We investigate projectile electron-loss, target electron-loss, and target electron capture in H + H₂⁺ collisions using classical trajectory Monte Carlo (CTMC) and quantum-corrected classical trajectory Monte Carlo (QCTMC) methods within a fully four-body model. Energy-dependent cross sections are reported for all three channels. For projectile electron-loss, we additionally analyze impact-parameter-dependent probabilities and representative trajectories. The QCTMC method incorporates Heisenberg corrections, improving accuracy at low impact energies. Projectile and target electron-loss cross sections peak at intermediate energies and exhibit good agreement with H + H₂⁺ data at high energies. At low to intermediate energies, projectile electron-loss exceeds target electron-loss because the projectile electron experiences a stronger, unscreened Coulomb perturbation; both channels converge at high energies. Target electron capture displays a sharp low-energy maximum due to transient molecular-state formation, followed by a rapid decrease with energy. While CTMC and QCTMC results agree well at intermediate and high energies, QCTMC enhancements at low energies confirm the critical role of Heisenberg corrections. This study provides the first comprehensive four-body cross-section dataset for H + H₂⁺ collisions-distinguishing all three channels-which has been previously unavailable. These findings are directly relevant to fusion plasma modeling and nonequilibrium hydrogen plasmas.
Sunspots are cool regions where the magnetic field is significantly stronger than in the surrounding photosphere, and their darkest central regions are known as the umbra. Umbral dots (UDs) are small, bright features within the umbra that represent convective motions partially suppressed by strong magnetic fields. In this study, high-resolution images from the Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO) were used to analyze four sunspots at different evolutionary stages, including two developed and two undeveloped sunspots. The physical properties of UDs, including brightness, diameter, eccentricity, lifetime, and dynamic velocity, were measured and compared across these sunspots to evaluate the influence of magnetic field on UD behavior. The results show that undeveloped sunspots have a higher mean umbral brightness than developed ones, while UDs in developed sunspots exhibit longer lifetimes, and UDs in undeveloped sunspots move with higher dynamic velocity. Furthermore, relationships between UD physical properties are more stable in developed sunspots, indicating more tightly coupled physical parameters. These findings demonstrate that UD characteristics are strongly controlled by the magnetic field and evolutionary stage of the sunspot.
Machine learning interatomic potentials (MLIPs) provide an efficient approach to large-scale molecular dynamics simulations of high-entropy alloys. In this work, an MLIP trained on AIMD data was developed for the CrMnFeCoNi Cantor alloy and validated against DFT calculations. In addition, we compared MLIP results with those obtained using the classical modified embedded atom method (MEAM)-based interatomic potential for the CrMnFeCoNi Cantor alloy. The model achieved energy and force RMSEs of 7.39 x 10-6 meV/atom and 4.46 x 10-3 eV/A, respectively. Tensile simulations demonstrated that the MLIP accurately reproduced the stress–strain response, elastic constants, and deformation mechanisms observed in DFT calculations. The predicted maximum strain reached 0.104, compared with 0.098 obtained using the MEAM potential. This highlights the potential of MLIP to extend DFT accuracy to realistic large-scale simulations of complex HEA systems, enabling the efficient exploration of their mechanical performance and practical engineering applications.
Nonlinear magnetization dynamics in confined nanostructures provide a rich platform for fundamental studies and advanced spintronic applications. Here, we investigate the driven dynamics of a perpendicularly magnetized permalloy nanodisk using high-resolution micromagnetic simulations. In the linear regime, geometric confinement quantizes the uniform precession into discrete radial standing spin-wave (SSW) modes. As the excitation amplitude increases, the fundamental Kittel mode exhibits a pronounced nonlinear frequency shift relative to the higher-order SSW modes, ultimately leading to a critical mode crossing associated with nonlinear resonance overlap and the breakdown of modal orthogonality. Time-domain analysis reveals that strong mode hybridization governs the ensuing dynamics, giving rise to amplitude beating and delayed switching mediated by nonlinear bistability. In the frequency domain, phase-coherent, equidistant magnetic frequency combs emerge near the mode-crossing point, which we attribute to multimagnon scattering processes. At still higher drive amplitudes, these frequency combs are followed by a collapse of the modal structure. These results elucidate the role of nonlinear mode interactions in confined magnetic systems and establish magnetic nanodisks as highly tunable platforms for integrated microwave signal processing and precision metrology.
The recent trend of incorporating fractional derivatives into Lagrangians prompts a critical assessment of their physical viability. This work analyzes such theories through the lenses of classical mechanics and quantum field theory (QFT), revealing profound foundational issues. While the nonlocal nature of fractional operators circumvents Ostrogradsky's theorem, it introduces more severe problems. Classically, these include a complicated variational principle and the violation of energy conservation due to broken time-translation symmetry. In QFT, the pathologies are fatal: the propagator violates the K\"all\'en--Lehmann spectral representation, leading to acausal propagation, ghost states, complex poles, and a loss of unitarity. We conclude that Lagrangians with fractional derivatives in the kinetic term are fundamentally incompatible with the core principles of a physical, unitary quantum field theory.
