Brazilian Journal of Physics最新文献

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.

Determining whether a quantum state is separable or entangled—the separability problem—is a fundamental challenge in quantum information science. Despite decades of research yielding numerous separability criteria, the problem is known to be NP-hard in the general case, lacking an efficient, scalable solution for high-dimensional systems. In recent years, machine learning (ML) has emerged as a powerful, data-driven paradigm to tackle this challenge. By leveraging its capacity to identify subtle patterns and approximate complex functions from data, ML offers a promising computational alternative to purely analytical methods. In this work, we synthesize the diverse applications of machine learning to the entanglement problem. We systematically review the use of key ML models—such as Neural Networks, Support Vector Machines, and K-Nearest Neighbors— for two primary tasks: the binary classification of states (the separability problem) and the quantitative estimation of entanglement measures like concurrence and negativity.

This study uses Molecular Dynamics (MD) simulations to investigate how temperature, dimensions, and vacancy defects affect the mechanical properties of a planar net-τ nanotube. Non-equilibrium MD simulations are performed using the AIREBO interatomic potential, which is chosen for its ability to model bond breaking and long-range van der Waals interactions. The mechanical properties of interest consist of Young’s modulus, ultimate strength, fracture strain, strain at ultimate stress, stress-strain curve and fracture process. The results indicate that longer nanotubes exhibit increased stiffness due to enhanced interatomic bonding and fewer defects, leading to a slower change in elastic modulus with length. Zigzag configurations have higher elastic modulus than armchair but converge beyond 70 Å. Increasing radius raises the modulus, reaching about 650 GPa for armchair at 3 Å and 735 GPa for zigzag at 6 Å, indicating anisotropy. Higher temperatures reduce ductility, especially in zigzag configurations. Increasing defect percentage significantly reduces elastic modulus and ultimate stress, with armchair exceeding zigzag above 2.5% defects.

Inspired by string theory topics, we investigate the Reissner–Nordström–AdS black holes in noncommutative geometry with Lorentzian-smeared distributions. Concretely, we study certain thermodynamic properties including the criticality behaviors by computing the relevant quantities. For large radius approximations, we first derive the asymptotic expansions of the mass and charge functions appearing in the metric function of such black holes. Then, we approach the thermodynamical behavior in the extended phase space. After the stability discussion, we inspect the P–V criticality in noncommutative geometry by calculating the corresponding thermodynamic quantities. As a result, we show that the proposed black holes exhibit certain similarities with Van der Waals fluid systems. Finally, we present a discussion on the Joule–Thomson expansion showing perfect universality results appearing in charged AdS black holes.