Search Results (258)

We consider a model problem of polarization-dependent light bending and time delays in the vicinity of neutron stars endowed with magnetar-strength magnetic fields (B∼${10}^{15}\mathrm{G}$), combining an effective-metric formulation of Heisenberg–Euler nonlinear electrodynamics with general-relativistic ray tracing. The spacetime geometry is analyzed using both the Kerr metric and a quadrupole-deformed q-metric, characterized by a quadrupole parameter varying in the range $q\in [{10}^{-3},0.5]$. In addition, the impact of complex magnetic-field topologies is examined by introducing a magnetic quadrupole component alongside the dipole configuration. The simulations performed in this study demonstrate that the inclusion of the quadrupole deformation parameter significantly modifies photon trajectory deflections compared to the standard Kerr solution. We further quantify the geometric dilution of the photon beam, finding a cross-section expansion ratio of approximately $4.7\times {10}^{13}$ for rays reaching Earth. This strong dilution imposes stringent constraints on the detectability of polarization-dependent signatures and time-delay echoes. Finally, characteristic illustrations are presented for trajectory distortions, bending-angle distributions, and intensity valleys produced by the combined gravitational and magnetic lensing effects. Full article

Microwave ablation, as a minimally invasive technique used for the treatment of tumors, is highly dependent on the performance of ablation antennas for its therapeutic effect. Clinically, antennas are required to form roughly spherical ablation zones with sufficient volume within a limited time. To meet this requirement, this paper establishes finite element models and conducts multi-objective optimization on fully water-cooled dipole antenna and partially water-cooled choke dipole antenna based on different water-cooled structures. On the premise of minimizing reflection coefficient and maximizing ablation volume, a three-dimensional objective space is constructed by introducing the minimization of roundness error, and the set of Pareto solutions is solved. The CRITIC-TOPSIS method is used to balance multi-objective conflicts and select the unique optimal solution from the Pareto set. By analyzing the optimal solution, simulation results show that the optimized antennas can effectively form near-spherical ablation shapes while minimizing the reflection coefficient and maximizing the ablation volume. Among these, the partially water-cooled antenna exhibits superior electromagnetic characteristics and ablation profile, whereas the fully water-cooled antenna demonstrates better temperature field behavior. Full article

We present a comprehensive computational study of the UV-visible absorption spectra of 7-methoxycoumarin and Nile red in aqueous solution. Our fully atomistic workflow couples classical molecular dynamics (MD) with polarizable QM/MM based on fluctuating charges (QM/FQ) and dipoles (QM/FQF$\mu$). Ensemble-averaged spectra are constructed from the snapshots extracted from the MD, embedding solvent fluctuations and specific solute–solvent interactions in the electronic response of organic dyes. The spectral profiles, obtained at the various levels, reflect the underlying solute–solvent interactions and dynamics, and we rationalize them in terms of hydrogen bonding and frontier molecular orbitals involved in the main electronic transitions. Finally, the simulated spectra and solvatochromic shifts are compared with the available experimental data, showing an overall good agreement and demonstrating the robustness of the computational protocol. Full article

A-site-ordered quadruple perovskite manganites, AMn₇O₁₂, show many interesting physical phenomena, including orbital and spin modulations, spin-induced multiferroic properties, and competitions between different magnetic ground states. Doping with Cu²⁺ can result in colossal magnetoresistance properties, ferrimagnetism, and additional structural modulations producing electric–dipole helicoidal textures. Many previous works have focused on large-concentration doping, reaching ACu₃Mn₄O₁₂ compositions. Small-concentration doping has been investigated in a limited number of systems, e.g., in BiCu_xMn_7−xO₁₂. In this work, we investigated solid solutions of NdCu_xMn_7−xO₁₂ with x = 0.1, 0.2, and 0.3, prepared at 6 GPa and 1500 K. Specific heat measurements detected three magnetic transitions at x = 0 (at T_N3 = 9 K, T_N2 = 12 K, and T_N1 = 84 K) and two transitions at x = 0.1 (at T_N2 = 10 K and T_N1 = 78 K), while only one transition was found at x = 0.2 (T_N1 = 72 K) and x = 0.3 (T_N1 = 65 K). Differential scanning calorimetry (DSC) measurements showed sharp and strong peaks near T_OO = 664 K at x = 0, corresponding to an orbital-order (OO) structural transition from I2/m to Im-3 symmetry. DSC anomalies were significantly broadened and their intensities were significantly reduced at x = 0.1–0.3, and structural transitions were observed near T_OO = 630 K at x = 0.1, T_OO = 600 K at x = 0.2, and T_OO = 570 K at x = 0.3. The x = 0.1 sample clearly showed double-peak features on the DSC curves near T_OO because of the presence of two close phases. High-resolution synchrotron powder X-ray diffraction studies gave strong evidence that phase separation phenomena took place in the x = 0.1–0.3 samples, where two I2/m phases with an approximate ratio of 1:1 were present (e.g., a = 7.47143 Å, b = 7.36828 Å, c = 7.46210 Å, and β = 90.9929° for one phase and a = 7.46596 Å, b = 7.37257 Å, c = 7.45756 Å, and β = 90.9328° for the second phase at x = 0.3). The Curie–Weiss temperature changed from negative (for x = 0, 0.1, and 0.2) to positive (for x = 0.3). T_OO, T_N1, the Curie–Weiss temperature, and magnetization (at 5 K and 70 kOe) changed almost linearly with x. Full article

The rapid accumulation of space debris poses a serious threat to operational spacecraft, with the capture and removal of rapidly tumbling non-cooperative targets being a primary challenge. Non-contact electromagnetic de-tumbling technology is a promising solution due to its enhanced safety. This paper addresses the issue of torque modeling and validation in the electromagnetic de-tumbling process for a specific configuration involving a magnetic dipole and a spherical shell under a symmetrically distributed magnetic field. Based on the principles of electromagnetic induction, an approximate analytical expression for the electromagnetic eddy current torque on a rotating spherical shell within a dipole magnetic field is first derived. A high-fidelity finite element model is then established, which reveals a systematic discrepancy between the initial theoretical model and numerical simulation results. A distance-dependent power-law correction factor is introduced to calibrate the theoretical model, significantly improving its accuracy and reducing the average error to 1.5 percent. Finally, a ground-based experimental platform is designed and implemented. The experimental results demonstrate that the corrected approximate analytical model agrees well with the empirical data, verifying its validity and accuracy under the given conditions and providing a reliable theoretical basis for the design of future space debris de-tumbling controllers. Full article

Giant dielectric oxides are attractive for next-generation capacitors and related applications, but their practical use is limited by high loss tangent (tanδ), strong temperature dependence of dielectric permittivity (ε′), and the need for energy-intensive high-temperature sintering. To address these challenges, this study focuses on the development of (In_0.5Ta_0.5)_xTi_1−xO₂ (ITTO, x = 0.02–0.06) ceramics via a green egg-white solution route, targeting high dielectric performance at reduced processing temperatures. The as-calcined powders exhibited the anatase TiO₂ phase with particle sizes of ~20–50 nm. These powders promoted densification at a sintering temperature of 1300 °C, significantly lower than those of conventional co-doped TiO₂ systems. The resulting ceramics exhibited refined grains, high relative density, and homogeneous dopant incorporation, as confirmed by XRD, SEM/TEM, EDS mapping, and XPS. Complementary density functional theory calculations were performed to examine the stability of In³⁺/Ta⁵⁺ defect clusters and their role in electron-pinned defect dipoles (EPDDs). The optimized ceramic (x = 0.06, 1300 °C) achieved a high ε′ of 6.78 × 10³, a low tanδ of 0.038, and excellent thermal stability with Δε′ < 3.9% from 30 to 200 °C. These results demonstrate that the giant dielectric response originates primarily from EPDDs associated with Ti³⁺ species and oxygen vacancies, in agreement with both experimental and theoretical evidence. These findings emphasize the potential of eco-friendly synthesis routes combined with rational defect engineering to deliver high-performance dielectric ceramics with reliable thermal stability at reduced sintering temperatures. Full article

This study proposes a semi-analytic harmonic modeling method that significantly improves the accuracy and efficiency of complex magnetic field modeling by integrating numerical and analytical approaches. Compared to traditional methods such as the equivalent charge method and finite element method, this approach optimizes the distribution of surface and body charges in the magnetic dipole model and introduces a finite and variable permeability model to accommodate material non-uniformity. Through harmonic expansion and analytical optimization, the method more accurately reflects the characteristics of real magnets, providing an efficient and precise solution for complex magnetic field problems, particularly in the design of high-performance magnets such as Halbach arrays. In this study, the effectiveness of the new modeling method is verified through the combination of simulation and experiment: the magnetic field distribution of the new Halbach array is accurately simulated, and the applicability of the model in the description of complex magnetic fields is analyzed. The dynamic response ability of the optimized model is verified by modeling and simulating the variation of the permeability under actual conditions. The distribution of scalar potential energy with permeability was simulated to evaluate the adaptability of the model to the real physical field. Through the comparative analysis of simulation and experimental results, the advantages of the new method in modeling accuracy and efficiency are clearly pointed out, and the effectiveness of the semi-analytic harmonic modeling method and its wide application potential in the design of new magnetic fields are proved. In this study, a semi-analytic harmonic modeling method is proposed by combining numerical and analytical methods, which breaks through the efficiency bottleneck of traditional modeling methods, and achieves the unity of high precision and high efficiency in the magnetic field modeling of the new Halbach array, providing a new solution for the study of complex magnetic field problems. Full article

Gallium oxide (Ga₂O₃) is a promising ultra-wide-bandgap (UWBG) material with exceptional transport properties, including a large breakdown voltage, making it ideal for high-voltage power device applications. Recently, Ga₂O₃ has gained significant attention as a next-generation material for electronic device fabrication aimed at advancing power electronics. In this paper, we investigate the effect of a Ga₂O₃ buffer layer on a GaN-based high electron mobility transistor (HEMT), focusing on output I–V characteristics and surface charge effects. Furthermore, we explore an advanced approach to enhance HEMT performance by utilizing polarization-induced two-dimensional electron gas (2DEG), as an alternative to conventional doping methods. A III-N/Ga₂O₃ heterostructure is proposed as a distinctive electrical property and a cost-effective UWBG solution. To evaluate the associated effects, we simulate a two-dimensional (2D) Ga₂O₃/GaN HEMT structure incorporating surface charge models. Our results confirm that 2DEG formation near the surface creates a conductive channel due to polarization-induced dipoles at the interface. The simulations also show a negative shift in the threshold voltage, a condition typically unattainable without oxidation layers or doping. Finally, we analyze the potential of AlGaN/Ga₂O₃-based HEMTs for future power electronic applications. Full article

This paper shows that low concentrations of either a low-molecular-weight or a recyclable polymeric cosolvent can be used to design recyclable, tunable alkane polymeric solvent systems. We have shown that dyes experience a microheterogeneous environment that is ca. 40–50% like that of a polar solvent with as little as 0.1 M added cosolvent. Dyes like Nile red or a polyisobutylene (PIB)-bound dansyl fluorophore both detected microheterogeneity in macrohomogeneous mixtures of heptane or a poly(α-olefin) (PAO) with 0.1–2.0 M added polar solvents. H-Bonding cosolvents have greater effects than cosolvents that only interact with dyes by dipole–dipole interactions. Microheterogeneity was also seen when a PIB-bound version of a low-molecular-weight solvent is used as the added polar cosolvent. These microheterogeneous environments can advantageously be used in synthetic and catalytic reactions. This was demonstrated in transesterification and S_N2 chemistry. Reactions in PAO solutions polarized by 2 M added THF or by 0.5 M of a PIB-bound HMPA analog both had enhanced reactivity versus reactions in a PAO solution without added cosolvent. In the latter case, the catalyst, the PAO solvent, and the PIB-bound cosolvent were all fully recyclable. Full article

The substation grounding grid, as the primary path for fault current dissipation, is crucial for ensuring the safe operation of the power system and requires regular inspection. The pulsed eddy current method, known for its non-destructive and efficient features, is widely used in grounding grid detection. However, during the parameter identification process, it is prone to local minima or no solution. To address this issue, this paper first develops a pulsed eddy current forward response model for the substation grounding grid based on the magnetic dipole superposition principle, with accuracy validation. Then, a variable dimensional Bayesian parameter identification method is introduced, utilizing the Reversible-Jump Markov Chain Monte Carlo (RJMCMC) algorithm. By using nonlinear optimization results as the initial model and introducing a dual-factor control strategy to dynamically adjust the sampling step size, the model enhances coverage of high-probability regions, enabling effective estimation of grounding grid parameter uncertainties. Finally, the proposed method is validated by comparing the forward response model with field test results, showing that the error is within 10%, demonstrating both the accuracy and practical applicability of the proposed parameter identification method. Full article

Electromagnetic wave pollution has become a growing concern in recent decades. Biomass-derived carbon materials have attracted significant attention as wave-absorbing materials due to their easy availability, low cost, and environmental friendliness. In this study, the invasive plant Solidago canadensis (Canada goldenrod) in China was used as the carbon source, and a two-step pyrolysis and hydrothermal process was applied to create a porous composite material with magnetic CoFe₂O₄ particles. This improved the impedance matching of the biomass carbon and introduced multiple loss mechanisms. The combination of magnetic loss, interfacial polarization, dipole polarization, and multiple reflections in the biomass carbon produced a material with excellent microwave absorption properties. At 16.76 GHz with a thickness of 2.5 mm, the material achieved a minimum reflection loss of −35.21 dB and an effective absorption bandwidth of 7.76 GHz. This study presents a promising method for developing biomass-based absorbers and offers an efficient, cost-effective, and environmentally friendly solution for managing invasive species. Full article

Unmanned Aerial Vehicles (UAVs), commonly known as drones, are becoming increasingly important in multiple areas and various applications, including communication, detection, and monitoring. This review paper examines the development of antennas for UAVs, with a particular focus on miniaturization techniques, polarization strategies, and beamforming solutions. It explores both structural and material-based methods, such as meander lines, slots, high-dielectric substrates, and metasurfaces, which aim to make the antenna more compact without compromising performance. Different antenna types including dipole, monopole, horn, vivaldi, and microstrip patch are explored to identify solutions that meet performance standards while respecting UAV constraints. In terms of polarization strategies, these are often implemented in the feeding network to achieve linear or circular polarization, and beamforming techniques like beam-steering and beam-switching enhance communication efficiency by improving signal directionality. Future research should focus on more lightweight, structurally integrated, and reconfigurable apertures that push miniaturization through conformal substrates and programmable metasurfaces, extending efficient operation from 5/6 GHz into the sub-THz regime and supporting agile beamforming for dense UAV swarms. Full article

This research focuses on designing polymer membranes as biocompatible materials using home-built electrospinning equipment, offering alternative solutions for tissue regeneration applications. This technological development supports cell growth on biomaterial substrates, including hepatocellular carcinoma (Hep-G2) cells. This work researches the compatibility of polymer membranes (fiber mats) made of polyvinylidene difluoride (PVDF) for possible use in cellular engineering. A standard culture medium was employed to support the proliferation of Hep-G2 cells under controlled conditions (37 °C, 4.8% CO₂, and 100% relative humidity). Subsequently, after the incubation period, electrochemical impedance spectroscopy (EIS) assays were conducted in a physiological environment to characterize the electrical cellular response, providing insights into the biocompatibility of the material. Scanning electron microscopy (SEM) was employed to evaluate cell adhesion, morphology, and growth on the PVDF polymer membranes. The results suggest that PVDF polymer membranes can be successfully produced through electrospinning technology, resulting in the formation of a dipole structure, including the possible presence of a polar β-phase, contributing to piezoelectric activity. EIS measurements, based on R_ct and C_dl values, are indicators of ion charge transfer and strong electrical interactions at the membrane interface. These findings suggest a favorable environment for cell proliferation, thereby enhancing cellular interactions at the fiber interface within the electrolyte. SEM observations displayed a consistent distribution of fibers with a distinctive spherical agglomeration on the entire PVDF surface. Finally, integrating piezoelectric properties into cell culture systems provides new opportunities for investigating the influence of electrical interactions on cellular behavior through electrochemical techniques. Based on the experimental results, this electrospun polymer demonstrates great potential as a promising candidate for next-generation biomaterials, with a probable application in tissue regeneration. Full article

A four-vector potential of an external test electromagnetic field in a Schwarzschild background is described in terms of a combination of dipole and quadrupole magnetic fields. This combination is an interior solution of the source-free Maxwell equations. Such external test magnetic fields cause the dynamics of charged particles around the black hole to be nonintegrable, and are mainly responsible for chaotic dynamics of charged particles. In addition to the external magnetic fields, some circumstances should be required for the onset of chaos. The effect of the magnetic fields on chaos is shown clearly through an explicit symplectic integrator and a fast Lyapunov indicator. The inclusion of the quadrupole magnetic fields easily induces chaos, compared with that of the dipole magnetic fields. This result is because the Lorentz forces from the quadrupole magnetic fields are larger than those from the dipole magnetic fields. In addition, the Lorentz forces act as attractive forces, which are helpful for bringing the occurrence of chaos in the nonintegrable case. Full article

Synchronous fluorescence spectra are presented to investigate solute–solvent interactions in liquids. To this end, the spectra of 2-amino-7-nitro-fluorene (ANF) in six different solvents—acetic anhydride, acetone, acetonitrile, benzene, chlorobenzene, and ethyl acetate—are presented. The study also examines ANF’s synchronous fluorescence signals at five temperatures from 25 °C to 5 °C, providing a comprehensive analysis of its fluorescence characteristics in different environments and temperatures. An ANF sample dissolved in benzene at 5 °C produced a synchronous band with the largest intensity and smallest frequency shift. The results show that higher-intensity peaks are obtained at lower temperatures with solvents with a small dipole moment and dielectric constant. This suggest that the best conditions to detect ANF and similar molecules at very low concentrations are with non-polar solvents at low temperatures. Full article