Vol. 64 No. 6 Nov-Dec (2018): Revista Mexicana de Física

The crystal structure of the quaternary compound CuVInSe₃ belonging to the system (CuInSe₂)_1-x(VSe)_x with x= ½, was analyzed using X-ray powder diffraction data. This material was synthesized by the melt and anneal method and crystallizes in the tetragonal space group P2c (Nº 112), with unit cell parameters a = 5.7909(4) Å, c = 11.625(1) Å, V = 389.84(5) ų. The Rietveld refinement of 25 instrumental and structural variables led to R_exp = 6.6 %, R_p = 8.7 %, R_wp = 8.8 % and S = 1.3 for 4501 step intensities and 153 independent reflections. This compound has a normal adamantane structure and is isostructural with CuFeInSe₃. The DTA indicates that this compound melts at 1332 K.

We present a systematic first-principles study of the structural, magnetic and optical properties of perovskite-structure EuTiO₃. This compound exists in different structures: cubic, tetragonal and presents multiferroic properties. Comparing the formation energy between tetragonal and cubic structures, the system has a tendency to symmetry lowering structural deformations composed of rotations of the oxygen octahedral, especially the I4/mcm phase is the most stable structure. Our calculations of the high symmetry cubic structural prototype show an antiferromagnetic order type G. We discuss the dynamical stability of Pm-3m, P4mm and I_4-mcm structures, and the influence of some parameters on the magnetic coupling and the electrical polarization.

The charge transport mechanisms in nanostructured porous silicon (PS) films were studied through current-voltage (I-V) measurements of planar Au/PS/Au structures at 300 K. The films were formed by electrochemical etching of 1-5 Ω-cm p-type Si (100) wafers producing PS layers of 4.48 x 10⁹ Ω-cm. The charge transport is limited both by the space charge limited currents (SCLC) and the carrier trapping-detrapping kinetics in the inherent localized PS energy levels. I-V characteristics evolve according to the trapping-detrapping carrier kinetics in the PS films showing that the electrical current can be controlled by applying external electric fields. An equivalent trap filling limiting voltage (V_TFL) was identified that shifts between 1 and 3 volts by the carrier trapping-detrapping kinetics from the PS intrinsic defect states. An energy band diagram for the PS films is schematically depicted including the influence of the intrinsic PS defect states. To give a reasonable explanation of the found behavior the existence of a thin silicon oxide film covering the network-like-silicon-nanocrystallites is required, in agreement with the widely accepted PS structural models.

In a previous paper, we reported that thin films of ZnO:Al [aluminum-zinc oxide (AZO)] deposited after achieving a very low base pressure [from 4.0×10^–7 Torr (5.6×10^–5 Pa) to 5.7×10^–7 Torr (7.6×10^–5 Pa)] result dark yellow in color and are resistive. These are undesirable characteristics for the application of AZO thin films as front electrodes in solar cells. However, given the increasingly tendency in the acquisition of equipment that allow us to reach excellent vacuum levels, it is necessary to find the deposition conditions that lead to an improving of transmittance without greatly impacting the electrical properties of materials deposited after achieving these levels of vacuum. In this way, the present work is focused on AZO thin films deposited after achieving a very low base pressure value: 4.2×10^–7 Torr (5.6×10^–5 Pa). For this, we studied the effect of the variation of the oxygen volume percent in the argon/oxygen mixture (by maintaining the deposition pressure constant) and the effect of deposition pressure with only argon gas on the main properties of AZO thin films. The depositions were done at room temperature on glass substrates by direct-current magnetron sputtering with a power of 120 W (corresponding to a power density of 2.63 W/cm²). As results, we found that the variation of deposition pressure with only argon gas is a good option for the control of optical and electrical properties, since the addition of oxygen, although improves transmittance, greatly impacts on the electrical properties. Furthermore, an interesting correlation was found between the optical and electrical properties and the chemical composition of the AZO films, the latter depending on the argon pressure (for this, a careful X-ray photoelectron spectroscopy analysis was performed). Also, the inverse relationship between crystallinity and deposition rate was confirmed, in which deposition rate inversely depends on argon pressure.

In this letter, we study some relevant parameters of the massless Gross-Neveu (GN) model in a
finite spatial dimension for different boundary conditions. It is considered the standard homogeneousHartree-Fock solution using zeta function regularization for the study the mass dynamically generated and its respective beta function. It is found that the beta function does not depend on the boundary conditions. On the other hand, it was considered the Casimir effect of the resulting effective theory. There appears a complex picture where the sign of the generated forces depends on the parameters used in the study.

In this paper, we will present a Dvali-Gabadadze-Porrati stable model in order to perform an observational test using $H(z)$ data and radial BAO scale in the galaxy distribution. In this vein, we study the tension between constraints on the cosmological constant $\Lambda$ and the crossover scale $r_c$, which is associated with the DGP model. Our results show that observations do not favor the DGP stable model as a possible candidate to fit to the observations of the late cosmic acceleration.

In this work, the traveling wave solutions of a mathematical modeling of DNA vibration dynamics proposed by Peyrard-Bishop, that takes into consideration the inclusion of nonlinear interaction between adjacent displacements along the Hydrogen bonds, is investigated by both $(G'/G)$-expansion and $F$-expansion methods. Using these methods, some new explicit forms of traveling wave solutions of present nonlinear equation are given. The methods come in to be easier and faster by means of a symbolic computation and yield powerful mathematical tools for solving nonlinear evolution equations in many branches of sciences, especially physics, biology and etc.

We consider the efficacy of the scaling Born positron (SBP) approach, in calculating reliable integral cross sections (ICS) for positron impact excitation of electronic states in atoms. We will demonstrate, using specific examples as H, He, Hg, and Mg, that this relatively simple procedure can generate quite accurate ICS when compared with more sophisticated methods. In the absence of the experimental data, comparisons are made with analogous electron scattering.

We study some peculiarities of the classical variational treatment that applies Jaynes’ maximum entropy principle. The associated variational treatment is usually called MaxEnt. We deal with it in connection with thermodynamics’ reciprocity relations. Two points of view are adopted: (A) One of them is purely abstract, concerned solely with ascertaining compliance of the variational solutions with the reciprocity relations in which one does not need here to have explicit values for the Lagrange multipliers. The other, (B) is a straightforward variation process in which one explicitly obtains the specific values of these multipliers. We focus on the so called q-entropy because it illustrates
a situation in which the above two approaches yield different results. We detect an information loss in extracting the explicit form of the normalization-associated Lagrange multipliers.

We solved the Schrodinger equation with Quadratic Exponential-Type Potential (QEP) model in D-dimensions using the Modified factorization method. The energy eigenvalues and total wavefunctions were obtained in a Gauss hypergeometric form. The thermodynamic properties including vibrational partition function, vibrational mean energy, vibrational mean free energy and vibrational entropy have been calculated for the electronic state of (X1 Σ +g) Rubidium (Rb2) dimer. The QEP discussed can be applied extensively in Physics and Chemistry, especially in molecular dynamics.

The present manuscript shows linear cavity fiber laser experimental analysis. The fiber laser arrangement, use an all fiber mechanical long period grating (MLPG) to control single and dual laser emission.  Here, single laser emission centered at 1562nm with a signal to noise ratio of 27dB and a linewidth less than 0.1nm is obtained and tuned from 1562nm to 1546nm, by twisted a conventional section of single mode fiber that was set into the MLPG. Thus, when the twist is around 120º dual emission is obtained with centered wavelengths at 1533nm and 1546nm, these emissions can be switched between them when the twist is incremented and the peak centered at 1546nm is tuned in backward direction. The proposed laser offers wavelength and power stability with minimal variations of   0.5dB and 0.5nm respectively.  This laser can be applied in several fields such as sensing, optical signal analysis and optical communications. 

Ocean-atmosphere exchange processes are known to decisively determine the sea state, the weather and our planet´s climate. With the ultimate goal of a better understanding of the processes contributing with turbulent kinetic energy into both boundary layers above and below the sea surface, we approach the ocean surface wave phenomenon, and in particular we study the non-breaking waves potential effect.  Therefore, measurement of particle velocities were made in a fluid under non-breaking waves, for the purpose of detecting turbulence and its association with the wave steepness. A total of 184 experiments were analyzed, each one with a duration of 3.5 minutes and the presence of approximately monochromatic waves with varying steepness (0.012-0.273). The measurements were carried out in a wave tank with dimensions of 12.26 m × 0.55 m x 0.32 m using an acoustic velocimeter (Vectrino Profiler, Nortek). The u, v and w components of velocity were measured on a 3.5 cm long fluid column in 35 cells of 1 mm in height. During the experiments, the voleocity profile between 1.5 cm and 8.6 cm depth was obtained,  referred to the water level in the wave tank (h = 48.6 cm  ± 0.5 cm). The waves propagated in deep waters (h/λ > 0.5), where λ is the wave length. Only u(t, z) and w(t, z) components were considered for the analysis since, waves are practical two-dimensional (v(t, z) = 0). Power spectra were calculated in as function of frequency corresponding to u'(t, z) and w'(t, z) turbulent fluctuations, an inertial subrange (isotropic turbulence) was detected in the most of the spectra, for certain depths regardless of the wave steepness. Results from turbulent fluctuations frequency spectra show that eddy size involved in transferring energy to smaller ones, increases with the wave slope.

 

This study characterizes the properties of Newtonian heat and mass species conditions in three-dimensional Jeffrey nanoliquid flow generated by the movement of thermally radiative surface. The liquid flow is electrically conducting through the consideration of magnetic field. The aspects of heat absorption, generation and thermal radiation are considered in the equation of energy conservation. The boundary layer phenomenon is employed to obtain the mathematical expressions of considered physical model. These equations are solved via homotopic scheme. The convergence of homotopic solutions is validated by the numerical data. The importance of physical constraints on temperature and nanoparticle concentration of liquid is visualized by the graphical results.

We computationally investigate the hydrogen storage properties of C₁₂ carbyne structure decorated with one and up to six calcium (Ca) atoms adsorbed to outer surface. The calculations are carried out by density functional theory DFT with the generalized gradient approximation PW91 (Perdew and Wang) as implemented in the modeling and simulation Materials Studio software. Dmol³ is used to calculate, total energies, charge density HOMO-LUMO and Mulliken population analysis. Based on these results, up to six H₂ molecules per Ca atom can be physisorbed with an average binding energy of 0.1272 eV per H₂ molecule. The study is extended to a system with six calcium atoms, which can adsorb up to 36 H₂ molecules. This leads to 15.87 weight percentage (wt %) for the gravimetric hydrogen storage capacity. According to these results, the calcium-coated carbyne C₁₂ structure is a good candidate for hydrogen storage with application to fuel cells.

We study the performance of an electromechanical oscillator as an energy harvester driven by
finite-bandwidth random vibrations under the influence of both a stiffness-type nonlinearity and a
nonlinear damping that has recently been found to be relevant in the dynamics of submicrometer
mechanical resonators. The device was numerically simulated and its performance assessed by means
of the net electrical power and the efficiency of the conversion of the supplied power by the noise
into electrical power for exponentially correlated noise. We tune the parameters to achieve a good
performance of the device for non-negligible amplitudes of the nonlinearity of the oscillator and the
damping.

This work is devoted to investigate solutions to RC circuits using four different types of time fractional diferential operators of order  0 < γ ≤ 1. The fractional derivatives considered are, Caputo, Caputo-Fabrizio, Atangana-Baleanu and the conformable derivative. It is shown that Atangana-Baleanu fractional derivative (non-local), and the conformable (local) derivative could describe a wider class of physical processes then the Caputo and Caputo-Fabrizio. The solutions are exactly equal for all four erivatives only for the case γ=1.

By means of the I-V characteristics measured at room temperature, the height of the Schottky barrier established by the conductive Pt-Ir tip of an Atomic Force Microscope on the aluminum doped ZnO thin films were estimated in the range of 0.58-0.64 eV. The ideality factors were in the range of  2.11-1.39, respectively. These values are in accordance with those reported by other authors that measured the height of the Pt Schottky barrier on ZnO by means of several methods. The procedure detailed in this work suggests that the scanning time for obtaining I-V Schottky characteristics is of the order of 2 ms.

Local available quantum correlations (LAQCs), as dened by Mundarain et al. [19], are analytically determined for Bell Diagonal states. Using the Kraus operators formalism [10], we analyze the dissipative dynamics of 2-qubit LAQCs under Markovian decoherence. This is done for Werner states under the depolarizing [20] and phase damping channels [21]. Since Werner states are among those that exhibit the so called entanglement sudden death [27], the results are compared with the ones obtained for Quantum Discord [22], as analyzed by Werlang et al. [24], as well as for entanglement, i.e. Concurrence[7]. The LAQCs quantier, as Quantum Discord does, only vanishes asymptotically.