforcement likely when sympatric and allopatric populations exchange migrants?

When partially reproductively isolated species come back into secondary contact, these taxa may diverge in mating preferences and sexual cues to avoid maladaptive hybridization, a process known as reinforcement. This phenomenon often leads to reproductive character displacement (RCD) between sympatric and allopatric populations of reinforcing species that differ in their exposure to hybridization. Recent discussions have reinvigorated the idea that RCD may give rise to additional speciation between conspecific sympatric and allopatric populations, dubbing the concept “cascade reinforcement.” Despite some empirical studies supporting cascade reinforcement, we still know very little about the conditions for its evolution. In the present article, we address this question by developing an individual-based population genetic model that explicitly simulates cascade reinforcement when one of the hybridizing species is split into sympatric and allopatric populations. Our results show that when sympatric and allopatric populations reside in the same environment and only differ in their exposure to maladaptive hybridization, migration between them generally inhibits the evolution of cascade by spreading the reinforcement alleles from sympatry into allopatry and erasing RCD. Under these conditions, cascade reinforcement only evolved when migration rate between sympatric and allopatric populations was very low. This indicates that stabilizing sexual selection in allopatry is generally ineffective in preventing the spread of reinforcement alleles. Only when sympatric and allopatric populations experienced divergent ecological selection did cascade reinforcement evolve in the presence of substantial migration. These predictions clarify the conditions for cascade reinforcement and facilitate our understanding of existing cases in nature.