Brazilian Journal of Physics最新文献

An annealed version of the quenched mean-field model for epidemic spread is introduced and investigated analytically and assisted by numerical calculations. The interaction between individuals follows a prescription that is used to generate a scale-free network, and we have adjusted the number of connections to produce a sparse network. Specifically, the model’s behavior near the infection threshold is examined, as well as the behavior of the stationary prevalence and the probability that a connection between individuals encounters an infected one. We found that these functions display a monotonically increasing dependence on the infection rate. Subsequently, a modification that mimics the mitigation in the probability of encountering an infected individual is introduced, following an old idea rooted in the Malthus-Verhulst model. We found that this modification drastically changes the probability that a connection meets an infected individual. However, despite this change, it does not alter the monotonically increasing behavior of the stationary prevalence.

This research is aimed at investigating the potential of using fluorescent dyes, specifically fluorescein sodium salt dye (FSSD), embedded within epoxy resin (EP) as a material for luminescent solar concentrators (LSCs). LSCs are devices that capture and concentrate sunlight onto a smaller area, increasing the efficiency of solar cells. The study revealed a relationship between FSSD at different concentrations and EP molecules. The FSSD might be well-dispersed on a molecular level within the EP matrix. This homogeneous distribution is supported by the lack of distinct peaks in the XRD pattern, indicating the absence of any ordered crystal structure of FSSD within the composite. FSSD significantly influences the optical properties of the EP. It reduces light transmittance while increasing absorbance at specific wavelengths, which is crucial for efficient light capture in LSCs. The high polarizability of FSSD molecules and their random orientation can give rise to nonlinear optical phenomena. The enhanced light sensitivity of the FSSD@EP composite materials affects its nonlinear optical properties. These nonlinearities describe how the material’s response to light changes with increasing light intensity. The peak observed suggests a frequency range (10⁵–10⁷ rad/s) where this energy dissipation is most prominent. This points towards a relaxation process occurring within the material at those frequencies. As the frequency of the electric field increases, both the real part (M′) and the imaginary part (M″) of the complex modulus (M*) are expected to change in the FSSD@EP composite materials.

Cobalt-doped bismuth ferrite (Bi_1-xCo_xFeO₃) nanoparticles (NPs) at x = 0.00, 0.03, 0.07 were synthesized using the sol–gel auto-combustion method and studied the influence of cobalt (Co) on structural, surface, chemical, optical, magnetic, and photocatalytic properties on Bi_1-xCo_xFeO₃ nanoparticles. X-ray diffraction (XRD) studies revealed the rhombohedral structure of the synthesized nanoparticles with a mean crystallite size of 51 nm. Using Tauc’s relation, the optical band gap was calculated, and it decreased from 2.13 to 1.6 eV with increased Co concentration. The iron-oxygen (Fe–O) stretching vibrations were confirmed from Fourier transform infrared (FT-IR) spectra. Magnetic properties of the nanoparticles were studied using a vibrating sample magnetometer and found that Bi_1-xCo_xFeO₃ nanoparticles are ferromagnetic at room temperature. The strength of magnetization increased with an increase of Co doping concentration. Photocatalytic properties of the Bi_1-xCo_xFeO₃ nanoparticles were studied using a UV–Vis-NIR spectrophotometer, and it was found that Bi_1-xCo_xFeO₃ nanoparticles can be used as a photocatalyst in the visible region of the spectrum. Methyl blue (MB) dye was used to study the dye degradation property of pure and Co-doped BiFeO₃ nanoparticles, and the results were explained in detail. The Bi_1-xCo_xFeO₃ nanoparticles at x = 0.07 exhibited the highest photocatalytic activity (97.9%).

We present a numerical study of the erosion of channel walls applied to the PHALL Hall thruster being developed at the Plasma Physics Laboratory at the University of Brasilia. The simplified two-dimensional model retains the axial and radial directions to evaluate the erosion rate due to energetic ions. Plasma particles and fields are solved using the particle-in-cell and Monte Carlo collision techniques. The electrostatic potential and ion density profiles are obtained after the simulation reaches the steady state. We compute the channel wall erosion rate from the time series of the number of sputtered particles. We also perform and analyze a simulation of an SPT-100 Hall thruster. Our results show that the erosion acts mainly near the middle of the channel walls of the PHALL thruster, in agreement with patterns of wall material degradation observed in the laboratory device after experimental tests. The implications of the erosion pattern predicted for the PHALL thruster are discussed.

This study aimed to investigate the atherogenicity (quality) of LDL particles in patients with acute and recovered from COVID-19 infection. The participants were adults, aged 18 years or older of both sexes. Those with positive RT-PCR results at baseline were included in the Acute COVID-19 group (n = 33), and those with negative RT-PCR six months after acute infection, were included in the Recovered COVID-19 group (n = 30). The LDL quality was evaluated using three validated methods: Z-scan, UV–visible spectroscopy, and Lipoprint system. The Recovered COVID-19 group showed significantly higher numbers of large LDL particles (less atherogenic) than the Acute COVID-19 group (P < 0.05). We also found that COVID-19 infection was associated with the oxidative modification of LDL particles. D-dimer and CRP levels were correlated with Z-scan results and antioxidant-amount estimate. Moreover, we noticed that the infection left a sequel in LDL quality, even after six months of recovery. These findings highlight the importance of monitoring lipids during and after recovery from COVID-19 infection, and their potential deleterious effect on the LDL profile might correlate with the progression of atherosclerosis and poor clinical outcomes.

This research investigates the structural, morphological, and optical properties of SnO₂ and Cu-doped SnO₂ films deposited on glass and FTO substrates using the RF sputtering technique. The Cu-doped samples were subsequently annealed in an oxygen flow at temperatures of 400 and 500 °C. X-ray diffraction (XRD) analysis revealed that pure SnO₂ thin films exhibited an amorphous structure on both substrates. In contrast, the Cu-doped SnO₂ sample on the glass substrate maintained its amorphous state, while the sample on the FTO substrate transitioned to a tetragonal rutile structure. Thermal annealing further induced a transformation to a tetragonal rutile structure for samples on both substrates. Energy dispersive spectroscopy (EDS) confirmed the incorporation of Cu into the SnO₂ films. The estimated crystallite sizes ranged from 16 to 20 nm for the glass substrate and from 20 to 31 nm for the FTO substrate. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) analyses demonstrated uniform surface morphology with a smooth texture and strong adhesion to the substrates. The band gap energy (Eg) values for the unannealed samples were found to be 3.52, 3.40, 3.50, and 3.29 eV. Notably, Cu doping and the deposition of SnO₂ films on FTO substrates resulted in a reduction of the band gap. These findings provide valuable insights into the effects of Cu doping, annealing, and substrate type on the structural and optical properties of SnO₂, paving the way for advanced optoelectronic applications.

The collisionless, unmagnetized relativistic dusty plasma system comprising relativistic positively charged ions, energetic ion beams, and Maxwellian electrons is considered to study the fundamental properties of dust ion-acoustic waves (DIAWs) and formation of rogue waves (RWs). The Korteweg-de Vries (KdV) equation and nonlinear Schrödinger equation (NLSE) are derived using the well-known reductive perturbation and derivative expansion approaches, respectively. The concerned parameters play a crucial role in the fundamental properties (such as phase velocity, nonlinearity, dispersion, production, and propagation) of DIAWs. In addition, the rational solution of rogue waves (RWs) exists due to the effect of concerned parameters except the ion beam for argon.

The cylindrical dust-acoustic (DA) solitary structures in an unmagnetized four-component dusty plasma medium whose constituents are negatively charged dust particles, electrons, ions, and positrons are investigated. A set of equations has been considered to describe the plasma system in a cylindrical geometry and reduced those equations into a nonlinear partial differential equation so-called ((3+1))-dimensional cylindrical Kadomstev-Petviashvili (cKP) equation with the help of reductive perturbation method. The analytical solitary wave solution of the cKP equation has been reported. The existence and non-existence of solitary wave solution with the plasma parameters have been discussed. The effects of the dust grains and the nonthermality of other particles on the amplitude and width of solitary waves are examined. It is found that our plasma system supports both the positive and negative solitary waves. These results may improve our understanding of DA solitary structures in space and laboratory plasma devices in non-thermal dusty plasma medium.

The self-organization of a four-component partially ionized dusty plasma consisting of mobile electrons, positive ions, neutrals, and negatively charged static dust particles into a non-force-free double Beltrami (DB) state is explored. The DB state is a linear superposition of two distinct single Beltrami fields, characterized by two self-organized vortices. By considering the plasma species densities typical to earth’s ionosphere, the analysis of the relaxed state shows that densities and invariant helicities of plasma species can control the nature as well as the size of self-organized vortices. Additionally, the potential implications of these DB states are highlighted.

This study introduces a highly sensitive one-dimensional (1D) photonic crystal (PC)-based biosensor for detecting urea concentrations using a defect layer approach. The sensor is designed with alternating layers of BaF₂ and TiO₂, featuring a central defect layer filled with urea samples. Using the transfer matrix method (TMM), the transmission spectra were analyzed to detect urea concentrations ranging from 50 to 800 mM. This study examines the effects of varying layer thicknesses on key performance parameters, including sensitivity, Q-factor, FWHM, FoM, and LoD, successfully optimizing the sensor design. The results demonstrate a redshift in the defect mode wavelength with increasing urea concentrations, achieving sensitivities as high as 212.75 nm/RIU. Unlike conventional methods that require extensive sample preparation and lengthy analysis times, this sensor provides a rapid, cost-effective, and scalable solution. Its capability to operate near the pathophysiological range of urea in human blood makes it particularly suitable for medical diagnostics.