Long-range magnetic order in graphene has been the subject of study for many researches due to its fundamental and technological prospects. Many theoretical researches have predicted the long-range ...ferromagnetic exchange in porphyrin-like graphene due to doping the transition metals centers in their structures. However, the experimental observations for such structures, have been rarely reported. In the present work the ferromagnetic properties of iron doped porphyrin-like graphene (Fe–N-Gr) are experimentally considered and the results are reported. At first, the Fe–N-Gr sample is synthesized by mechanochemical mixing of Fe, N, and C sources followed by high temperature pyrolysis treatment. The sample shows the high saturation magnetization of 140 emu/gr at room temperature. Raman, XRD, and XPS results as well as microscopic images revealed that the observed ferromagnetic behavior is induced by doped Fe–N sites in graphene, however the exact microscopic origin cannot be explained and further investigation is needed.
These results pave the way for the synthesis of magnetic porphyrin-like graphene with structurally deformed structure.
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•The magnetic properties of Fe/N doped graphene lattice was investigated.•Fe/N doped graphene shows the high saturation magnetization of 140 emu/gr at RT.•The long-range ferromagnetic order in graphene creates ferromagnetic behavior.
This study focused on the microscopic strengthening and failure mechanisms of the Al matrix reinforced by graphene (Gr), and the effects of number of layers, chirality, and arrangement of Gr were ...calculated based on the molecular dynamics simulation. The results revealed that the Young's modulus and yield strength were significantly enhanced by the addition of Gr. In Gr/Al composites with monolayer Gr, the zigzag Gr exhibited a better ultimate strain than the armchair Gr, indicating that the plastic deformation was affected by the chirality, and the dislocation hindrance and load transfer were the dominated strengthening mechanisms. The crack in the armchair Gr was limited to follow straight paths, while that in zigzag Gr extended in a petal-like manner across multiple directions. In Gr/Al composite with multilayer Gr, the dispersing Gr showed a stronger strengthening effect than the stacking Gr, and the strengthening effect increased with increasing the volume fraction of Gr. The dispersing Gr strongly hindered the movement of dislocations, while the Al matrix in the composite with stacking Gr could retard the folding of Gr. Moreover, Gr nanosheets fractured layer by layer rather than simultaneously fractured in the Gr/Al composites with multilayer Gr.
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•The distinct strengthening of zigzag and armchair Gr is caused by the difference in the way of carrying stress.•The crack in armchair Gr followed straight paths, while that in zigzag Gr extended in a petal-like manner.•Dispersing Gr showed stronger strengthening than stacking Gr in Gr/Al composites with multilayer Gr.•Gr with good interface bonding and capacity of load transfer strongly inhibited dislocation movement.
This review summarizes the advancement on sintered composites based on tungsten–copper (W–Cu) reinforced with graphene (Gr) or its derivative such as reduced Gr oxide (rGO) for use as electrical ...contacts applied in switching devices. Main synthesis approaches for preparing Gr or rGO reinforced W–Cu composite powders and their consolidations by using various powder metallurgy techniques are presented. The nature of the initial materials, synthesis conditions, processing parameters, and the relevant findings are disclosed and discussed. The improvement in microstructure and technical characteristics like density, electrical conductivity, hardness, coefficient of friction, wear rate, and arc ablation behavior of W–Cu–Gr/rGO composites are highlighted comparatively with that of unreinforced composites. This review reveals an insight into a novel class of composites as candidates for electrical contact applications.
Abstract The inspiral merger time for two black holes captured into a nonrelativistic bound orbit by gravitational radiation emission has been often calculated by a formula of Peters that assumes the ...adiabatic approximation that the changes per orbit are small. However, initially this is not true for the semimajor axis and period of most of the initially highly eccentric orbits, which change significantly during closest approach and much less elsewhere along the orbit. This effect can make the merger time much longer (using other formulas from Peters that do not assume the adiabatic approximation) than that calculated by the adiabatic formula of Peters.
There has recently been renewed interest in the possibility that the dark matter in the Universe consists of primordial black holes (PBHs). Current observational constraints leave only a few PBH mass ...ranges for this possibility. One of them is around 10^{-12} M_{⊙}. If PBHs with this mass are formed due to an enhanced scalar-perturbation amplitude, their formation is inevitably accompanied by the generation of gravitational waves (GWs) with frequency peaked in the mHz range, precisely around the maximum sensitivity of the LISA mission. We show that, if these primordial black holes are the dark matter, LISA will be able to detect the associated GW power spectrum. Although the GW source signal is intrinsically non-Gaussian, the signal measured by LISA is a sum of the signal from a large number of independent sources suppressing the non-Gaussianity at detection to an unobservable level. We also discuss the effect of the GW propagation in the perturbed Universe. PBH dark matter generically leads to a detectable, purely isotropic, Gaussian and unpolarized GW signal, a prediction that is testable with LISA.
Abstract
In Atazadeh and Hadi (
JCAP
01
(2024) 067), the authors proposed that black bounce solutions, such as the Simpson-Visser and the Bardeen-type spacetimes, can be obtained from Rastall ...gravity. To achieve these spacetimes, the authors consider the presence of a phantom scalar field with nonlinear electrodynamics. However, in this comment, we obtained different electromagnetic Lagrangians from the original work. The most problematic issue is not the incorrect expression of the electromagnetic Lagrangian itself. We show that the method obtains electromagnetic functions that are inconsistent. To obtain the black bounce spacetimes as solutions of Rastall gravity, it is necessary to consider an isotropic fluid, combined with the nonlinear electrodynamics and the phantom scalar field.
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By conducting first-principles calculations with van der Waals correction, we investigate the structural, electronic, and magnetic properties of Gr/MnF3(4)/Gr sandwiched structures. ...The calculated binding energies were within 54–79meV per carbon atom for the four Gr/MnF3(4)/Gr sandwiched structures tested in our study. This result implies that the MnF3(4) clusters stabilize the AA and AB stacking in the Gr/MnF3(4)/Gr sandwich structures because of the high electronegativity of the clusters. Charge transfer from bilayer graphene to the MnF3(4) clusters is noted to induce effective hole doping in graphene. Spin polarization in the graphene layer is approximately 100% along the majority direction on the position of the Fermi level in the MnF3-doped structures. The band gaps of the spin-up and spin-down branches and the magnetic moments of the C atoms are increased by compression interlayer spacing. The MnF3 clusters affect the electronic and magnetic properties of graphene in the vicinity of the Fermi level; thus, the clusters are highly suitable for use as a dopant in spintronic applications.
Black-bounce to traversable wormhole Simpson, Alex; Visser, Matt
Journal of cosmology and astroparticle physics,
02/2019, Letnik:
2019, Številka:
2
Journal Article
Recenzirano
So-called “regular black holes” are a topic currently of considerable interest in the general relativity and astrophysics communities. Herein we investigate a particularly interesting regular black ...hole spacetime described by the line element This spacetime neatly interpolates between the standard Schwarzschild black hole and the Morris-Thorne traversable wormhole; at intermediate stages passing through a black-bounce (into a future incarnation of the universe), an extremal null-bounce (into a future incarnation of the universe), and a traversable wormhole. As long as the parameter a is non-zero the geometry is everywhere regular, so one has a somewhat unusual form of “regular black hole”, where the “origin” r=0 can be either spacelike, null, or timelike. Thus this spacetime generalizes and broadens the class of “regular black holes” beyond those usually considered.
Ultralight bosons and axion-like particles appear naturally in different scenarios and could solve some long-standing puzzles. Their detection is challenging, and all direct methods hinge on unknown ...couplings to the Standard Model of particle physics. However, the universal coupling to gravity provides model-independent signatures for these fields. We explore here the superradiant instability of spinning black holes triggered in the presence of such fields. The instability taps angular momentum from and limits the maximum spin of astrophysical black holes. We compute, for the first time, the spectrum of the most unstable modes of a massive vector (Proca) field for generic black-hole spin and Proca mass. The observed stability of the inner disk of stellar-mass black holes can be used to derive direct constraints on the mass of dark photons in the mass range 10 −13 eV≲ m V ≲ 3× 10 −12 eV. By including also higher azimuthal modes, similar constraints apply to axion-like particles in the mass range 6×10 −13 eV≲ m ALP ≲ 10 −11 eV. Likewise, mass and spin distributions of supermassive BHs—as measured through continuum fitting, Kα iron line, or with the future space-based gravitational-wave detector LISA – imply indirect bounds in the mass range approximately 10 −19 eV≲ m V , m ALP ≲ 10 −13 eV, for both axion-like particles and dark photons. Overall, superradiance allows to explore a region of approximately 8 orders of magnitude in the mass of ultralight bosons.
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