Traversable braneworld wormholes supported by astrophysical observations

In this study, we investigate the characteristics and properties of a traversable wormhole constrained by the current astrophysical observations in the framework of modified theories of gravity (MOG). As a concrete case, we study traversable wormhole space–time configurations in the Dvali–Gabadadze–Porrati (DGP) braneworld scenario, which are supported by the effects of the gravity leakage of extra dimensions. We find that the wormhole space–time structure will open in terms of the 2σ confidence level when we utilize the joint constraints supernovae (SNe) Ia + observational Hubble parameter data (OHD) + Planck + gravitational wave (GW) and z < 0:2874. Furthermore, we obtain several model-independent conclusions, such as (i) the exotic matter threading the wormholes can be divided into four classes during the evolutionary processes of the universe based on various energy conditions; (ii) we can offer a strict restriction to the local wormhole space–time structure by using the current astrophysical observations; and (iii) we can clearly identify a physical gravitational resource for the wormholes supported by astrophysical observations, namely the dark energy components of the universe or equivalent space–time curvature effects from MOG. Moreover, we find that the strong energy condition is always violated at low redshifts.     

On the improvement of signal repeatability in laser-induced air plasmas

The relatively low repeatability of laser-induced breakdown spectroscopy (LIBS) severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation (plasma evolution) and signal collection. Timeintegrated spectra and images were obtained under different laser energies and focal lengths to investigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 mm. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation (RSD) to increase with laser energies of > 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O(I), N(I), and H(I) lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.     

Exotic ferromagnetism in the two-dimensional quantum material C<Subscript>3</Subscript>N

The search for and study of exotic quantum states in novel low-dimensional quantum materials have triggered extensive research in recent years. Here, we systematically study the electronic and magnetic structures in the newly discovered two-dimensional quantum material C₃N within the framework of density functional theory. The calculations demonstrate that C₃N is an indirect-band semiconductor with an energy gap of 0.38 eV, which is in good agreement with experimental observations. Interestingly, we find van Hove singularities located at energies near the Fermi level, which is half that of graphene. Thus, the Fermi energy easily approaches that of the singularities, driving the system to ferromagnetism, under charge carrier injection, such as electric field gating or hydrogen doping. These findings not only demonstrate that the emergence of magnetism stems from the itinerant electron mechanism rather than the effects of local magnetic impurities, but also open a new avenue to designing field-effect transistor devices for possible realization of an insulator–ferromagnet transition by tuning an external electric field.     

Ultrathin nanosheets of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>: A new two-dimensional ferromagnetic material with strong magnetocrystalline anisotropy

Two-dimensional (2D) materials with robust ferromagnetism have played a key role in realizing nextgeneration spin-electronic devices, but many challenges remain, especially the lack of intrinsic ferromagnetic behavior in almost all 2D materials. Here, we highlight ultrathin Mn₃O₄ nanosheets as a new 2D ferromagnetic material with strong magnetocrystalline anisotropy. Magnetic measurements along the in-plane and out-of-plane directions confirm that the out-of-plane direction is the easy axis. The 2D-confined environment and Rashba-type spin-orbit coupling are thought to be responsible for the magnetocrystalline anisotropy. The robust ferromagnetism in 2D Mn₃O₄ nanosheets with magnetocrystalline anisotropy not only paves a new way for realizing the intrinsic ferromagnetic behavior in 2D materials but also provides a novel candidate for building next-generation spin-electronic devices.     

All-dielectric bowtie waveguide with deep subwavelength mode confinement

Plasmonic waveguides and conventional dielectric waveguides have favorable characteristics in photonic integrated circuits. Typically, plasmonic waveguides can provide subwavelength mode confinement, as shown by their small mode area, whereas conventional dielectric waveguides guide light with low loss, as shown by their long propagation length. However, the simultaneous achievement of subwavelength mode confinement and low-loss propagation remains limited. In this paper, we propose a novel design of an alldielectric bowtie waveguide, which simultaneously exhibits both subwavelength mode confinement and theoretically lossless propagation. Contrary to traditional dielectric waveguides, where the guidance of light is based on total internal reflection, the principle of the all-dielectric bowtie waveguide is based on the combined use of the conservation of the normal component of the electric displacement and the tangential component of the electric field, such that it can achieve a mode area comparable to its plasmonic counterparts. The mode distribution in the all-dielectric bowtie waveguide can be precisely controlled by manipulating the geometric design. Our work shows that it is possible to achieve extreme light confinement by using dielectric instead of lossy metals.     

The facile in situ preparation and characterization of C/FeOF/FeF<Subscript>3</Subscript> nanocomposites as LIB cathode materials

C/FeOF/FeF₃ nanocomposite was synthesized by a facile in situ partial oxidation method. High-resolution transmission electron microscopy (HR-TEM) showed a special texture comprised of interpenetrating nanodomains of FeOF and FeF₃. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements revealed that the introduction of nanodomain FeOF enhanced both the electronic and ionic conductivity of the composite material. Therefore, the improvement of electron and lithium-ion dynamics resulted in the significant enhancement of the electrochemical performances of the material at ambient temperature. At a current density of 20 mA g^?1 within potential range 1.5–4.5 V, the specific capacities of the first ten circles were maintained at about 400 mAh g^?1 _. This material also exhibited excellent cycling capacity retention capability especially for high C rates. When the current density further increased to 100 and 200 mA g^?1, a steady capacity of 80 and 60 mAh g^?1 was observed, respectively. Furthermore, nearly no capacity loss was observed for the followed cycles. The discharge platforms based on intercalation and conversion reaction were also heightened by about 0.4 V, which increased the contribution of high voltage capacities. Compared to C/FeF₃, C/FeOF/FeF₃ is showing more of capacitive behavior, which also contributes to the high specific capacity delivered and is believed to be closely related to the enlarged nanodomain interfaces between two electrochemical active materials. An expansion-cracking-oxidation mechanism was proposed to explain the formation of this interpenetrating nanodomains of FeOF and FeF₃.     

Engineering beneficial structures and morphologies of M-N-C oxygen-reduction catalysts derived from different metal-containing precursors

Morphologies and structures of M-N-C catalysts are the key factor for controlling the formation of catalytic active sites, which are directly connected with the electrocatalytic activities for oxygen reduction reaction (ORR). By combining different metal sources (metal-free, Co, and Fe) with polyaniline (PANI) and para-phenylenediamine functionalized GO (PGO), morphologies and structures are tuned to accelerate the ORR activity. Compared with metal-free catalyst, metal-containing catalysts show better ORR performance because of the possible synergistic effect between metal and N atoms. In particular, the improved ORR activity of Fe-PANI-PGO catalyst is obtained by rotating disc electrode (RDE) at 1600 rpm in 0.1 M KOH electrolyte. The Fe-PANI-PGO electrocatalyst has the enhanced half-wave potential of 0.89 V and the high stability with only decreasing 7 mV of half-wave potential after 10,000 cycles, implying increased number and strengthened structures of active sites. Combined with various means of characterization, advantageous morphologies and structures including large electrochemically active surface area, high graphitization degree, and thick carbon structure with more pyridinic nitrogen boned with metal atoms can greatly enhance the ORR activity and stability of the catalyst.

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Electrochemical properties of modified acetylene black/sulfur composite cathode material for lithium/sulfur batteries

Lithium/sulfur (Li/S) batteries have a high theoretical specific capacity of 1672 mAh g^?1. However, the insulation of the elemental sulfur and polysulfides dissolution could result in poor cycling performance of Li/S batteries, thus restricting the industrialization process. Here, we prepared sulfur-based composite by thermal treatment. The modified acetylene black (H-AB) was used as a carrier to fix sulfur. The H-AB could interact with polysulfides and reduce the dissolution of polysulfides in the electrolyte. Nonetheless, the conductivity of H-AB relatively reduced. So the conductivity of the sulfur electrode would be improved by the addition of the conductive agent (AB). In this paper, the different content of conductive agent (AB) in the sulfur electrode was studied. The electrochemical tests indicate that the discharge capacity of the sulfur electrode can be increased by increasing the conductive agent (AB) content. The H-AB@S composite electrode with 30 wt.% conductive agent has the best cycle property. The discharge capacity still remains at 563 mAh g^?1 after 100 cycles at 0.1 C, which is 71% retention of the highest discharge capacity.