Static Configuration Research Articles

Density functional thermodynamic description of spin, phonon and displacement degrees of freedom in antiferromagnetic-to-paramagnetic phase transition in YNiO3

This work demonstrates a direct density functional description of the finite-temperature thermodynamic properties of solids exhibiting phase transitions through positional and spin symmetry breaking degrees of freedom. A classic example addressed here is the rare-earth (R) nickelates RNiO3 where the ground state is characterized by crystallographic and magnetic (e.g., antiferromagnetic) long-range order (LRO), whereas the higher temperature paramagnetic phase manifests a range of local spin and positional symmetry breaking motifs with short-range order (SRO). Unlike time-dependent simulations of spin and positional degrees of freedom, in the present work, phases are described via a superposition of static configurations constructed by populating a periodic base lattice supercell allowing for the formation of energy lowing distribution of positional and spin local motifs. The thermal populations of the configurations in such a superposition phase are obtained from the energy-minimized Density Functional Theory (DFT)-calculated partition functions at different temperatures. This approach offers flexible inclusion of different physical contributions to the free energy, such as elastic, electronic and phonon free energies, all obtained from the same underlying DFT total energy calculations of periodic structures. The thermodynamic and magnetic properties of both LRO and SRO crystallographic and spin phases, including antiferromagnetic (AFM) to paramagnetic (PM) Néel phase transition in YNiO3 are studied. Including spin and phonon contributions, we find a DFT-calculated Néel temperature to be 144 K in satisfactory agreement with the experimental value of 145 K; whereas omitting the phonon contribution, one obtains a Néel temperature of 81 K. We present phonon contributions to the DFT-calculated temperature-dependent SRO, heat capacities, and the polymorphous distribution of nonzero local magnetic moments in the PM phase. This approach thus extends to finite temperatures the symmetry-broken DFT description of both the AFM and PM phases, demonstrating that a thermodynamic superposition approach based on symmetry broken configurations evaluated by a mean-field like DFT is sufficient to obtain a consistent description of the thermal physics of the AFM, PM phases and their interconversion in 3d oxides illustrated by YNiO3.

From predicting to learning dissipation from pair correlations of active liquids.

Active systems, which are driven out of equilibrium by local non-conservative forces, can adopt unique behaviors and configurations. An important challenge in the design of novel materials, which utilize such properties, is to precisely connect the static structure of active systems to the dissipation of energy induced by the local driving. Here, we use tools from liquid-state theories and machine learning to take on this challenge. We first analytically demonstrate for an isotropic active matter system that dissipation and pair correlations are closely related when driving forces behave like an active temperature. We then extend a nonequilibrium mean-field framework for predicting these pair correlations, which unlike most existing approaches is applicable even for strongly interacting particles and far from equilibrium, to predicting dissipation in these systems. Based on this theory, we reveal a robust analytic relation between dissipation and structure, which holds even as the system approaches a nonequilibrium phase transition. Finally, we construct a neural network that maps static configurations of particles to their dissipation rate without any prior knowledge of the underlying dynamics. Our results open novel perspectives on the interplay between dissipation and organization out of equilibrium.

Thin-shell wormhole supported by exotic dust in gravity coupled with nonlinear electrodynamics

In the context of the spontaneously broken scale-invariant nonlinear electrodynamic introduced by Guendelman et al with a magnetic dominance feature, we construct a thin-shell wormhole. The surface tension of the thin-shell wormhole is zero in static configuration implying the exotic matter present at the throat is a cloud of exotic dust. We study the dynamic of the throat as well as the radial linear perturbation to investigate its mechanical stability. In the former case, the equation of state of the fluid on the shell is kept unchanged and upon our calculation the TSW is unstable. For the latter case, we consider a variable equation of state for the dynamic phase of the wormhole and with detailed analysis, it is shown that the TSW may be stable.

Study of gravitational collapse for anisotropic Karmarkar star in minimally coupled [formula omitted] gravity

The current study investigates a class of an exact analytical solutions of shear-free and spherically symmetric gravitational collapse of Karmarkar star in the minimally coupled f(R) theory of gravity by incorporating the properties of anisotropic radiating matter. The collapse onsets from an early static configuration obeying the class 1 Karmarkar approach and progresses with dissipation of immense energy in the form of outgoing radiation and radial heat flux. We have formulated the f(R) field equations under the specified Karmarkar approach and extracted dynamical evolution equation through the boundary matching hypersurface between the exterior Vaidya solution of radiative star and the interior spacetime. We have analyzed our solutions for a system of field equations with two specific f(R) models like f(R)=R+ηR2 and f(R)=R+ηR(R+ϵR2), which are famous and cosmologically viable for the present and the late epoch of cosmic inflation. The physical viability of the proposed solutions has manifested in various graphical plots via material variables, stability tests, anisotropy of pressure, effective mass, compactness, energy conditions and relaxational effects through temporal profile. These phenomenological evidences ensure the viable physical conduct of our radiative star model that seems in good agreement with the possible features of gravitational collapse (GC).

A new class of charged spherically symmetric and static superdense star configurations

In this paper, we have obtained a new family of interior solutions of the Einstein–Maxwell field equations in general relativity for static, spherically symmetric distribution of charged fluid. The electric density is assumed to be [Formula: see text] where [Formula: see text] is a positive constant. The solution is well behaved for a wide range of [Formula: see text] hence, it is suitable for modeling a superdense star. The surface density is assumed to be [Formula: see text], for the validation of the solution set present, a comparison has also been made with the stars EXO 1785-248, SMC X-1, Her X-1, 4U 1538-52. For these particular stars, the maximum value of surface redshift is obtained to be 0.3251 for the star EXO 1785-248 and the minimum value is obtained as 0.2027 for the star Her X-1. We subject our solution to rigorous physical viability tests. In the absence of charge, we obtain the regular and well-behaved sixth model of Durgapal.

How liquid–liquid phase separation induces active spreading

The interplay between phase separation and wetting of multicomponent mixtures is ubiquitous in nature and technology and recently gained significant attention across scientific disciplines, due to the discovery of biomolecular condensates. It is well understood that sessile droplets, undergoing phase separation in a static wetting configuration, exhibit microdroplet nucleation at their contact lines, forming an oil ring during later stages. However, very little is known about the dynamic counterpart, when phase separation occurs in a nonequilibrium wetting configuration, i.e., spreading droplets. Here we show that liquid-liquid phase separation strongly couples to the spreading motion of three-phase contact lines. Thus, the classical Cox-Voinov law is not applicable anymore, because phase separation adds an active spreading force beyond the capillary driving. Intriguingly, we observe that spreading starts well before any visible nucleation of microdroplets in the main droplet. Using high-speed ellipsometry, we further demonstrate that the evaporation-induced enrichment, together with surface forces, causes an even earlier nucleation in the wetting precursor film around the droplet, initiating the observed wetting transition. We expect our findings to improve the fundamental understanding of phase separation processes that involve dynamical contact lines and/or surface forces, with implications in a wide range of applications, from oil recovery or inkjet printing to material synthesis and biomolecular condensates.

Near wall effects on the premixed head-on hydrogen/air flame

The paper focuses on laminar and turbulent premixed head-on hydrogen/air flames interacting with inert, non-reacting walls. Both adiabatic and isothermal (450K) walls are considered in a counterflow-like, statistically stationary configuration. In the laminar condition, the flame-wall distance and isothermal-wall heat flux are studied under different strain rates. In the turbulent condition, interactions between turbulent, thermal and chemical effects are analyzed via direct numerical simulations. Influences on the flame dynamics due to near wall effects are characterized by conditional statistics on the flame fronts, such as the flame front temperature, thickness and the heat release rate. Compared with the single-step chemistry results, the detailed chemical reaction mechanism brings new and more complicated physics. Because of the differential diffusion between heat and mass, the flame front topology associated with major species changes with the wall boundaries. Accounting for contributions from various reacting species, a newly defined mean flame displacement speed has been proposed to quantify the movement of the turbulent flame zone. Interestingly, the wall heat flux model developed from the single-step chemistry remains applicable even for the detailed chemistry case with good predictability for both laminar and turbulent flames.

Dynamic System Diversification for Securing Cloud-based IoT Subnetworks

Remote exploitation attacks use software vulnerabilities to penetrate through a network of Internet of Things (IoT) devices. This work addresses defending against remote exploitation attacks on vulnerable IoT devices. As an attack mitigation strategy, we assume it is not possible to fix all the vulnerabilities and propose to diversify the open-source software used to manage IoT devices. Our approach is to deploy dynamic cloud-based virtual machine proxies for physical IoT devices. Our architecture leverages virtual machine proxies with diverse software configurations to mitigate vulnerable and static software configurations on physical devices. We develop an algorithm for selecting new configurations based on network anomaly detection signals to learn vulnerable software configurations on IoT devices, automatically shifting towards more secure configurations. Cloud-based proxy machines mediate requests between application clients and vulnerable IoT devices, facilitating a dynamic diversification system. We report on simulation experiments to evaluate the dynamic system. Two models of powerful adversaries are introduced and simulated against the diversified defense strategy. Our experiments show that a dynamically diversified IoT architecture can be invulnerable to large classes of attacks that would succeed against a static architecture.

HyTM-AP Hybrid Transactional Memory Scheme Using Abort Prediction and Adaptive Retry Policy for Multi-Core In-Memory Databases

Recently, works on integrating HTM with STM, called hybrid transactional memory (HyTM), have intensively studied. However, the existing works consider only the prediction of a conflict between two transactions and provide a static HTM configuration for all workloads. To solve the problems, we proposes a hybrid transactional memory scheme based on both abort prediction and an adaptive retry policy, called HyTM-AP. First, our HyTM-AP can predict not only conflicts between concurrently running transactions, but also the capacity and other aborts of transactions by collecting the information of transactions previously executed. Second, our HyTM-AP can provide an adaptive retry policy based on machine learning algorithms, according to the characteristic of a given workload. Finally, through our experimental performance analysis using the STAMP benchmark, our HyTM-AP shows 12~13% better performance than the existing HyTM schemes.

Superfluid vortex multipoles and soliton stripes on a torus

We study the existence, stability, and dynamics of vortex dipole and quadrupole configurations in the nonlinear Schr\"odinger (NLS) equation on the surface of a torus. For this purpose we use, in addition to the full two-dimensional NLS on the torus, a recently derived [Phys. Rev. A 101, 053606 (2021)] reduced point-vortex particle model which is shown to be in excellent agreement with the full NLS evolution. Horizontal, vertical, and diagonal stationary vortex dipoles are identified and continued along the torus aspect ratio and the chemical potential of the solution. Windows of stability for these solutions are identified. We also investigate stationary vortex quadrupole configurations. After eliminating similar solutions induced by invariances and symmetries, we find a total of 16 distinct configurations ranging from horizontal and vertical aligned quadrupoles, to rectangular and rhomboidal quadrupoles, to trapezoidal and irregular quadrupoles. The stability for the least unstable and, potentially, stable quadrapole solutions is monitored both at the NLS and the reduced model levels. Two quadrupole configurations are found to be stable on small windows of the torus aspect ratio and a handful of quadrupoles are found to be very weakly unstable for relatively large parameter windows. Finally, we briefly study the dark soliton stripes and their connection, through a series of bifurcation cascades, with steady state vortex configurations.

Characterization of tandem aerosol classifiers for selecting particles: implication for eliminating the multiple charging effect

Abstract. Accurate particle classification plays a vital role in aerosol studies. Differential mobility analyzers (DMAs), centrifugal particle mass analyzers (CPMAs) and aerodynamic aerosol classifiers (AACs) are commonly used to select particles with a specific mobility diameter, aerodynamic diameter or mass, respectively. However, multiple charging effects cannot be entirely avoided when using either individual techniques or tandem systems such as DMA–CPMA, especially when selecting soot particles with fractal structures. In this study, we calculate the transfer functions of the DMA–CPMA and DMA–AAC in static configurations for flame-generated soot particles. We propose an equation that constrains the resolutions of the DMA and CPMA to eliminate the multiple charging effect when selecting particles with a certain mass–mobility relationship using the DMA–CPMA system. The equation for the DMA–AAC system is also derived. For DMA–CPMA in a static configuration, our results show that the ability to remove multiply charged particles mainly depends on the particle morphology and resolution settings of the DMA and CPMA. Using measurements from soot experiments and literature data, a general trend in the appearance of the multiple charging effect with decreasing size when selecting aspherical particles is observed. As for DMA–AAC in a static configuration, the ability to eliminate particles with multiple charges is mainly related to the resolutions of the classifiers. In most cases, the DMA–AAC in a static configuration can eliminate the multiple charging effect regardless of the particle morphology, but multiply charged particles will be selected when decreasing the resolution of the DMA or AAC. We propose that the potential influence of the multiple charging effect should be considered when using the DMA–CPMA or DMA–AAC systems in estimating size- and mass-resolved optical properties in field and lab experiments.

Cell motility as an energy minimization process.

The dynamics of active matter driven by interacting molecular motors has a nonpotential structure at the local scale. However, we show that there exists a quasipotential effectively describing the collective self-organization of the motors propelling a cell at a continuum active gel level. Such a model allows us to understand cell motility as an active phase transition problem between the static and motile steady-state configurations that minimize the quasipotential. In particular, both configurations can coexist in a metastable fashion and a small stochastic disorder in the gel is sufficient to trigger an intermittent cell dynamics where either static or motile phases are more probable, depending on which state is the global minimum of the quasipotential.

TurboNet: Faithfully Emulating Networks With Programmable Switches

Faithfully emulating networks is critical for verifying the correctness and effectiveness of new networking-related designs. Existing network experiment platforms either cannot faithfully emulate the functionality and performance of production networks or cannot scale well due to cost constraints. In this paper, we propose <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TurboNet</i> , a new network emulator that utilizes one or more programmable switches to achieve faithful emulation of the network data plane and control plane. For data plane emulation, we propose a series of key designs, such as port mapper, queue mapper, and delayed queue, to emulate network topologies and performance metrics with high flexibility and accuracy. For control plane emulation, we support static routing configurations, distributed routing agents, and the centralized routing controllers. Meanwhile, we provide APIs for operators to simplify network emulation tasks. We implement <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TurboNet</i> on Tofino switches. Evaluation results show that: (1) On the data plane, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TurboNet</i> can flexibly emulate various topologies, such as an 8-ary fat-tree with only one programmable switch and a 10-ary fat-tree with four programmable switches; (2) On the control plane, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TurboNet</i> supports about 200 BGP agents on a single programmable switch with a CPU usage of 25%; (3) <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TurboNet</i> can accurately emulate different network performance metrics such as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−8</sup> link loss, and microsecond to millisecond link delay.

Schwarzschild quasi-normal modes of non-minimally coupled vector fields

We study perturbations of massive and massless vector fields on a Schwarzschild black-hole background, including a non-minimal coupling between the vector field and the curvature. The coupling is given by the Horndeski vector-tensor operator, which we show to be unique, also when the field is massive, provided that the vector has a vanishing background value.We determine the quasi-normal mode spectrum of the vector field, focusing on the fundamental mode of monopolar and dipolar perturbations of both even and odd parity, as a function of the mass of the field and the coupling constant controlling the non-minimal interaction. In the massless case, we also provide results for the first two overtones, showing in particular that the isospectrality between even and odd modes is broken by the non-minimal gravitational coupling.We also consider solutions to the mode equations corresponding to quasi-bound states and static configurations. Our results for quasi-bound states provide strong evidence for the stability of the spectrum, indicating the impossibility of a vectorization mechanism within our set-up. For static solutions, we analytically and numerically derive results for the electromagnetic susceptibilities (the spin-1 analogs of the tidal Love numbers), which we show to be non-zero in the presence of the non-minimal coupling.

Ab initio point defect calculations for structural properties of a model austenitic steel alloy

We present first-principles density functional theory calculations of atomic properties relevant to the material strength of an austenitic (face-centered-cubic) steel alloy, ${\mathrm{Fe}}_{0.70}{\mathrm{Ni}}_{0.12}{\mathrm{Cr}}_{0.18}$. While alloys in this class appear nonmagnetic macroscopically because their local magnetic moments are disordered, significant deviation from experiment can result when calculating properties using nonmagnetic calculations or magnetically ordered calculations. To overcome these difficulties, in this paper we employ a magnetic sampling method with spin-polarized density functional theory to approximate magnetically disordered states by averaging over several static configurations. Using this approach, we calculate elastic and point defect properties, which are then used to evaluate the dominant solid-solution strengthening contribution to the yield strength. Moreover, since the local composition can interact with strain fields, whether externally applied or induced by dislocations, diffusion can give rise to local changes in mechanical properties. We therefore calculate contributions to the vacancy-mediated diffusivity from the formation and migration terms in the activation energy, and the harmonic-transition-state-theory prefactor which depends on the vibrational states of the initial and transition states. While the prefactor is very sensitive to noise and therefore only loosely constrained by our calculations, the activation energies and yield stress values obtained from paramagnetic calculations are in good agreement with available experimental results.

Understanding seasonal and diurnal variations of inter-city tourism destination network

ABSTRACT This study characterizes a destination network based on travel flow. Although the tourism literature has mostly discussed the static configurations of network structures, this study presents a dynamic destination network considering four seasons and daily periods. Given the advancement of connected technology, mobile sensor big data collected from international travelers visiting Korea were analyzed to explore movement behaviors across 250 cities in the country. Results demonstrate the dynamics of complex network systems in tourism destinations over time, such as seasons and diurnal variations. Its findings are crucial for developing vibrant destination management and contextual marketing.

Exponentially generalized vortex

In this work, we propose an exponentially generalized Abelian model. We investigated the presence of vortex structures in models coupled to Maxwell and Chern-Simons fields. We chose to investigate the dynamics of the complex scalar field in models coupled separately to the Maxwell term and the Chern-Simons term. For this, we analyze the Bogomol'nyi equations in both cases to describe the static field configurations. An interesting result appears when we note that scalar field solutions generate degenerate minimum energy configurations by a factor of in Maxwell's case. On the other hand, in the Chern-Simons case, the solutions in this sector are degenerate by a factor of . Finally, we solve the Bogomol'nyi equations numerically and discuss our results.

Target Capturing in an Ellipsoidal Region for a Swarm of Double Integrator Agents

In this paper we focus on the target capturing problem for a swarm of agents modelled as double integrators in any finite space dimension. Each agent knows the relative position of the target and has only an estimation of its velocity and acceleration. Given that the estimation errors are bounded by some known values, it is possible to design a control law that ensures that agents enter a user-defined ellipsoidal ring around the moving target. Agents know the relative position of the other members whose distance is smaller than a common detection radius. Finally, in the case of no uncertainty about target data and homogeneous agents, we show how the swarm can reach a static configuration around the moving target. Some simulations are reported to show the effectiveness of the proposed strategy.

Exploring circulations yielding no vortex equilibria

A system of n planar point vortices with prescribed real circulations Γi≠0 may be arranged so that the interaction between vortices causes them all to move with a common velocity. Such solutions are called vortex equilibria, where the required configurations are referred to as stationary or translating depending on whether the velocity is zero or not. The necessary condition, ∑i<jΓiΓj=0, for stationary configurations is not sufficient, for general n. The necessary condition, ∑iΓi=0, for translating configurations is also not sufficient in general. For example, for n≥5, the set of circulations (24−n,⋯,24−n,2(2−n)n(4−n),1) yields no stationary configurations. For n≥4, the set of circulations (−1,⋯,−1,n−3,1) yields no translating configurations. In this paper, we generalize such results and find new classes of sets of circulations satisfying the respective necessary conditions, but yielding no vortex equilibria.Let α and m be two independent parameters of natural numbers. The sets of circulations we consider are of the form (x,…,x,y,1,…,1) with α 1's, no less than three x=−2m, and one y such that ∑i<jΓiΓj=0 or ∑iΓi=0 holds. Using the minimal polynomial systems defined by O'Neil, we obtain new systems involving only part of the variables in the minimal polynomial systems. Sufficient conditions for no vortex equilibria are derived from them. We explore α and m such that the n-vortex system has no vortex equilibria for general n with the help of a computer algebra system. As a byproduct of the derived systems, they also help to find vortex equilibria for some exceptional n's where no information is obtained from our sufficient conditions.

Cubic Anisotropy for a Reconfigurable Magnonic Crystal Based on Co2MnSi Heusler Alloy

We explore the possibility offered by magnetic materials with cubic anisotropy to realize reconfigurable magnonic crystals with a very simple geometry working at zero magnetic field. As a proof of concept, we use micromagnetic simulations to calculate the static and dynamic magnetic configurations of a squared antidot lattice made with ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$ Heusler alloy having an anisotropy constant of about 17 \ifmmode\times\else\texttimes\fi{} ${10}^{3}\phantom{\rule{0.25em}{0ex}}{\mathrm{J}/\mathrm{m}}^{3}$. We show that the cubic anisotropy allows very different magnetic states to be obtained at remanence as a function of the direction of a saturation magnetic field, including quasiuniform remanent states that cannot be obtained in materials with the same magnetic parameters but without crystal anisotropy. This leads to the possibility to excite or extinct several quantized spin-wave modes whose frequencies can be tuned with the antidot dimensions. Reconfigurable magnetic states are demonstrated for antidot sizes in the range 300--350 nm for a ratio between the antidot size and spacing equal to 1/3. The transition between two different remanent states can be obtained with low-amplitude magnetic field (down to 3.5 mT) and with a switching time faster than 1 ns, which is of great interest for applications. Oppositely, for a particular orientation of the antidot lattice with respect to the cubic anisotropy axes, we also obtain a single stable remanent state independent of the saturation field direction. Finally, propagation properties and frequency band gaps in an antidot lattice with lateral antidot size of 100 nm are studied for frequency-filtering applications at remanence. Both magnetostatic surface waves and magnetostatic backward-volume wave configurations are explored for different positions of the microwave excitation with respect to the magnonic crystal.