Brazilian Journal of Physics最新文献

This study investigates the influence of a non-ideal external force on the complexity of attraction basins in a system of coupled Van der Pol oscillators. By introducing a phase-modulated force (varvec{Phi (t) = f}_{varvec{0}}, varvec{cos [omega t + a}_{varvec{0}}, varvec{sin (b}_{varvec{0}} varvec{omega t)]}), we analyze how parameters (varvec{a}_{varvec{0}}), (varvec{b}_{varvec{0}}), and (varvec{omega }) affect the system’s dynamics, particularly the structure and fractal properties of attraction basins. Using numerical simulations, we compute the topological entropy ((varvec{h}_{varvec{top}})) and uncertainty coefficient ((varvec{alpha })) to quantify boundary complexity and sensitivity to initial conditions. Our results reveal that variations in (varvec{omega }) induce transitions between regular and chaotic regimes, with peak entropy values ((varvec{h}_{varvec{top}} varvec{approx 5.95}) for (varvec{a}_{varvec{0}} varvec{= b}_{varvec{0}} varvec{= 0.5})) corresponding to the emergence of multiple attractors and fractal basin boundaries. These findings highlight the critical role of external forcing in controlling synchronization and bifurcations, with direct implications for applications such as cardiac pacemakers and robust control systems. The proposed metrics ((varvec{h}_{varvec{top}}), (varvec{alpha })) provide a robust framework for predicting dynamical transitions in nonlinear coupled oscillators.

The notion of electron spin emerges from the problematic of the anomalous Zeemann effect. As a strictly quantum property of electrons, the development of this concept was only possible through severe conceptual constraints, since the mechanism for producing magnetic dipole moments cannot be conceived without considering rigid-body dynamics, which, in this case, implied attributing spatial dimensions to the electron. Some degree of relativistic kinematics was also required to explain the electron’s intrinsic magnetism, requiring, however, adjustments to the Lorentz transformations to be applied to the non-inertial frame of reference of a self-rotating electron. Despite the obvious contradictions - the electron is, after all, point-like - the proposed solution to the problem accounted for the experimental data, although it led to a new puzzle. Paradoxically, the concept of electron spin emerged by applying classical knowledge to a context in which it could not be employed, except metaphorically.

In this work we study unitary symplectic representations of space-time symmetries associated with the Husimi function method. For the non-relativistic (Galilei) symmetry, a symplectic Schrödinger equation is derived. As application, cubic and quartic oscillators are addressed perturbatively. First-order corrected eigen functions for the ground and first excited states are obtained. Then the corresponding Husimi functions are computed. In the same context, the symplectic Klein-Gordon and Dirac equations are derived closely associated with a relativistic Husimi distribution. In this case, gauge symmetry is studied. As a basic result, gauge fields are accommodated within the context of the Husimi method. Specifically, the U(1)- gauge symmetry is preliminarly analyzed.

This work presents a comprehensive overview of variational quantum computing and their key role in advancing quantum simulation. This work explores the simulation of quantum systems and sets itself apart from approaches centered on classical data processing, by focusing on the critical role of quantum data in Variational Quantum Algorithms (VQA) and Quantum Machine Learning (QML). We systematically delineate the foundational principles of variational quantum computing, establish their motivational and challenges context within the noisy intermediate-scale quantum (NISQ) era, and critically examine their application across a range of prototypical quantum simulation problems. Operating within a hybrid quantum-classical framework, these algorithms represent a promising yet problem-dependent pathway whose practicality remains contingent on trainability and scalability under noise and barren-plateau constraints. This review serves to complement and extend existing literature by synthesizing the most recent advancements in the field and providing a focused perspective on the persistent challenges and emerging opportunities that define the current landscape of variational quantum computing for quantum simulation.

CdSe_1 − xTe_x (x = 0.0, 0.3, 0.6, and 1.0) thin films were grown on glass substrates by spray pyrolysis method. Numerous characterization techniques such as X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray (EDX), and UV-Vis spectrophotometer were employed to study the structural, morphological, compositional and optical analysis of the deposited CdSe_1 − xTe_x thin films. XRD data revealed that as the Te composition increases, there could be a phase transition from a CdSe-rich phase to a CdTe-rich phase or the formation of an alloy. The average crystallite size of the films was calculated using the Scherrer’s formula and varies from 16 nm to 24 nm. Scanning electron micrographs of CdSe_1 − xTe_x films demonstrated the uniform deposition of spherical-shaped grains. EDX analysis confirms the formation of CdSeTe thin films. The optical band gap values of deposited films were found to be decreased from 1.79 eV to 1.51 eV illustrating the potential viability for photovoltaic and optoelectronic devices.

In this work, we investigated the properties of an atmospheric pressure plasma jet (APPJ) generated using a (mathrm {Ar-H_2}) gas mixture. For this purpose, electrical, thermal, and optical characterization were employed to obtain discharge power and current, gas temperature ((T_g)), rotational and vibrational temperatures ((T_r) and (T_v), respectively), electron density ((n_e)), and chemical species formed in the gas phase of the plasma jet. All parameters were analyzed as a function of the (mathrm {H_2}) content in the gas mixture, for two different kinds of targets (a conductor and an insulator). Notably, large differences in discharge power, (T_g), (T_r), (T_v) and (n_e) were found with and without (mathrm {H_2}) in the gas composition. Additionally, as an example for the (mathrm {Ar-H_2}) gas mixture application, surface modification of a polymer was performed. As a result of the (mathrm {H_2}) addition, there was a slight improvement in the polymer surface composition compared to the condition without hydrogen.

This study presents a comparative investigation of the thermal stability and crystallization behavior of Fe₉₂Si₆C₂ and Fe₉₃Si₆C₁ amorphous ribbons. Differential Scanning Calorimetry (DSC) analysis revealed that the Fe₉₂Si₆C₂ ribbon exhibited higher crystallization onset (Tx ≈ 457 °C) and peak (Tp ≈ 488 °C) temperatures than Fe₉₃Si₆C₁. According to Thermogravimetric Analysis (TGA), the Fe₉₂Si₆C₂ sample also showed a delayed onset of mass loss (T_onset ≈ 520 °C) and a lower total mass loss (1.8%). A comparative assessment of both methods demonstrates that higher carbon content stabilizes the amorphous structure, suppressing crystallization and enhancing resistance to thermal oxidation. These results suggest that Fe₉₂Si₆C₂ ribbons are more suitable for use in high-temperature environments such as transformers, electromagnetic devices, and power conversion systems. The findings confirm that the thermal stability of amorphous materials can be effectively tuned by controlling their chemical composition.

Popular extensions of the standard model of particle physics feature new fields and symmetries which could, for example, dynamically generate neutrino masses from (B-L) spontaneous symmetry breaking. If a new light scalar that decays into dark radiation appears in the spectrum of the theory, it could significantly modify the cosmological observables. In this case, cold dark matter could have a stable and a decaying component and limits on its decay rate (Gamma _textrm{dcdm}) can be used to put constraints on the new energy scales of a given model. We illustrate this idea using a gauged (B-L) model where the dark radiation is in the form of light neutrinos.

Quantum dots are versatile systems for exploring quantum transport, electron correlations, and many-body phenomena such as the Kondo effect. While equilibrium properties are well understood through methods like the numerical renormalization group and density matrix renormalization group, nonequilibrium transport remains a major theoretical challenge. From the experimental point of view, recent advances in nanofabrication and measurement techniques have enabled the investigation of far-from-equilibrium regimes. These conditions give rise to new transport phenomena, where strong correlations and nonequilibrium dynamics interplay in complex ways — beyond the reach of conventional linear response theory. To meet these challenges, new approaches such as nonequilibrium Green’s functions, real-time NRG, and time-dependent DMRG have emerged. This work reviews the established results for quantum dot transport in and beyond the linear regime, highlights recent theoretical and experimental advances, and discusses open problems and future prospects.

The phase transition to the state of a phonon gas with pairwise correlations of interacting phonons with opposite momenta is studied. A method for describing such phonon systems within the framework of the self-consistent field model is developed and their thermodynamic characteristics are calculated. It is shown that a phonon gas with pair correlations can exist in a state of unstable thermodynamic equilibrium. The possibility of experimental observation of a solid in such a phase is discussed.