Anisotropic Collapse Research Articles

Polarized Light from Massive Protoclusters (POLIMAP). I. Dissecting the Role of Magnetic Fields in the Massive Infrared Dark Cloud G28.37+0.07

Magnetic fields may play a crucial role in setting the initial conditions of massive star and star cluster formation. To investigate this, we report SOFIA-HAWC+ 214 μm observations of polarized thermal dust emission and high-resolution GBT-Argus C18O(1-0) observations toward the massive Infrared Dark Cloud (IRDC) G28.37+0.07. Considering the local dispersion of B-field orientations, we produce a map of the B-field strength of the IRDC, which exhibits values between ∼0.03 and 1 mG based on a refined Davis–Chandrasekhar–Fermi method proposed by Skalidis & Tassis. Comparing to a map of inferred density, the IRDC exhibits a B–n relation with a power-law index of 0.51 ± 0.02, which is consistent with a scenario of magnetically regulated anisotropic collapse. Consideration of the mass-to-flux ratio map indicates that magnetic fields are dynamically important in most regions of the IRDC. A virial analysis of a sample of massive, dense cores in the IRDC, including evaluation of magnetic and kinetic internal and surface terms, indicates consistency with virial equilibrium, sub-Alfvénic conditions, and a dominant role for B-fields in regulating collapse. A clear alignment of magnetic field morphology with the direction of the steepest column density gradient is also detected. However, there is no preferred orientation of protostellar outflow directions with the B-field. Overall, these results indicate that magnetic fields play a crucial role in regulating massive star and star cluster formation, and therefore they need to be accounted for in theoretical models of these processes.

Coevolution of superconductivity and Hall coefficient with anisotropic lattice shrinkage in compressed KCa2Fe4As4F2

Abstract The stability of superconductivity in superconductors is widely recognized to be determined by various factors, including charge, spin, orbit, lattice, and other related degrees of freedom. Here, we report our findings on the pressure-induced coevolution of superconductivity and Hall coefficient in KCa2Fe4As4F2, an iron-based superconductor possessing a hybrid crystal structure combining KFe2As2 and CaFeAsF. Our investigation, involving high-pressure resistance, Hall effect and x-ray diffraction (XRD) measurements, allows us to observe the connection of the superconductivity and Hall coefficient with the anisotropic lattice shrinkage. We find that its ambient-pressure tetragonal (T) phase presents a collapse starting at around 18 GPa, where the sign of the Hall coefficient (R H) changes from positive to negative. Upon further compression, both superconducting transition temperature (T c) and R H exhibit a monotonous decrease. At around 41 GPa, the superconductivity is completely suppressed (T c = 0), where the parameter a begins to decline again and the Hall coefficient remains nearly unchanged. Our experiment results clearly demonstrate that the pressure-induced anisotropic lattice collapse plays a crucial role in tuning the interplay among multiple degrees of freedom in the superconducting system and, correspondingly, the stability of the superconductivity.

Hierarchical reconstruction of the cosmic web, the H-Spine method

ABSTRACT The cosmic web consists of a nested hierarchy of structures: voids, walls, filaments, and clusters. These structures interconnect and can encompass one another, collectively shaping an intricate network. Here we introduce the Hierarchical Spine (H-Spine) method, a framework designed to hierarchically identify and characterize voids, walls, and filaments. Inspired by the geometrical and dynamical constraints imposed by anisotropic gravitational collapse, the H-Spine method captures the geometry and interconnectivity between cosmic structures as well as their nesting relations, offering a more complete description of the cosmic web compared to single-scale or multiscale approaches. To illustrate the method’s utility, we present the distribution of densities and sizes of voids, walls, and filaments identified in a three-level hierarchical space. This analysis demonstrates how each level within the hierarchy unveils distinctive densities and scales inherent to cosmic web elements.

A High-mass, Young Star-forming Core Escaping from Its Parental Filament

We studied the unique kinematic properties in massive filament G352.63-1.07 at 103 au spatial scale with the dense molecular tracers observed with the Atacama Large Millimeter/submillimeter Array. We find the central massive core M1 (12 M ⊙) being separated from the surrounding filament with a velocity difference of and a transverse separation within 3″. Meanwhile, as shown in multiple dense-gas tracers, M1 has a spatial extension closely aligned with the main filament and is connected to the filament toward both its ends. M1 thus represents a very beginning state for a massive, young star-forming core escaping from the parental filament, within a timescale of ∼4000 yr. Based on its kinetic energy (3.5 × 1044 erg), the core escape is unlikely solely due to the original filament motion or magnetic field but requires more energetic events such as a rapid intense anisotropic collapse. The released energy also seems to noticeably increase the environmental turbulence. This may help the filament to become stabilized again.

Cosmic web dissection in fuzzy dark matter cosmologies

ABSTRACT On large cosmological scales, anisotropic gravitational collapse is manifest in the dark cosmic web. Its statistical properties are little known for alternative dark matter (DM) models such as fuzzy dark matter (FDM). In this work, we assess for the first time the relative importance of cosmic nodes, filaments, walls, and voids in a cosmology with primordial small-scale suppression of power. We post-process N-body simulations of FDM-like cosmologies with varying axion mass m at redshifts z ∼ 1.0−5.6 using the NEXUS+ Multiscale Morphology Filter technique at smoothing scale Δx = 0.04 h−1 Mpc. The formation of wall and void halos is more suppressed than naively expected from the half-mode mass M1/2. Also, we quantify the mass- and volume-filling fractions of cosmic environments and find that 2D cosmic sheets host a larger share of the matter content of the Universe as m is reduced, with an ∼8−12 per cent increase for the m = 7 × 10−22 eV model compared to cold dark matter (CDM). We show that in FDM-like cosmologies, filaments, walls, and voids are cleaner and more pronounced structures than in CDM, revealed by a strong mid-range peak in the conditioned overdensity PDFs P(δ). At high redshift, low-density regions are more suppressed than high-density regions. Furthermore, skewness estimates S3 of the total overdensity PDF in FDM-like cosmologies are consistently higher than in CDM, especially at high redshift z ∼ 5.6 where the m = 10−22 eV model differs from CDM by ∼6σ. Accordingly, we advocate for the usage of P(δ) as a testbed for constraining FDM and other alternative DM models.

Construction of Planar Gliding Restriction Buffer and Kinetic Self-Accelerator Stabilizing Single-Crystalline LiNi0.9Co0.05Mn0.05O2 Cathode.

The single-crystalline Ni-rich cathode has aroused much attention for extenuating the cycling and safety crises in comparison to the polycrystalline cathode. However, planar gliding and kinetic hindrance hinder its chemo-mechanical properties with cycling, which induce delamination cracking and damage the mechanical integrity in single crystals. Herein, a robust Li2.64(Sc0.9Ti0.1)2(PO4)3 (LSTP) ion/electron conductive network was constructed to decorate single-crystal LiNi0.9Co0.05Mn0.05O2 (SC90) particles. Via physicochemical characterizations and theoretical calculations, this LSTP coating that evenly grows on the SC90 particle with good lattice matching and strong bonding effectively restricts the anisotropic lattice collapse along the c-axis and the cation mixing activity of SC90, thus suppressing planar gliding and delamination cracking during repeated high-voltage lithiation/delithiation processes. Moreover, such a 3D LSTP network can also facilitate the lithium-ion transport and prevent the electrolyte's corrosion, lightening the kinetic hindrance and triggering the surface phase transformation. Combined with the Li metal anode, the LSTP-modified SC90 cell exhibits a desirable capacity retention of 90.5% at 5 C after 300 cycles and stabilizes the operation at 4.3/4.5 V. Our results provide surface modification engineering to mitigate planar gliding and kinetic hindrance of the single-crystalline ultra-high Ni-rich cathode, which inspires peers to design other layered cathode materials.

An in-depth understanding of chemomechanics in Ni-rich layered cathodes for lithium-ion batteries

Anisotropic lattice variations in Ni-rich layered oxides of lithium-ion batteries (LIBs) have been investigated extensively to suppress the chemomechanics and achieve high energy density with long-term cycling stability. However, an in-depth understanding of the anisotropy is lacking and is very important in the design of high-performance Ni-rich cathodes. Therefore, we reinvestigate the fundamentals of anisotropic lattice variations in Li [Ni10/12Mn1/12Co1/12]O2 (NCM) to understand the correlation between cycling stability degradation at high rate and intergranular microcrack generation between the primary particles, which is confirmed as follows: first, the capacity retention of the NCM under 4.3 V cutoff voltage (NCM4.3V) is much poorer than that under 3.8 V (NCM3.8V); this is described by various electrochemical analyses showing the multiple phase transitions accompanying anisotropic lattice variations and structural collapse. These structural evolutions are clearly observed in the ex situ X-ray diffraction patterns. Second, the resistance of NCM4.3V increases at a faster rate than that of NCM3.8V upon cycling, which supports the direct evidence regarding intergranular microcracks in the cycled particles of NCM4.3V. Third, the nonlinear lattice change in the c direction plays a critical role in accelerating cycling stability degradation. Fourth, serious lattice changes originate from the cationic repulsions between the Li and Ni ions with the electrostatic repulsion of oxygen ions. This mechanism is universally expected in Ni-rich layered oxides; furthermore, these findings provide insights into design strategies that mitigate chemomechanical degradations caused by long-term cycling stabilities in LIBs.

How the magnetic field behaves during the motion of a highly conducting fluid under its own gravity: A new theoretical, relativistic approach

Within the context of general relativity we study in a fully covariant way the so-called Euler-Maxwell system of equations. In particular, on decomposing the aforementioned system into its 1 temporal and 1 + 2 spatial components at the ideal magnetohydrodynamic limit, we bring it in a simplified form that favors physical insight to the problem of a self-gravitating, magnetized fluid. Of central interest is the decomposition of Faraday's equation which leads to a new general solution governing the evolution of the magnetic field during the motion of the highly conducting fluid. According to the latter relation, the magnetic field generally grows or decays in proportion to the inverse cube law of the scale factor--associated with the continuous contraction or expansion of the fluid's volume respectively. The magnetic field's law of variation, which has remarkable implications for the motion of the whole fluid, is subsequently applied to homogeneous (anisotropic-magnetized) cosmological models--especially to the Bianchi I case--as well as to the study of homogeneous and anisotropic gravitational collapse in a magnetized environment. Concerning the cosmological application, we derive the evolution equations of Bianchi I spacetime permeated by large-scale magnetic fields (these equations reduce to their FRW counterparts at the small/large--scale limit). Also, the compatibility of the new evolution formula for the magnetic field with the standard cosmic nucleosynthesis constraint is examined. As for the application in astrophysics, our results predict that homogeneous gravitational implosion is impeded when the electric Weyl tensor (associated with tidal forces) along the magnetic forcelines overwhelms the magnetic energy density. Lastly, our model denotes that the satisfaction of the aforementioned criterion is ultimately driven into a problem of initial conditions.

Dynamics of charged anisotropic spherical collapse in energy-momentum squared gravity

This paper investigates the dynamics of charged spherical collapse with anisotropic matter configuration in the context of energy-momentum squared gravity. This newly developed proposal resolves the big-bang singularity and yields the physically viable cosmological results in the early time universe. We establish dynamical equations through Misner-Sharp technique and analyze the effects of charge, anisotropy, effective matter variables and dark source terms on the collapse rate. A relation between Weyl scalar, fluid parameters and dark source terms is also established. The spacetime is not conformally flat due to the presence of anisotropic pressure, multivariate functions and their derivatives. In order to obtain conformally flat spacetime, we consider a specific model of this gravity, neglect the impact of charge and assume the isotropic matter distribution which yields homogeneity of the energy density and conformally flat spacetime. We conclude that positive dark source terms, anisotropy and charge yield the action of a repulsive force which enhances the stability of the system and hence diminishes the collapse rate.

Models of non-static and inhomogeneous gravitating source in Rastall gravity

In this paper, we have explored the non-static anisotropic gravitational collapse and expansion solutions in Rastall theory of gravity. The field equations have been formulated for the non-static and inhomogeneous gravitating source. The Misner–Sharp mass function, auxiliary solution and trapped condition have been used to obtained a trapped surface. The auxiliary solutions have been used to obtain the expansion and collapse solutions; these solutions depend on [Formula: see text] and parameter [Formula: see text] (which appears due to parametric form of metric components); also the range of parameter [Formula: see text] has been examined. The expansion scalar [Formula: see text] depends on parameter [Formula: see text], in the case of expansion [Formula: see text] for [Formula: see text], while for collapse [Formula: see text] with [Formula: see text]. Also, the dynamics of the gravitating spherical source has been discussed graphically with the effects of Rastall parameter [Formula: see text]. For the physically reasonable fluid, the validity of energy conditions has been discussed for expansion and collapse solutions with the various values of [Formula: see text].

Charged anisotropic spherical collapse with heat flow

In this article, we study the shear-free gravitational collapse of a charged radiating star. The Einstein field equations of gravitational collapse for the charged stars are known to give rise to a high degree of non-linearity in the ordinary differential equation coming from junction conditions. The attempts to solve it analytically proved to be unfortunate. Numerical methods have been suggested in the past. However, the high degree of non-linearity tends to introduce fluctuations and large round off errors in the numerical calculation. A new ansatz is proposed in the present work to reduce the degree of non-linearity. An ordinary differential equation is derived by satisfying junction conditions, and its numerical solution is demonstrated. Physical quantities associated with the collapse process are plotted to observe the effect of charge on these quantities. It is concluded that the charge can delay the collapse of a star and can even prevent it depending upon the amount of charge. It is also verified that the solution satisfies all the energy conditions.

Stabilizing Effects on Structural and Thermal Properties of Al-Doped Ni-Rich Layered Oxides for Li Rechargeable Batteries

One of the key strategies to achieve high energy density for Li rechargeable batteries is through improved layered oxides cathode materials. Ni-rich layered oxides with Ni content above 80% have been under extensive attention as promising cathodes for next-generation Li-ion batteries due to the merits of high capacity, low cost, and low toxicity [1–3]. However, these materials still require further improvement to overcome inferior capacity retention and poor thermal stability originated from Ni-rich environments [1,4].We investigated Al-doped LiNi0.80Co0.15Mn0.05O2 and undoped LiNi0.80Co0.15Mn0.05O2 layered cathodes in view of the structural, electrochemical, and thermal properties. Synchrotron-based X-ray techniques were used to analyze their structural behavior systematically during electrochemical reactions and thermal decomposition. High resolution powder diffraction (HRPD) results show that Al-doping in Ni-rich NCM cathode alleviates anisotropic lattice distortion and structural collapse as well as preserves a wider LiO6 interslab thickness, even at highly deintercalated states comparing to the undoped cathode. Al-doped Ni-rich cathode exhibits lower polarization potential, better rate capability, and cyclability compared to undoped one. This can be attributed to the alleviation of anisotropic lattice changes and volume changes during cycling. More importantly, Al-doped Ni-rich cathode maintains a wider LiO6 interslab thickness without collapse at highly charged states, allowing Li-ions to be deintercalated/intercalated reversibly. This indicates that rigid structural integrity contributes to enhanced electrochemical performance. Moreover, in situ X-ray diffraction (XRD) during the heating process and differential scanning calorimetry (DSC) analysis demonstrate that Al-doped NCM cathode outperforms bare NCM cathode significantly in terms of thermal aspects. Specially, Al-doping improves thermal stability by delaying the onset temperatures of phase transformations during the heating process. These results demonstrate that Al-doping plays a major role in stabilizing the structure by suppressing abrupt lattice changes during cycling and the formation of a rock-salt phase during thermal decomposition reaction.Based on these experimental results, we will provide the structural evidence for the Al-doping effects on Ni-rich layered materials, thereby guiding for the strategical design factor in the development of layered cathodes with high performance. More detailed results and discussion will be presented in the PRiME 2020.

Stability of anisotropic spheres

This work is devoted to understanding the dynamical instability of spherically symmetric space–time in the background of Einstein-Λ gravity. For this purpose, we have considered a spherical geometric distribution and assumed that it is filled with an anisotropic fluid. To proceed with our analysis, we have calculated the corresponding field as well as the mass function. We found nonlinear behavior of physical variables. To deal with that situation, we have subjected our system to the radial perturbations. We assume that after a particular era, our structural quantities have the same time dependence parameter. After linearizing the basic expressions, we have studied the impact of the cosmological constant in the modeling of relativistic stars. It is concluded that Λ tends to slow down the rate of spherical anisotropic collapse.

Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars

In this work we construct families of anisotropic neutron stars for an equation of state compatible with the constraints of the gravitational-wave event GW170817 and for four anisotropy ansatze. Such stars are subjected to a radial perturbation in order to study their stability against radial oscillations and we develop a dynamical model to describe the non-adiabatic gravitational collapse of the unstable anisotropic configurations whose ultimate fate is the formation of a black hole. We find that the standard criterion for radial stability dM/drho _c >0 is not always compatible with the calculation of the oscillation frequencies for some anisotropy ansatze, and each anisotropy parameter is constrained taking into account the recent restriction of maximum mass of neutron stars. We further generalize the TOV equations within a non-adiabatic context and we investigate the dynamical behaviour of the equation of state, heat flux, anisotropy factor and mass function as an unstable anisotropic star collapses. After obtaining the evolution equations we recover, as a static limit, the background equations.

Magnetically driven giant negative thermal expansion covering room temperature in Hf0.875Ta0·125Fe2

Abstract Neutron diffraction is used to reveal the origin of the unusual thermal expansion properties of the itinerant-electron system Hf0.875Ta0.125Fe2. Hf0.875Ta0.125Fe2 shows temperature-induced magnetic transitions from ferromagnetic (FM) to antiferromagnetic (AFM) order and then to the paramagnetic (PM) state upon heating. The FM-AFM transition proceeds in a stepwise fashion, as a first-order phase transition, and is accompanied by an extremely anisotropic lattice collapse. The unit cell volume shrinks abruptly, but only in the basal plane; the dimension along the c-axis varies almost continuously. Hf0.875Ta0.125Fe2 exhibits a 0.37 % spontaneous volume contraction across the FM-AFM transition, where a giant negative thermal expansion (NTE), with a volumetric thermal expansion coefficient -74×10-6 K-1, is observed over a temperature interval of ΔT∼50 K around room temperature.

Final fate of charged anisotropic fluid collapse in f(R) gravity

In this paper, the spherically symmetric gravitational collapse of anisotropic fluid in the presence of charge in metric [Formula: see text] theory is analyzed. We consider the static and non static spherically symmetric spacetimes for outer and inner regions of collapsing object respectively. For the smooth matching of inner and outer regions, the Senovilla as well as Darmois matching conditions are utilized. The closed form solutions are obtained from field equations. Moreover, we examine the apparent horizons and their physical significance. The effect of cosmological constant and [Formula: see text] term is same and the collapsing rate speeds up as compared to that of anisotropic fluid case when the electromagnetic field is introduced. Electromagnetic charge also affects the time interval of singularities and cosmological horizons.

Collapse of LiNi1- x- yCo xMn yO2 Lattice at Deep Charge Irrespective of Nickel Content in Lithium-Ion Batteries.

Volume variation and the associated mechanical fracture of electrode materials upon Li extraction/insertion are a main cause limiting lifetime performance of lithium-ion batteries. For LiNi1- x- yCo xMn yO2 (NCM) cathodes, abrupt anisotropic collapse of the layered lattice structure at deep charge is generally considered characteristic to high Ni content and can be effectively suppressed by elemental substitution. Herein, we demonstrate the lattice collapse is a universal phenomenon almost entirely dependent on Li utilization, and not Ni content, of NCM cathodes upon delithiation. With Li removal nearing 80 mol %, very similar c-axis lattice shrinkage of around 5% occurs concurrently for NCMs synthesized in-house regardless of nickel content (90, 70, 50, or 33 mol %); meanwhile, the a-axis lattice contracts for high-Ni NCM, but it expands for low-Ni NCM. We further reveal Co-Mn cosubstitution in NCM barely, if at all, affects several key structural aspects governing the lattice distortion upon delithiation. Our results highlight the importance of evaluating true implications of compositional tuning on high-Ni layered oxide cathode materials to maximize their charge-storage capacities for next-generation high-energy Li-ion batteries.

Generating solutions for charged geodesic anisotropic spherical collapse with shear and heat radiation

In this paper we give a generating function for solutions of the type of collapse mentioned in the title. It satisfies a simple Riccati equation, derived from a formula, which holds in the interior of the star. The generating function is unique, unlike in the neutral case. Every neutral solution has a charged companion with the same generating function. The charged solution has bigger radius, surface redshift, mass and compactness than the neutral one. Its rate of collapse is slower. A class of known exact neutral solutions, containing generalised travelling waves, is charged.

Towards conformally flat, non-Kasner vacuum solution in infinite derivative gravity

In this paper, we will show that the equations of motion of the quadratic in curvature, ghost free, infinite derivative theory of gravity will not permit an anisotropic collapse of a homogeneous Universe such as Kasner-type vacuum solution. However, such a solution is shown to be admissible in the local quadratic curvature gravity. Therefore pointing towards the crucial role played by the non-local infinite derivatives. We will show that it is possible to obtain conformally flat and singularity-free spacetime metrics.

Shape evolution of cometary nuclei via anisotropic mass loss

Context.Breathtaking imagery recorded during the European Space Agency Rosetta mission confirmed the bilobate nature of the nucleus of comet 67P/Churyumov-Gerasimenko. The peculiar appearance of the nucleus is not unique among comets. The majority of cometary cores imaged at high resolution exhibit a similar build. Various theories have been brought forward as to how cometary nuclei attain such peculiar shapes.Aims.We illustrate that anisotropic mass loss and local collapse of subsurface structures caused by non-uniform exposure of the nucleus to solar irradiation can transform initially spherical comet cores into bilobed cores.Methods.We derived a mathematical framework to describe the changes in morphology resulting from non-uniform insolation during the spin-orbit evolution of a nucleus. We solved the resulting partial differential equations that govern the change in the shape of a nucleus subject to mass loss and consequent collapse of depleted subsurface structures analytically for simple insolation configurations and numerically for more realistic scenarios.Results.The proposed mechanism is capable of explaining why a large percentage of periodic comets appear to have peanut-shaped cores and why light-curve amplitudes of comet nuclei are on average larger than those of typical main belt asteroids of the same size.