Vol. 69 No. 4 Jul-Aug (2023): Revista Mexicana de Física

In the present study, radiation shielding characteristics of the borogypsum, which is a waste generated during the boric acid production in Turkey, was analyzed. For this purpose, we used recently developed Phy-X/PSD software, which is provided to calculate shielding parameters such as mass attenuation coefficient, linear attenuation coefficient, half-value layer, tenth-value layer, effective atomic number, total atomic cross section, total electronic cross section, effective conductivity, effective electron number, buildup factors and fast neutron removal cross section in a wide photon energy range. Additionally, mass attenuation coefficients of borogypsum were compared with those of other radiation shielding materials (ordinary concrete, obsidian, pumice, clay) in order to give a significant evaluation about the radiation shielding capability of borogypsum. Half value layer and fast neutron removal cross section values are also evaluated by other materials. It is noticed that borogypsum has higher shielding potential than other reported shielding materials.  

 In this work, we adopt the three-order perturbation formulae for g-factors (g_//, g_^) of d¹ ions in the octahedral environment to calculate the g-factors of W⁵⁺ ions in tungsten phosphate glasses containing lithium (P₂O₅-Li₂WO₄-Li₂O). In the light of the high valence state of the studied W⁵⁺ centers and hence the strong covalency of the studied octahedral [WO₆]^7- cluster, we consider the contributions to g-factors from the ligand orbital and spin-orbit (SO) coupling interactions based on the cluster approach. The required tetragonal crystal-field parameters are calculated from the local structure of W⁵⁺ ions based on the superposition model. According to the theoretical calculations, we find that the octahedral [WO₆]^7- clusters possess the tetragonally compressed distortion with a shorter W-O bond length (≈1.54 Å) and a longer one (≈2.26 Å) along C₄ axis and four normal W-O bond length (≈1.94 Å) in the perpendicular plane, which infers that the W⁵⁺ ions are in the form of tungstyl ions (i.e., WO^3-). Based on the local structural data, the theoretical values of g_// and g_^ agree well with the experimental values.

We present a theoretical formalism based on fluctuational electrodynamics and the Maxwell-stress tensor for describing the impact of radiative and multiple reflections corrections on the van der Waals force between two nanoscale spherical particles and a pair of atoms in the dipolar approximation. Particularly, we examine the van der Waals forces for two metallic particles whose dielectric constant is represented by the Drude model, for two dielectric particles in which their material response has phononic resonances, and for two atoms with dynamic polarizabilities containing a single resonant frequency. For the metallic particles, in relation to the case in which the aforementioned effects are omitted, the van der Waals force is unchanged by the radiative corrections of the polarizabilities, whereas the mechanism of multiple reflections perturbs force about a few percentage points when the spheres nearly touch each other. In contrast to the the metallic case, the radiative corrections of the polarizabilities of the dielectric particles modify peculiarly the van der Waals force in comparison to the case where such corrections are neglected; there is a critical interparticle separation that divides two regimes: when the interparticle separation is smaller (larger) than this critical distance the force with radiative corrections is smaller (greater) than that without these corrections. Moreover, the van der Waals force is practically unchanged when effect of multiple reflections is taken into account. For the atomic case, the deviation of the van der Waals force due to multiple reflections is about a few percentage points when the interatomic separation corresponds to twice the van der Waals radius, and this deviation can reach about seventeen percent at a separation of 2.5 times the atomic radius. This work might have implications concerning the fine-tuning between theoretical and experimental outcomes of the van der Waals forces.

The motivation for this theoretical paper comes from recent experiments of a heat transfer system of two thermally coupled rings rotating in opposite directions with equal angular velocities that present anti-parity-time (APT) symmetry. The theoretical model predicted a rest-to-motion temperature distribution phase transition during the symmetry breaking for a particular rotation speed. In this work we show that the system exhibits a parity-time ($\mathcal{PT}$) phase transition at the exceptional point in which eigenvalues and eigenvectors of the corresponding non-Hermitian Hamiltonian coalesce. We analytically solve the heat diffusive system at the exceptional point and show that one can pass through the phase transition that separates the unbroken and broken phases by changing the radii of the rings. In the case of unbroken $\mathcal{PT}$ symmetry the temperature profiles exhibit damped Rabi oscillations at the exceptional point. Our results unveils the behavior of the system at the exceptional point in heat diffusive systems.

From a macroscopic point of view, the edges of a type II superconductor are degraded non-homogeneous regions compared with the bulk of the sample. This paper presents numerical simulations of a long superconducting bar with square cross section subjected to an axial external magnetic field, the edges effect on its electromagnetic properties is studied. The edges of the sample are modeled as finite width regions with lower critical current density than the rest of the material. The simulations are based on a continuum electrodynamics model which describes the magnetic induction dynamics of partially penetrated states. Unlike a simpler homogeneous superconductor, in an inhomogeneous material rough flux fronts are formed. It was established that the stochastic profile of the current density produces jets of magnetic flux near a macrodefect. 

We show that the orthochronous proper Lorentz transformations that preserve the condition z = 0 can be parametrized by (two-component) SU(1, 1) spinors in such a way that the Wigner angle associated with a pair of non-collinear boosts is given by one of the scalar products defined between these spinors

By applying the generalized fractional-Nikiforov-Uvarov (GF-NU) approach, the radial Schrodinger equation is analytically solved. The energy eigenvalues and associated functions are calculated by extending the interaction potential to an anisotropic hot-dense QCD medium. For heavy quarkonium masses like charmonium and bottomonium, the special cases of the mass of quarkonium are obtained at α=β=1. The effect of the fraction parameter is investigated within the context of the QCD medium on binding energy and dissociation temperature when baryonic chemical potential is included. Recent works are compared and studied. Therefore, the fractional quark model correctly represents heavy mesons in an anisotropic hot-dense QCD medium, according to the current findings.

The time resolution (TR) is one of the most important characteristic of a detector. The particular case of a scintillator, the collection of light also is important, it depends of the sensitive area of the photo-sensor (Scorer). Normally, the Scorer of Photomultipliers Tubes (PMTs) is greater than the Scorer of Silicon Photomultipliers (SiPMs). Other differences are the voltage of operation, their size and cost, in some cases, the large size of PMTs can be difficult to place, if small space is required, in which case, it is preferable to use SiPMs. The value of TR also depends of the size and geometry of the
scintillator, number of photo-sensors and the electronic part.
In this work, we study the mean optical photon arrival time distribution (AT) to a Scorer from a SiPM of 6X6 mm2. We define the variation of AT as the intrinsic time resolution (ITR). In Geant4, we simulated two different geometries: square and hexagonal, for a BC-404 plastic scintillator coupled to one Scorer. The sources simulated were Sr90, Co60, Cs137, Na22 and μ^- of 1 GeV. It is shown that AT and ITR depends of the geometry and size of the plastic scintillator, the location of the Scorer, the incident particle and its energy. Then, the ITR and therefore the TR is not a constant for a detector. Finally, we show the relation between AT and the deposited energy by the particle incident, which are related in the experiment to the response time event of the detector and the deposited charge by the incident particle, respectively.

This research aims to study the impact of some physical and structural parameters on the I–V characteristics of a high electron mobility transistors (HEMTs) based on Al_xGa_1-x N/GaN, we investigate the effect of the GaN buffer layer thickness and the impact of other properties of the materials such as aluminum mole fraction and doping concentration, the Al_0.2Ga_0.8 N/GaN heterostructures with 400 nm of buffer layer and a layer doped with  n = 4 x 10¹⁸ cm^-3 , for this structure we find the maximum saturation current of 420 mA/mm . The proposed model included GaN buffer layer and Al content were derived from our developed I-V characteristics. The proposed model is in excellent agreement with the simulated I-V characteristics and experimental results.

An ab initio study using density functional theory (DFT) is carried out to explore the structural, electronic, and optical
properties of Zn2NbN3 compound. The structural properties of these compound are determined by using the approximation
(GGA-PBE) as implemented in WIEN2k. The calculated lattice parameters of the Zn2NbN3 compound are found to be
a = 9.91 °A, b = 5.81 °A and c = 5.44 °A . The calculated electronic band structure and density of states indicate that the
Zn2NbN3 compound is a wide gap semiconductor with a direct band gap of 2.5 eV. The different contributions of the electronic
orbitals are discussed using the total and partial DOS with PBE and TB-mBJ approximations , which shows significant
contribution from the Nb-d and N-p. The optical properties such as dielectric function, refractive index, absorption coefficient,
and extinction coefficient are calculated and discussed.

In this present paper, we obtain a general version of constant angle surfaces constructed concerning any direction in three dimensional Euclidean space. This constant angle surface is the special case of developable ruled surfaces whose direction is a spherical circle. Here, we obtain the constant angle surfaces by taking the circles (small circles) whose radius is less than the radius of the sphere, as the base curve. Also, the relationship between the isophote curve and this surface and its physical interpretation is mentioned. When we beam from a light source in a constant direction, the intensity of the light will be the same at every point on this constant angle surface. This study is very important in terms of associating optics, a branch of physics, with geometry, a branch of mathematics. Finally, we classify the singular points of these constant angle surfaces.

An introduction to the study of Noise Shaping type successive approximation (SAR) analog-to-digital converters is presented, as well as a general description of the fundamentals of Noise Shaping SAR (NS SAR), its basic principles of operation and its main architectures. Open problems are addressed, fundamental challenges and main sources of error in processing circuits are reviewed, and state-of-the-art architecture developments are summarized, addressing various problems including loop filter, its passive/active implementations, and the mismatch in the elements of the DAC network, among others. Additionally, future trends and challenges are exposed.

Se presenta una introducción al estudio de los convertidores analógico-digital de aproximaciones sucesivas tipo Noise Shaping, así como una descripción general de los fundamentos del Noise Shaping SAR, sus principios básicos de operación y principales arquitecturas. Se abordan los problemas abiertos, se revisan los desafíos fundamentales y las fuentes de error principales en los circuitos de procesamiento, y se resumen los desarrollos de las arquitecturas de vanguardia, que hacen frente a diversos problemas que incluye el filtro de lazo, sus implementaciones pasivas/activas, y el mismatch presente en los elementos de red del DAC, entre otros. Adicionalmente, se exponen las tendencias y retos futuros.

This paper presents the structural, magnetic properties, and spin moment cancelation phenomenon of the zirconium oxide with a gradual change of zirconium for Iridium. The density functional theory (DFT) is shown to be a key feature of magnetic properties of solid materials treatment.  It is shown that a small adjustment of the spin moment (less than 6%) is allowed. The Zr_1-xIr_xO₂ crystal structure deformation leads to a magnetic compensation that occurs at x=0.06. Spin and orbital moments behaviors are discussed. The stability of the alloy compounds is confirmed by energy calculations. The material presents ferromagnetic stability and an indirect exchange coupling with a hybridization effect that permitted the evolution from a non-magnetic to a host material with magnetic properties. The Ir orbitals are set at the Fermi level and are spin polarized which indicates a half metallic behavior then makes the material a good candidate for spintronics’ applications.

Flat band electronic modes are responsible for superconductivity in twisted bilayer graphene (TBG) rotated at magic angles. From there other magic angles can be found for any multilayered twisted graphene systems. Eventually, this lead to the discovery of the highest ever known electron-electron correlated material. Moreover, the quantum phase diagram of TBG is akin to those observed among high-Tc superconductors and thus there is a huge research effort to understand TBG in the hope of clarifying the physics behind such strong correlations. A particularity of the TBG is the coexistence of superconductivity and the fractional Quantum Hall effect, yet this relationship is not understood. In this work, a simple 2 × 2 matrix model for TBG is introduced. It contains the magic angles and due to the intrinsic chiral symmetry in TBG, a lowest energy level related to the quantum Hall effect. The non-Abelian properties of this Hamiltonian play a central role in the electronic localization to produce the flat bands and here it is proved that the squared Hamiltonian of the chiral TBG model is equivalent to a single electron Hamiltonian inside of a non-Abelian pseudo-magnetic field produced by electrons in other layers. Therefore, the basic and fundamental elements in the physics of magic angles are determined. In particular, a study is made on these fundamental energy contributions at the Γ-point due to its relation to the recurrence of magic angles and its relationship with the Quantum Hall effect.

Herein we report on the magneto-transport properties of disordered three dimensional inverse opals (3D-IOs) fabricated by a standard three-probe electrodeposition technique into the interstices of porous membranes made of 150 nm diameter self assembled poly(methyl methacrylate) spheres. This approach has allowed the synthesis of large scale nanocomposites with exact ferromagnetic NixCo1−x alloy compositions and complex interconnected structure. Particularly, the microstructure of Co-rich 3D-IOs is consistent with the hexagonal close packed hcp texture and its corresponding magnetoresistance is explained in terms of the hcp Co magnetocrystalline anisotropy contribution. Conversely, the magnetoresistive behavior of Ni-rich 3D-IO networks is explained in terms of only their magnetostatic field. The control of these features is made possible by the reduced dimensions of necks and walls, in the 40 nm to 60 nm range, of the 3D-IO structure. Despite the disordered morphology of these 3D-IO nanoarchitectures, their microstructural and magneto-transport properties can be fine tuned due to the reduced nanoscale dimensions of the electrical interconnections. These properties have been found to be comparable to those obtained in other 3D networks, making them interesting systems for their potential use for magnetic sensing and spintronic applications.

In CdS/CdTe solar cells, the dependence on the frequency of the applied voltage is essential to improve theoretical results. Our model is based on the conservation of energy and charge and considering a ternary layer and the existence of plasmons in the interface. In this work, the capacitance dependence as a function of the frequency of the induced field in the heterojunction is observed; furthermore, a plasmon was formed in the interface in the surface semiconductor. The results provided with the theoretical model were compared with the experimental data, and a better adjustment was obtained.

Building micro and macro sized structures using compacted magnetic nanoparticles is a widely used approach that has proven a great potential as the basis for novel materials made by design. These materials are made by compactation of soft magnetic particles in the nano o micrometer sizes and their macroscopic properties are mostly governed by magnetostatic effects and the combination of the intervening shapes, namely those of the individual particles and that of the piece made with these particles. Herein a simplified mean-field model is presented to describe the magnetostatic effects in soft magnetic composites with cylindrical macroscopic shape made of densely packed ideal spherical soft magnetic particles. The model allows calculating the main magnetic parameters of the system as well as their most relevant tendencies as a function of its main parameters. Furthermore, the model has also been successfully applied to arrays of interacting macroscopic shapes, which provides a further controllable magnetic parameter.

In this work, we tested ab initio methods combined with evolutionary algorithms for searching stable crystalline structures for solid fluorine in the Tera-Pascal (TPa) regime. We performed several structural searches using the USPEX code, at pressures that spanned the range from 1 to 5 TPa, considering up to 16 atoms per unit-cell for selected pressures. Our findings partially support recent studies by validating the transformation of fluorine from a molecular form, Cmca, into an intermediate polymeric form before its eventual dissociation. Enthalpy comparisons between candidate structures of fluorine at high pressure show a direct transition from the molecular phase Cmca into a Pm¯3n extended structure at 2.7 TPa, the later consisting of linear chains and independent atoms, which disagrees with previous conflicting reports that proposed two other intermediate phases to also exist as stable crystalline forms close to 3 TPa at zero temperature.

Hexagonal boron nitride (h-BN) is a 2D material with excellent properties, such as large band gap, high thermal and chemical stability, transparency, and high resistance to oxidation and corrosion. These properties make h-BN a suitable candidate to be used in the development of advanced coatings. However, as for other nanomaterials, tailor and control the properties of h-BN is a fundamental key for their application into several fields. Here, the wetting properties of h-BN when is exfoliated by ultrasonic cavitation in different solvents including isopropyl alcohol (IPA), dioxane (Dx), N-methyl pyrrolidone (NMP) and dimethyl formamide (DMF) were investigated. The wetting properties of the different h-BN materials were determined by measuring the water contact angle (WCA) of h-BN thin films deposited on silicon dioxide, different contact angles were observed for each sample, the different WCA values are explained by the differences in the structure and roughness of the thin film surfaces obtained just by changing the solvent during exfoliation. These surface properties were characterized by optic and transmission electronic microscopy (TEM) as well as atomic force microscopy (AFM).

The equation of state to predict solid-liquid-vapor coexistence of heavy n-alkane is developed. In the equation of state, their parameters are functions of the n-alkane molecular weight and were correlated with their corresponding values from methane (16.04g/mol) to eicosane (280.53g/mol) published in reference Chem. Eng. Commun., 209:2, 171, (2022); and the experimental melting temperature value of some heavy n-alkanes (from 100g/mol until 600g/mol).