Vol. 66 No. 5 Sept-Oct (2020): Revista Mexicana de Física

Magnetic fields appear at all scales in the Universe, spanning many orders of magnitude in their strength, and intervening in the development of many astrophysical processes. In particular, in compact objects, magnetic fields can reach huge intensities and play a fundamental role in the evolution of the star and its surroundings. In this review, the most relevant ideas about their generation mechanisms and their eects on the composition and evolution of compact stars are summarized. The review highlights the role played by anisotropic pressures, induced by the presence of strong magnetic fields, in the equation of state and in the macroscopic observables of compact objects. Anisotropies demand to solve Einstein equations beyond the spherical
symmetry. In this regard, two models are analyzed, one using a metric in cylindrical coordinates and another one considering a metric, which allows to take into account small deformations of the objects. These results are relevant for the description of magnetized white dwarfs and hypothetical quark and Bose-Einstein condensate stars. Some related astrophysical phenomena, as pulsar kick velocities and jets associated to compact objects, are also addressed as a consequence of the presence of strong magnetic fields.

This article analyzes the behavior of electric charge carriers (electrons and holes) in a semiconductor material with metal contacts under thermodynamic equilibrium condition. Expressions were obtained for the concentrations of the electrons and holes, as well as for the Debye Radius () both in the general bipolar case, and in the particular cases of n, p and intrinsic type materials. Based on the previous expressions, the case is analyzed when the quasi-neutrality phenomenon appears in a semiconductor material.

Franck–Condon factors and r-centroids were computed for the D²S⁺ - A²Π_i and D²S⁺ - B²S⁺ band systems of the aluminium oxide (AlO) molecule for the v' = 10; v" = 10 matrix using the method developed by Jarmain and Nicholls. The latest Fourier-transform Spectrometer molecular constants of ground and excited state are used. The intensities of these bands are discussed and the Franck–Condon factors and r-centroids obey the established relationships

Chromium substituted copper base spinel ferrites (Ni_0.3Cu_0.7)Cr_xFe_2-xO₄ were prepared by Sol-gel method. Structural, magnetic and electrical properties were studied by utilizing X-ray diffractometer, Vibrating sample magnetometer, and precision LCR-meter using lab-tracer software. The X-ray diffraction analysis confirmed the formation of single phase fcc structure of the samples. The lattice constant and crystallite size decreased from 8.37-8.23Å and 53.09-35.36nm respectively with increasing content of Cr³⁺ ions. From MH loops it was observed that the saturation magnetization decreased and coercivity increased with increasing content of Cr³⁺ ions. Dc resistivity increased with increase of Cr³⁺ concentration and decreased with increase in temperature which shows the semiconducting behavior of ferrites. The dielectric constant, dielectric loss and tangent loss decreased with increase of Cr³⁺ concentration. The dielectric constant follows bilayer Maxwell Wagner model and Koop’s phenomenological theory while ac conductivity increased with increasing frequency following Jonscher’s power law.

We prepared 42P₂O₅–40ZnO–(18-x)Na₂O–(x)NiO glasses doped with xNiO where 1 ≤ x ≤ 6 mol.%. The optical absorption indicates that the nickel ions exist in both sites tetrahedral and octahedral as the concentration of NiO is increased. This investigation has provided the increasing degree of disorder in the glass network at higher concentrations of NiO. The magnetic characteristics investigated by two techniques electron spin resonance (ESR) spectra and magnetic properties using vibrating sample magnetometer (VSM) at room temperature. The ESR spectra exhibit resonance signal without hyperfine interaction of all the investigated glass samples. It is found that Ni²⁺ ions were in octahedral sites and it has ₃A^2g as ground state. The spin-Hamiltonian parameters are evaluated from the ESR spectra and we found a variation of   and   with different nickel concentration. Interesting results are found for the number of Ni²⁺ ions participating in resonance (N) and paramagnetic susceptibility (χ). The magnetic properties were performed at room temperature under an applied field of 20 kOe and the hysteresis loops of the powders were extracted for all the considered samples.

We analyze the two-dimensional motion of a rigid body due to a constant torque generated by a force acting on the body parallel to the surface on which the body moves extending an old note of Ferris-Prabhu [Am. J. Phys. 38, 1356-1357 (1970)] and supplementing it with a short discussion of the jerking properties.

Currently, non-linear loads are found virtually anywhere with the promise of high electrical efficiency. Examples of this type of non-linear loads are compact fluorescent lamps and light-emitting diode lamps which can now be found in any home. However, they produce highly distorted currents that pollute the power grid and cause stability problems, and making the measurement of the distorted electrical current a non-trivial issue. For the reliable measurement of distorted waveforms within a wide bandwidth, magnetic current transducers present disadvantages over resistive current transducers, such as those caused by the magnetic material which attenuates the high-frequency components while producing heating on the magnetic material. This research presents the design principles to develop a thin-film wideband current transducer. Principles such as the selection of high-purity materials, high-symmetry coaxial design, size, geometry, and aspect ratios were used to obtain a linear relationship between its input and output, i.e.: a flat frequency response from DC to 100 kHz, and the ability to operate continuously with a custom passive thermal system for heat dissipation and reliable measurement. An exhaustive effort has been made on the refinement of the design aimed at understanding the effects that govern the frequency behavior of the transducer and the ways to compensate them. The manufacturing feasibility of the proposed design is well confirmed by the results obtained from the simulation process.

In this study, we have investigated the structural, electronic, and magnetic properties of the Rb₂NaVF₆ compound. We have performed our calculations by the use of first-principle methods based on spin-polarized density functional theory, where the electronic exchange-correlation potential is treated by the generalized gradient approximation GGA- PBEsol coupled with the improved TB-mBJ approach. The calculated structural parameters of Rb₂NaVF₆ are in good agreement with the available experimental data. Rb₂NaVF₆ exhibits a half-metallic ferromagnetic feature with a spin polarization of 100 % at the Fermi level and a direct large half-metallic gap of 3.582 eV. The total magnetic moments are 2 μ_B. This material is half-metallic ferromagnets, and it can be potential candidates for spintronics applications at a higher temperature.

Titanium dioxide has been extensively investigated as a photocatalyst for water purification, presenting limitations such as recombination of electron-hole pairs generated by photons. The titania / graphene nanocomposites are promising materials to overcome these limitations due to the high specific area of graphene and unique electronic properties. In this work, an anatase-graphene nanocomposite was synthesized by a simple mixture assisted by ultrasound. Graphene was obtained by electrochemical exfoliation of graphite using the electrolysis technique. On the other hand, anatase was synthesized using the sol gel method. The obtained graphene, anatase and the nanocomposite material, were characterized with the X-ray diffraction technique (DRX), scanning electron microscopy (MEB) and transmission electron microscopy (MET). Using Raman spectroscopy, it was possible to verify that the graphite exfoliated correctly producing few layer-graphene. The lamellar nano-structure of the exfoliated graphite has crystallographic planes characteristic of graphite, graphene and graphene oxide. The presence of the anatase phase is shown in the diffraction spectrum of titania. The images obtained with SEM and TEM of the graphene sample show a layered lamellar structure and the TiO₂ images show agglomerates of ellipsoidal nanoparticles. Obtained titania nanoparticles have a size of about 6 nm. Band gap value for such extremely low particle size nanocomposite is around 3.6 eV and presumably corresponds to the TiO₂ (anatase) phase that completely surrounds the graphene. A nanocomposite model based on HRTEM observations is proposed. Considering the graphene electrical properties and the photocatalytic properties of TiO₂, this nanocomposite promises to have applications in photocatalysis.

We experimentally measured the effect of nature and concentration of crosslinker on the photopolymerized time of the poly(hydroxy-butyl-methacrylate-co-2-ethyl-hexyl-acrylate)/5CB system. Initial mixtures are composed of monofunctional monomers hydroxy-butyl-methacrylate (HBMA) and 2-ethyl-hexyl-acrylate (2-EHA), and one of the three bifunctional monomers, poly-propylene-glycol-di-acrylate (PPGDA), tri-propylene-glycol-di-acrylate (TPGDA), or 1,6-hexane-diol-di-acrylate (HDDA), and 2-hydroxy-2-methylpropiophenone (Darocur 1173) as a photoinitiator. The copolymers were elaborated via UV irradiation of reactive formulation. The central composite face-centered design of experiments (DoE) has been used to determine the influence of temperature, crosslinking density and their interactions on swelling behavior of poly(HBMA-co-EHA/crosslinker) networks in liquid crystal 5CB. The experimental results and the predicted responses indicate a good correlation and therefore the validity of the used model.

In this study, we have employed the first-principle methods based on density functional theory to investigate the structural, elastic, electronic and magnetic properties of BBi_0.75Mn_0.125N_0.125. The exchange and correlation potential are described by the generalized gradient approximation of Perdew, Burke and Ernzerhof (GGA-PBEsol) + SOC coupled with TB-mBJ approaches. The studied structure show that the compound BBi_0.75Mn_0.125N_0.125 is stable in ferromagnetic phase, the elastic property indicate that the structure is brittle and mechanically stable. The half metallic description is predicted with energy spin band gap in spin up channel. The structure attributed half-metallic ferromagnetism could be suitable for spintronics devices. To our knowledge, this is the first time that a study has been done on this alloy and we would like it to serve as a reference for the next studies.

The classical and modified equations of Kolmogorov-Johnson-Mehl-Avrami are compared with the equations of conventional Gompertz and
Montijano-Bergues-Bory-Gompertz, in the frame of growth kinetics of tumors. For this, different analytical and numerical criteria are used
to demonstrate the similarity between them, in particular the distance of Hausdorff. The results show that these equations are similar from
the mathematical point of view and the parameters of the Gompertz equation are explicitly related to those of the Avrami equation. It is
concluded that Modified Kolmogorov-Johnson-Mehl-Avrami and Montijano-Bergues-Bory-Gompertz equations can be used to describe the
growth kinetics of unperturbed tumors.

Presentamos un algoritmo complejo de promediación diferencial espacial-temporal que incrementa la capacidad de detección de un sistema de reflectrometría óptica en el dominio del tiempo sensible a la fase (φ-OTDR), el cual emplea un láser DFB estándar de telecomunicaciones para mediciones distribuidas en la fibra de prueba. El láser DFB es encadenado por auto-inyección a través de un anillo resonador de fibra óptica de polarización preservada, que genera una coherencia y estabilidad frecuencial de la emisión del láser para mediciones precisas. El algoritmo de promediación presentado es capaz de mejorar la SNR en 5-6 dB, incrementando significativamente la capacidad de detección del sistema φ-OTDR. Se demuestra la localización de perturbaciones a la frecuencia de 815 Hz en una distancia aproximada de 850 m con una precisión de 20 m.

We present a new spatial-temporal differential averaging algorithm that improves the capacity of our phase-sensitive optical time-domain reflectometry (φ-OTDR) system to vibration detection. Specifically, the system employs the standard DFB laser self-injection locked through an external feedback loop comprising a PM fiber-optic ring resonator that enhances coherence and provides enough frequency stability to the laser operation. The reported algorithm is responsible for 5-6 dB increase of the SNR thus improving the system ability to detect vibrations. We demonstrate the vibrations with the frequency of ~815 Hz at the distance of   ~ 850 m localized with a spatial resolution of ~ 20 m.

Recently, chaotic behavior has been studied in dynamical systems that generates hidden attractors. Most of these systems have quadratic nonlinearities. This paper introduces a new methodology to develop a family of three-dimensional hidden attractors from the switching of linear systems. This methodology allows to obtain strange attractors with only one stable equilibrium, attractors with an infinite number of equilibria or attractors without equilibrium. The main matrix and the augmented matrix of every linear system are considered in Rouché-Frobenius theorem to analyze the equilibrium of the switching systems. Also, a systematic search assisted by a computer is used to find the chaotic behavior. Basic chaotic properties of the attractors are verified by the Lyapunov exponents.

In order to explore the impact of distance between spins on quantum correlation, we compute trace ditance discord (TDD) and spin squeezing in an anisotropic Heisenberg XYZ model with Dzyaloshinskii-Moriya interaction in the presence of the external magnetic field. It is valuable to investigate that how we can protect quantum correlations in system when the distance between the spins is promoted. We find that rich Dzyaloshinskii-Moriya interaction and low temperature can be effective for quantum correlations with increasing distance between spins. As, at sufficeintly low temperature In addition, the generated correlated channels are inspected to interchange the information between the system qubits applying the standard dense coding protocol; then, the dense coding capacity of the transmitted information is quantified. It is found that the strength Dzyaloshinskii-Moriya interaction and magnetic field have a great impact on the dynamics of the quantum correlations and, consequently, the quality of the generated channels to exchange the information.  Therefore, the effect of Dzyaloshinskii-Moriya interaction for various strengths of temperature needs to be considered to have valid dense coding when distance of spin increases.

In this paper, a statistical analysis of high frequency fluctuations of the IPC, the Mexican Stock Market Index, is presented. A sample of tick--to--tick data covering the period from January 1999 to December 2002 was analyzed, as well as several other sets obtained using temporal aggregation. Our results indicates that the highest frequency is not useful to understand the Mexican market because almost two thirds of the information corresponds to inactivity. For the frequency where fluctuations start to be relevant, the IPC data does not follows any $\alpha$-stable distribution, including the Gaussian, perhaps because of the presence of autocorrelations. For a long range of lower-frequencies, but still in the intra-day regime, fluctuations can be described as a truncated L\'evy flight, while for frequencies above two-days, a Gaussian distribution yields the best fit. Thought these results are consistent with other previously reported for several  markets, there are significant differences in the details of the corresponding descriptions.

This paper is devoted to investigating the description of the q-deformed multiple-trapping equation for charge carrier transport in amorphous semiconductors. For this, we at first modified the multi–trapping model (MTM) of charge carriers in amorphous semiconductors from time-of-flight (TOF) transient photo-current in the framework of the q-derivative formalism, and then, we have constructed, our simulated current by using a method based on the Laplace method. This method is implemented in a program proposed recently by [14] which allows us to construct a current using the Padé approximation expansion.

A non-cubic equation of state is used to predict the solid-liquid, solid-vapor and liquid-vapor coexistences of pure substances. The equation of state is obtained using as input data the critical point, the boiling temperature, the triple point temperature and the acentric factor of the substance. In this work, some examples of phase diagrams predicted with the equation of state are reported in order to show its capabilities. Finally, a database with the parameters for different pure substances is presented.

AbstractIn this paper, we investigated the influence of energy-dependent potentials on the thermodynamic properties of the Klein-Gordon oscillator(KGO): in this way all thermal properties have been determinate via the well-know Euler-Maclaurin method. After this, we extend our study to the case of superstatistical properties of our problem in question. The probability densityf(β)followsχ2− superstatistics (=Tsallis statistics or Gamma distribution). Under the approximation of the low-energy asymptotics of superstatistics, the partition function, at first, has been calculated. This approximation leads to a universal parameterqfor any superstatistics, not only for Tsallis statistics. By using the desired partition function, all thermal properties have been obtained in terms of the parameterq. Also, the influence of the this type of potentials on these properties, via the parameterγ, are well discussed.