Scielo RSS <![CDATA[Revista mexicana de física]]> vol. 62 num. 5 lang. en <![CDATA[SciELO Logo]]> <![CDATA[Analysis of nuclear structure effects in sub-barrier fusion dynamics using energy dependent Woods-Saxon potential]]> Abstract The fusion dynamics of various projectiles ( O 8 16, A l 13 27 and C l 17 37) with the G e - i s o t o p e s 32 70,72 is analyzed using the energy dependent Woods-Saxon potential model (EDWSP model) and the coupled channel formulation. The impacts of the inelastic surface excitations of fusing nuclei have been examined using the coupled channel model and by inclusion of appropriate number of the intrinsic channels, the observed fusion enhancements can be reasonably explained for all fusing systems. The magnitude of the sub-barrier fusion enhancement is found to be increasing with the increase of deformation parameter associated with the colliding systems. Furthermore, the optimum choice of the static Woods-Saxon potential and the EDWSP model are simultaneously tested along with the Wong’s approximation for explanation of the fusion of O 8 16 + G e 32 70,72, A l 13 27 + G e 32 70,72 and C l 17 37 + G e 32 70,72 reactions. The theoretical predictions obtained by using the static Woods-Saxon potential model are found to be substantially smaller than the experimental data particularly at below barrier energies. In contrast, the EDWSP model based calculations provide an adequate description of the observed fusion enhancement of the chosen reactions. This unambiguously reveals that the discrepancies between theoretical results obtained through the single barrier penetration model and the experimental data can be partially or fully removed if either one makes the use of the EDWSP model along with the Wong’s approximation or includes the intrinsic channels associated with the fusing systems in coupled channel calculations. In addition, the EDWSP model based predictions are capable of recovering an agreement with the fusion data within 10%. For chosen reactions, only at 7 fusion data points out of 77 fusion data points does the deviation exceed 5% while 70 fusion data points lie within 5%. Therefore, the EDWSP model based calculations are able to provide close agreement with the fusion data points at above barrier energies within 5% with a probability more than 90%. <![CDATA[Management of the von Roos Operator in a Confined System]]> In this article, an analysis of the kinetic energy operator introduced by O. von Roos is performed. Such analysis allows for determining the energy of a particle when its mass is position-dependent within a specific medium. The work, focused on the discussion of this concept in quantum mechanics courses, reveals how to apply the operator to numerically solve the one-dimensional Schrödinger equation for an electron rigidly confined within a semiconductor structure having a specific width (considering that the particle is subject to the action of a position-dependent quadratic potential). The Schrödinger equation is solved considering the mass of the electron constant, allowing a comparison of the obtained energies for this case as well as the ones that correspond to the case involving a position-dependent mass and taking into consideration several values of the structure width where the particle is confined. Observations during the analysis reveal how the ambiguity of the parameters that appear in the von Roos operator leads us to conclude that several dynamic systems can be associated to a given form of the potential energy. <![CDATA[Simulating effective potential under strain for holes]]> The effective potential (Veff) evolution, is graphically illustrated for an increasing mixing of heavy- and light-holes (lh) under strain. The impact of the present study is enlarged provided we were able to deduce comprehensive analytic expressions for the valence-band offset both for zinc blende and wurtzite semiconductors, useful in current solid-state physics studies, whenever one manages to manipulate the accumulated pseudomorphic strain and mixing effects in a single shoot. We found permutations of Veff character for lh, that retrieve the hypothetically predicted striking “keyboard” effect. Interestingly, the strain diminishes the keyboard profile, and also makes it emerge or vanish occasionally. Due in-plane anisotropy the keyboard effect under pseudomorphic stress turns topologically tuned. We conclude that multiband-mixing and stress-induced events, are strong competitor mechanisms that can not be universally neglected by assuming a fixed-height Veff, as a reliable none-mutable test-run input for layered systems. Our results may be of relevance for promising tunable heterostructure’s design to enhance the hole mobility in semiconductor devices. <![CDATA[A comparison of the effect of multiple scattering on first and second order X-ray diffraction from textured polycrystals, for the investigation of secondary extinction]]> The integrated intensity of Debye-Scherrer (D-S) rings, arising from an eventual second diffraction process of a diffracted X-ray beam, was calculated. This represents the amount of intensity not arriving at the detector as oriented to register the first diffraction process, and as result, a measure of secondary extinction. Thus the objective is to investigate in this way if secondary extinction affects measurements of X-ray diffraction from textured polycrystals. This has been suggested by differences of pole density maxima observed between measured first and second order pole figures in strongly textured materials. Calculations are performed for a detector scan (varying only 2θ), and the integrated intensity is determined for first and second order diffraction conditions of a general plane (hkl). Normalization through corresponding powder is performed. It is found that this especial case of multiple scattering effect, indeed affects both orders essentially in the same way. If corresponding detector scan measurements verify this, then the observed differences between pole density maxima of pole figures of different order cannot be attributed to secondary extinction. Instead, they can be attributed to heterogeneous texture or error propagation. On the other hand, if the detector scans do exhibit a difference as that of pole density maxima, these differences can possibly be attributed to primary extinction. <![CDATA[Analysis of charge variation in fractional order LC electrical circuit]]> In this paper, the charge variation in time has been investigated in electrical LC circuit within the framework of fractional calculus. The second order differential equation related to the LC circuit has been re-solved by using Caputo fractional derivative. The solution of this new equation has been obtained in terms of Mittag-Leffler function which behaves in between power law and exponential law forms. The order of time-fractional derivative characterizes the time fractality effects in the system, and is considered in the interval 1 &lt; α ≤ 2. The obtained results have been compared with the other studies in the literature. It has been concluded that the Mittag-Leffler function and the order of time-fractional derivative have a special importance to take into account the non-local behaviour of the physical process in time. <![CDATA[Symmetry field breaking effects in Sr<sub>2</sub>RuO<sub>4</sub>]]> In this work, after reviewing the theory of the elastic properties of Sr2RuO4, an extension suitable to explain the sound speed experiments of Lupien et al. and Clifford et al. is carried out. It is found that the discontinuity in the elastic constant C66 gives unambiguous experimental evidence that the Sr2RuO4 superconducting order parameter Ψ has two components and shows a broken time-reversal symmetry state. A detailed study of the elastic behavior is performed by means of a phenomenological theory employing the Ginzburg-Landau formalism.<hr/>En este trabajo, luego de realizar una revisión de la teoría de las propiedades elásticas del compuesto Sr2RuO4, se presenta una extensión que permite explicar los resultados de los experimentos, sobre la rapidez del sonido realizados por Lupien y colaboradores y Clifford y colaboradores . Se muestra que la discontinuidad observada en la constante elástica C66 constituye una evidencia experimental directa de que el parámetro de orden Ψ tiene dos componentes y rompe la simetría de inversión temporal. También se realiza un estudio detallado del comportamiento elástico usando una teoría fenomenológica basada en el formalismo de Ginzburg-Landau. <![CDATA[Annealing effect on corrosion resistance of Bi<sub><em>x</em></sub>Ti<sub><em>y</em></sub>O<sub><em>z</em></sub> coatings]]> Bismuth titanate (BixTiyOz) has received widespread attention due to the fact that during recent times it has found important applications in strategic research fields such as optics and optoelectronic, and more recently studies have shown how their physicochemical properties may be harnessed in order to be able to use BixTiyOz, as an anticorrosive coating. In this work bismuth titanate (BixTiyOz) coatings were grown on titanium alloy (Ti6Al4V) substrates, using RF magnetron sputtering at room temperature. The main objectives of the work were quantify the evolution of crystallographic phase formation, as a function of the annealing temperature, and establish the chemical composition in order to characterize the behavior of the bismuth titanate coating as a protective coating of the corrosion. The morphology of the coating was observed via Scanning Electronic Microscopy (SEM); the crystalline structure was characterized by X-ray Diffraction (XRD) and the chemical composition was analyzed by Rutherford Backscattering Spectrometry (RBS). The corrosion resistance of the coatings was studied by Potentiodynamic polarization (PP) test (Tafel extrapolation). SEM results showed that the surface roughness of the coatings changed when the temperature of annealing increased. Similar change occurred after PP tests. The XRD analysis revealed a change in the coatings microstructure as a function of the annealing temperature, since they evolved from a completely amorphous phase to a polycrystalline phase. RBS results indicate that coatings growing at high temperature have a complex chemical composition. Finally, the electrochemical analysis showed that the corrosion resistance of the coating is much better in the amorphous phases of bismuth titanate than in the polycrystalline phases. <![CDATA[Growth of doped PbS:Co<sup>2+</sup> nanocrystals by Chemical Bath]]> Nanocrystalline PbS films grown by chemical bath at T = 40 ± 2 oC onto glass slides were modified by in situ Co2+ -doping and their structural and optical properties were examined. By FT-IR spectra, a sharp stretching mode can be seen at ~1384 cm-1 due to the vibration mode of C O 3 2 ions. XRD patterns shown the growth on the zinc blende crystalline face. The grain size was determined by using X-rays diffractograms and was found at ~28 nm and ~13 - 25 nm for undoped and doped samples, respectively. Optical absorption spectra was used for calculating the energy band gap, and displayed a shift in the ~1:21 - 2:21 eV range, associated with quantum confinement effect. Raman peaks at ~ 210, 271, and 451 cm-1, corresponding to a 1LO phonon mode, a two-phonon process, and a 2LO phonon mode respectively were also recorded. The surface and grain size of the films were measured by AFM studies. <![CDATA[The stability of the 1<em>u</em> state of <mml:math> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="bold">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> in magnetic fields with arbitrary orientations]]> The existence and stability of the van der Waals 1u state of the molecular ion H 2 + with fixed centers (infinitely massive nuclei) in the presence of a magnetic field with arbitrary orientations in the range of strengths 0 ≤ B ≤ 10 a.u. (1 a.u. ≡2.35× 109G) is studied within the non-relativistic framework. The study is based on the variational method with physical relevant trial functions. A particular emphasis to the gauge optimization through variational parameters is incorporated. It is shown that, for all the magnetic fields studied, the potential energy curve has a pronounced minimum for finite internuclear distances and the optimal configuration of minimal total energy is realized when the molecular axis is oriented along the magnetic field lines (parallel configuration). We found a domain of magnetic fields0 ≤ B ≲ 6.6 a.u. where the 1u state in parallel configuration is stable towards dissociation H 2 + ↛ H + p. The maximal stability is found to be at magnetic field B ∼ 2 a.u. where the potential well supports one vibrational state below the dissociation limit. <![CDATA[Dehydration of a gypsum mineral induced by shock compression: a micro-Raman analysis]]> In this article we report the phase transitions induced by shock-compression on a sample of gypsum mineral. The shock experiment was performed in a light gas gun until a pressure close to 14 GPa. The experimental technique employed to analyze the shock effects on the “recovered sample” was the Micro-Raman Spectroscopy (MRS). The results indicate the partial dehydration of gypsum as a result of impact. The MRS analysis also suggests the presence of micro-mixtures of gypsum, bassanite and anhydrite heterogeneously distributed throughout the “recovered sample”. <![CDATA[Analyses of low energy <em>π</em><sup>+</sup>−<sup>12</sup>C, <sup>16</sup>O elastic scattering data using inverse scattering theory and suggested scaling method]]> The elastic scattering data for low-energy (Tπ ≤ 80 MeV) positive pions from carbon-12 and oxygen-16 nuclei has been successfully analyzed by using our local optical potential based on the inverse scattering theory within the framework of the full Klein-Gordon equation. It is found that the same potential parameters used in the π−−12C, cases are still valid for π+−12C with a need to change the three parameters R0, V1 and W3 with the pion’s incident kinetic energy Tπ. This is also found to hold in analyzing successfully the measured angular distributions for π+−16O in the same energy region. The systematic trends in the changed three free potential parameters R0, V1 and W3 with Tπ are compared to their counterparts for π+−12C. The different relations obtained for the two cases under consideration, π+−12C and π+−16O, reveals their dependence on the atomic weight of the target nucleus. As such, this suggests the use of a scaling method to obtain the potential parameters for a certain pion-nucleus system, as π+−16O, from a similar nearby one, as π+−12C. Further studies are needed to confirm our new findings. <![CDATA[Estudio de propiedades fotoeléctricas de películas delgadas de SnS y SnS:Bi]]> En la actualidad las películas delgadas basadas en compuestos de Sn y S son de gran interés debido a sus aplicaciones potenciales en dispositivos fotovoltaicos y opto-electrónicos. El Sn y S son abundantes en la naturaleza, de bajo costo y mucho menos tóxicos que la mayoría de los materiales que se usan en la industria para la fabricación de celdas solares. En este trabajo se preparan películas delgadas de SnS y SnS:Bi con propiedades adecuadas para ser usadas en la fabricación de celdas solares, por una nueva técnica de sulfurización de sus precursores metálicos variando el contenido de Bi. A estas películas se les realizan estudios de fotoconductividad de transientes bajo iluminación y decaimiento, y de fotocorriente en función de la intensidad de la luz; también se caracterizaron micro-estructuralmente con la técnica Scanning Electron Microscopy (SEM). Los estudios permitieron establecer el efecto de la incorporación de Bi en los procesos de recombinación de portadores de carga libres que afectan la fotoconductividad de las películas depositadas, e identificar los tipos de recombinación que se presentan en ellas.<hr/>Thin films based on Sn-S compounds are currently of great interest because of their potential applications in photovoltaic and optoelectronic devices. S and Sn are abundant in nature, inexpensive and much less toxic than most of the materials used in the industry for manufacturing solar cells. In addition to their photovoltaic properties, the chalcogenide materials such as SnS, SnS2, Sn2S3, Sn3S4 and Bi2S3 are of great interest due to its applications in the fabrication of optoelectronic and thermoelectric devices, and as a holographic recording medium. In this work SnS and SnS:Bi thin films, with suitable properties for their use in manufacturing solar cells, were grown by a new technique of sulfurization of the metallic precursors. The deposited films were studied through transient photoconductivity measurements under illumination and decay, and photocurrent as a function of light intensity; their micro-structure was also characterized through Scanning Electron Microscopy (SEM) technique. The studies allowed establishing the effect of adding Bi on the recombination processes of free charge carriers which affect the photoconductivity of the deposited films, and to identify the types of recombination occurring in them. <![CDATA[Birefringence description of a helical fiber-coil]]> Helical coils are used to produce compact fiber devices with stable polarization properties. We have developed a matrix model for such a fiber, taking into account effects of photoelastically induced anisotropy and geometrical phase, on residual birefringence and diattenuation. We present here the theoretical basis used to construct the model and experimental results that validate it. <![CDATA[Synthesis of Cerium Oxide (CeO<sub>2</sub>) nanoparticles using simple CO-precipitation method]]> Synthesis of cerium oxide (CeO2) nanoparticles was studied by new and simple co-precipitation method. The cerium oxide nanoparticles were synthesized using cerium nitrate and potassium carbonate precursors. Their physicochemical properties were characterized by high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and UV-Vis spectrophotometer. XRD pattern showed the cubic structure of the cerium oxide nanoparticles. The average particle size of CeO2 was around 20 nm as estimated by XRD technique and direct HRTEM observations. The surface morphological studies from SEM and TEM depicted spherical particles with formation of clusters. The sharp peaks in FTIR spectrum determined the existence of Ce-O stretching mode and the absorbance peak of UV-Vis spectrum showed the bandgap energy of 3.26 eV.