Brazilian Journal of Physics最新文献

In this paper, we consider the effect of a topological Maxwell term (W(Phi )F_{mu nu }tilde{F}^{mu nu }) on holographic transport and thermodynamics in 2 + 1 dimensions, in the case with a dyonic black hole in the gravity dual. We find that for a constant W the modifications to the thermodynamics are easily quantified, and transport is affected only for (sigma _{xy}). If one considers also the attractor mechanism, and writing the horizon transport in terms of charges, the transport coefficients are affected explicitly. We also introduce the case of radially dependent W(z), in which case, however, analytical calculations become very involved. We also consider the implications of the two models for the S-duality of holographic transport coefficients.

A series of novel lead free tungsten bronze (T-B) structured Ba₄(La_2-xBi_x)Fe₂Nb₈O₃₀(x = 0, 0.05, 0.1, 0.15, 0.2, and 0.25) were formulated by traditional high temperature solid-state method and calcined at 1200 °C. The structural property was investigated by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Tetragonal structure was confirmed from Rietveld analysis. The scanning electron micrograph (SEM) shows uniform distribution of rod-shaped grains with less porosity. The UV–vis analysis shows that the materials have band gaps in the range 2.17–2.29 eV which makes them useful for photocatalytic applications. The dielectric measurement of the samples was carried out in a broad range of frequency as well as temperature. The impedance of the materials falling off with the rise in temperature showing their negative temperature coefficient (NTCR) behavior. The variation of polarization with electric field confirms the ferroelectric nature of the samples.

The growing rate of bump-on-tail instability has been investigated in unmagnetized plasma with Cairns distribution. The growing rate depends significantly on the number density ratio n₁/n₂, temperature ratio T₁/T₂, and the nonthermal parameter α. The influence of these parameters on the maximum growing rate is analyzed numerically by using solar data. The higher α can enhance the growing rate of bump-on-tail instability. The maximum growth rate also will increase with the decrease of number density ratio n₁/n₂ and the increase of temperature ratio T₁/T₂. These interesting results will be helpful for understanding some phenomena in space plasma.

Random packing of binary mixtures of spherical particles in viscous fluid is numerically investigated via CFD-DEM, where moderate size ratios (d/D) are specially considered. Results indicate that binary packing in fluid is much looser than that in the absence of fluid, and two distinct phenomena can be identified as the global packing density varies with the volume fraction of coarse particle (X_D). For small size ratios, the global packing density first increases with increasing X_D due to the occupation mechanism, and then it reaches the maximum value when X_D ≈ 0.6, beyond which the global packing density decreases as X_D increases further. However, for large size ratios, the global packing density always decreases with increasing X_D. These phenomena are further discussed by using the local packing density determined by Voronoi tessellation, the mean and local coordination number, and radial distribution function, with which the particle arrangements within binary mixtures are well identified.

In this communication, the design of broad band optical reflector and selective polarization filter using a one-dimensional photonic crystal (1D-PC) containing superconductor and semiconductor layer has been proposed and investigated theoretically. The proposed design consists of alternate layers of yttrium barium copper oxide YBa₂Cu₃O₇ (YBCO) and silicon (Si), respectively. The refractive indices of YBCO and Si layers are modulated as the function of temperature and wavelength both. This characteristic brings this study closer to actual physical implementation. To obtain the reflectance and transmittance properties of the proposed structure and to analyze the propagation characteristics of electromagnetic waves, the transfer matrix method (TMM) has been employed. The analysis of the reflectance spectra shows that near the critical temperature of YBCO layers no broad band reflection (from 0 to 85°) is observed. On the other hand, when the temperature is lowered below the critical temperature, ODR arises. The proposed YBCO/Si structure gives a broad band reflection band (from 1394 to 1621 nm (bandwidth 227 nm)) for both TE and TM modes of polarizations at 85 K. Also, at higher incident angles, the proposed structure acts as a selective polarization-based filter without introducing any defect in the geometry.

Narrowband bandpass filters (BPFs) are in great need in future terahertz (THz) technologies to suppress EM signals beyond the operating band. THz filters and antennas play a crucial role in controlling and transmitting THz signals for the application of high data rate (6G) communication, non-destructive sensing, imaging, switching, and filtering. The proposed filtenna is designed using a dual-band THz BPF with a complementary split ring resonator (CSRR) metamaterial (MM). This paper suggests the circle in the pentagon (CP) filtenna and the simulation results show the filtering characteristics of two pass bands with resonant frequency (f_r) between 6.3 and 7 THz and between 8 and 9.8 THz with a squared ratio of 0.54. Also, the maximum 3-dB bandwidth (BW) of 1.3 THz, low return loss (RL) of −28.46 dB, and low insertion loss (IL) of almost 0 were attained. The proposed antenna radiates at the frequency of 6.8 THz with an RL of −27.72 dB. The proposed CP filtenna applies to various wireless communication, especially in drone-to-drone communication with excellent data rates.

The investigation of plasma waves and instabilities in the context of relativistic, anisotropic drifting astrophysical plasma is the focus of this study. Utilizing fluid equations in the linear regime, the research reveals the emergence of novel instabilities as a result of drift velocity. Furthermore, the study uncovers a modification in the condition for the mirror instability due to the drift velocity. These findings potentially hold significant implications for comprehending plasma behavior in space environments, particularly in regions like the solar wind or the interstellar medium. The study proposes the further development of the theory to directly apply to space plasma, which could enhance our understanding of the intricate astrophysical systems that are driven by plasma dynamics.

In this note, we calculate susceptibilities, as derivatives of a thermodynamical potential, for the general perturbative holographic setup for transport with magnetic field, charge density and topological term, and compare with the quantities obtained in the standard (AdS_4) dyonic black hole analysis of Hartnoll and Kovtun. We find that the results do not match, despite previous expectations.

The present study focuses on investigating the shape evolution of neutron-rich even-even osmium (Os) transitional nuclei within the range of neutron number N = 82 to N = 190. The investigation is conducted using density-dependent meson-nucleon and point-coupling models within the framework of the covariant density functional theory (CDFT). Additionally, the results obtained from the CDFT calculations are compared with those obtained using the relativistic mean-field model with a nonlinear meson-nucleon interaction. The potential energy curve for Os isotopes (ranging from (^{158})Os to (^{260})Os) is analyzed in order to identify phase shape transitions, such as oblate-spherical-prolate. Furthermore, ground state bulk properties are calculated to gain insights into the structure of Os isotopes. The self-consistent calculations reveal a clear shape transition in the even-even Os isotopes, and overall, good agreement is observed among the different models employed as well as with the available experimental data.