Brazilian Journal of Physics最新文献

We investigate the properties of ion-acoustic waves (IAWs) in a magnetized plasma composed of cold ions and kappa-distributed electrons at two temperatures (cold and hot). The reductive perturbative method is implemented to derive the Davey-Stewartson (DS) equations. Through the analysis of the bifurcation theory, the effects of Coriolis force and superthermal parameter are addressed in the phase orbit analysis. Using numerical simulations, the impact of changes in various plasma parameters is discussed on the ion-acoustic periodic (IAP) and ion-acoustic super-periodic (IASP) nonlinear solutions of the DS equations. We further investigated the chaotic behaviors of the IAWs corresponding to the perturbed systems of the DS equations with the help of the time series plot, Poincaré section and Lyapunov exponent. In chaotic motions of perturbed IAWs via a quasi-periodic pathway to chaos, the frequency (omega ) and the amplitude of the external periodic disturbance (f_{0}) act as the switching parameters. This work may be relevant to understanding the dynamic of IAWs in astrophysical and space plasmas like in Saturn’s magnetosphere where the combined effects of high energy particles and the Coriolis force can exist.

In this study, we present a predator-prey model that incorporates a delay in the prey’s reproduction resulting from fear induced by predators. Next, we modify our model to a fractional-order system, incorporating the effects of memory. Establishing positivity and boundedness of the solutions demonstrates the well-posedness of the system. The local and global asymptotic stability of the positive equilibria are established under certain suitable parametric conditions. Additionally, we prove the existence and uniqueness of solutions for the fractional-order system while ensuring that they remain bounded. It is observed that, depending on constraints defined by the values of the model parameters, the breeding delay in the model system has both a stabilizing and destabilizing role in the system dynamics. The maximum length of delay that preserves the stability of the limit cycle is calculated. In the presence of delay, it is noticed that the fear factor in model system dynamics plays exactly the opposite role to that of the system without delay; more preciously, when the prey species delayed their breeding, fear acts as a destabilizing factor. We also consider the modified fractional order system to reveal the impact of the forgetting process on the system dynamics. Numerical simulations capture system dynamics and reveal that the delayed model system exhibits abundant dynamics, including several stability changes and chaotic behavior. Order of fractional derivative found to be involved in changing the stability property of the system near the coexistence equilibrium state.

The dynamics of electron–hole counter-streaming quantum semiconductor plasmas within an electrostatic framework is investigated by employing the quantum hydrodynamics (QHD) model, considering the effects of Bohm potential, exchange-correlation potential, and arbitrary degenerate pressure. The analysis covers both nearly degenerate and nearly non-degenerate scenarios, addressing distinct time-scale instabilities. Numerical investigations are carried out using typical parameter values for InP and GaN semiconductors to analyze the real frequency and growth rate of the two-stream instabilities. In both regimes, an inverse relationship is observed between species density and instability phase velocity. The system’s instability grows with increasing electron streaming velocity and shrinks with increasing hole streaming velocity. In nearly degenerate plasmas, growth rates are lower when species temperatures are equal compared to differing temperature ((T_{e}>T_{h})) and higher compared to (T_{e}<T_{h}). In nearly non-degenerate cases, temperature variation has a negligible effect on the growth rate, underscoring the dominance of other quantum effects. In both, the nearly degenerate and nearly non-degenerate regimes, the exchange-correlation potential enhances plasma instability, while tunneling recoil and degeneracy pressure significantly reduce instability at larger wave numbers. This comprehensive investigation provides valuable insights into the quantum behavior of semiconductor plasmas, informing applications in electronic devices and semiconductor physics.

This study explores the piezoelectric generator prototype, utilizing the PVA and graphene oxide composite for the first time, demonstrating superior energy conversion efficiency and power output under various mechanical stimuli, such as vibrations and pressure fluctuations. Graphene oxide and nanosheets were extracted by intercalating a 1 M HNO₃ solvent into the graphitic layers. This process involved applying a voltage bias of 2 V DC for 5 min, followed by increasing the voltage to 6 V DC for 5 h. Graphene oxide is identified via X-ray diffraction (XRD), ultraviolet–visible spectroscopy (UV–VIS), and Fourier transform infrared spectroscopy (FTIR). PVA typically peaks at around 2θ = 19.9°, which is the (101) plane. The direct energy bandgaps of the produced composite PVA/GO at 1 wt.% and 2 wt.% are 3.681 eV and 3.231 eV, respectively. FTIR spectra analysis of the PVA/graphene oxide composite reveals characteristic peaks from both components, indicating successful incorporation. The fabricated device, featuring a sandwich structure, exhibits superior performance with an impressive output voltage of approximately 15 Vpp. This device could be very effective in energy-harvesting applications. EDAX analysis shows that the carbon-to-oxygen ratio of approximately 3.86:1 is typical of well-oxidized graphene oxide, indicating a high oxidation level.

This study presents a comprehensive machine learning (ML) investigation into the design of ytterbium (Yb)–based materials with tunable bandgaps and enhanced stability. A dataset of 2062 Yb-based compounds was compiled from the literature, featuring structural and electronic properties. Their descriptors are designed to calculate their bandgap values through quantum chemical studies. Out of various machine learning models, Random Forest and Decision Tree regression models produce the more accurate prediction for their predicted values. The feature importance analysis reveals that their HeavyAtomCount, AliphaticRings, Ar_Rings, and TPSA emerge as important descriptors to influence their bandgap and stability. Their stability analysis by their Convex Hull Diagrams identifies 73% of data within its stability boundary. Further insights are gained through their clustering analysis, utilizing K-means clustering (k = 4), t-SNE, and elbow methods. The results show 85% accuracy to predict their stability and reveal distinct their chemical profiles as stable, moderately stable, and less stable compounds. The current study design approach is supposed to enable the discovery of Yb-based compounds with their tailored properties, like their tunable bandgaps (0.5–4.5 eV) with their enhanced stability (> 70%). The study can provide insights for materials scientists to identify promising optoelectronic materials.

In this work, plasma and magnetic field data from Venus Express (VEX) were analyzed through the entire mission span (2006-2014) in order to statistically study wave mode occurrence around Venus considering different solar wind conditions. The wave mode occurrence distribution in terms of solar activity (maxima and minimum periods), solar wind pressure, and interplanetary magnetic field (IMF) east-west component orientation ((B_{y}>0) and (B_{y}<0)) were computed. Our results reveal that mirror mode occurrence does not seems to be affected by changes in the analyzed parameters. The most significant influence was observed in the Alfvén mode by the solar wind dynamic pressure, which has its occurrence rate clearly increased, during periods of high solar wind dynamic pressure. The fast mode seems to be influenced by the three analyzed parameters and tends to occur more often during periods of positive IMF (B_{y}) and high solar activity. Further, a higher occurrence of fast wave mode during enhanced dynamic pressure was observed, which may be related to KH instability.

The CP violation problem has a long history emanating from its discovery 60 years ago in the decay of neutral kaons and subsequent experimental and theoretical studies over several decades. We review herein experimental data that observe indirect CP violation of the order of (sim 10^{-3}), as well as the discovery of direct CP violation of the order of (sim 10^{-6}). Despite improved experimental methods over the past half a century, the original CP violation numbers have remained the same. Verification of the CP violation in the decay of charged kaons was also observed. Data reflecting CP violation in the decays of B and D mesons have become very important and are also discussed in this review. The question of CP violation only with the participation of (sbar{s}), (cbar{c}), and (bbar{b}) quarks in the framework of the Standard Model or beyond it and its small magnitude remains open.

Proposed manuscript includes the implementation of all-optical transmission gate employing silicon microring resonator. A replacement for traditional CMOS technology, large-scale optical integrated circuits are being considered due to the increasing need for ultra-fast terahertz data processing and transfer. In addition, energy-efficient circuits are becoming more and more crucial. The architecture is analyzed at about 260 Gbps using MATLAB. Networks for optical signal processing can be constructed using all-optical transmission gates as building blocks. Proposed transmission gate is also utilized in the design of a 2:1 multiplexer. The evaluation of a few performance-indicating variables includes “extinction ratio (ER)”, “contrast ratio (CR)”, “amplitude modulation (AM)”, “on–off ratio (OOR)”, and “relative eye opening”. At low average pump power of 1.18 mW, proposed model has high ER, CR, and OOR of 26.78, 31.39, and 36.8 dB, respectively. Computed AM is recorded as 0.22 dB. Optimized design parameters are chosen to utilize the design practically.