Vol. 64 No. 5 Sept-Oct (2018): Revista Mexicana de Física

In the present paper, the linear and non-linear optical properties of GaAs/AlGaAs multiple quantum well with on-center and on-edge delta doping are studied within the effective mass approximation. The delta potential is analytically modeled within each quantum well potential to obtain the energy levels and the corresponding wave functions using the robust finite difference method. The linear and non-linear optical absorption coefficients and changes in the refractive index are studied in the presence of a static magnetic and a periodic laser field using the density matrix approach. The obtained results show that the position of resonances and the amplitude of the optical absorption coefficients and the refractive index changes can be modified by varying the magnetic field and strength and position of doping potential. Lastly, an increase of the optical intensity appreciably changes the total absorption coefficient, as well as the total refractive index changes. Obtained results are important for the design of various electronic components such as high-power FETs and infrared photonic devices based on the intersubband transition of electrons in δ-doped multiple quantum wells.

The effect of Li co-doping on the blue luminescence of Eu2+ doped (La-Al)2O3 phosphors and its incorporation into poly (methyl methacrylate) PMMA films are reported. The phosphors were synthesized by a simple two-step process in which the (La-Al)2O3 powders were obtained by an evaporation method, and the subsequent co-doping with Li+ and Eu2 were made through a solid state reaction in a reductive atmosphere at 1250 0C. The incorporation of those phosphors into PMMA matrix and the synthesis of films with thicknesses between 6 and 12 μm was made by drop-casting. The characteristic blue broad band emission centered at 440 nm associated with the 4f65d1→ 4F7 transition of Eu2+ ions (CIE coordinates 0.6731, 0.3343) was observed when these powders were excited with 297nm light. These luminescence intensity was enhanced when Li was incorporated as co-dopant up to 320% with respect to those phosphors without Li. The luminescent characteristics of these phosphors were preserved when they were embedded into the PMMA films.

Se utiliza un modelo de un circuito LC con carga discreta y una fuente AC de frecuencia w elevada para determinar su hamiltoniano efectivo aplicando la aproximación semiclásica y el método de promediación temporal de Kapitza hasta 1/(w^2). El resultado corresponde al de la propagación de una partícula en una red de enlace fuerte por el mecanismo de hopping bajo la acción simultánea de un campo homogéneo rápidamente oscilante y un campo estático lineal, donde la coordenada de posición de la partícula equivale a la carga eléctrica del condensador en el circuito LC. De manera análoga a resultados previos para el oscilador armónico en la red, en este caso también se encuentra una condición de bifurcación dada por la carga inicial del condensador que determina la aparición súbita de un voltaje DC. Otra predicción relevante es la supresión controlada de la corriente efectiva al elegir los parámetros de la fuente AC.

We study the Jost solutions for the scattering problem of a von Neumann-Wigner type potential, constructed by means of a two times iterated and completely degenerated Darboux transformation. We show that for a particular energy the unnormalizedJost solutions coalesce to give rise to a Jordan cycle of rank two. Performing a pole decomposition of the normalized Jost solutions we find the generalized eigenfunctions: one is a normalizable function corresponding to the bound state in the continuum and the other is a bounded, non-normalizable function. We obtain the time evolution of these functions as pseudo-unitary, characteristic of a pseudo-Hermitian system.

This paper is concerned with the investigation of the Electron Paramagnetic Resonance (EPR) signal response to Co⁶⁰ gamma-ray radiation on poly(L-lactic acid) (PLA). The aim of this study is to assess the usefulness of PLA as a high-dose dosimeter. The EPR-signal response of PLA has been investigated to determine some of its dosimetric characteristics such as: signal intensity versus gamma dose received, zero-dose response, signal fading, signal repeatability, batch homogeneity, detection threshold, and stability under simulated sunlight exposure. It is concluded that PLA might be used as a high-dose dosimeter.

 In this paper, an adaptive LDPC encoder for complete FSO/CV-QKD system using a COTS device for emulated dynamical atmospheric turbulence levels is presented. The experimental and emulation results show the maximum and minimal final secret key rates of  105 Kbps and 10 Kbps, respectively, for minimal and maximal throughput in a commercial network, 30 Mbps and 90 Mbps, respectively. Our proposal presents an adequate performance for weak and moderate atmospheric turbulence levels and a suitable option for improve the using of QKD systems.

The ground state magnetic properties of Co_N linear atomic chains with 1 ≤ N ≤ 10 are studied within density functional theory using the generalized gradient approximation . A linear scaling between the binding energy per atom and the inverse of the number of atoms in the chain is found. For the optimized geometries, our results show a dimerization effect for chains of few atoms but for bigger ones the phenomena disappear in the center but remains at the ends due to finite size effects. The spin moment, the orbital moment and the magnetic anisotropy energy were investigated. For large chains, the orbital and spin moments have a tendency to become uniform. Enhanced spin and orbital moments were found due to the reduced coordination number compared to the cobalt in bulk. The cobalt chain of five atoms has the biggest magnetic anisotropy energy with an outstanding 8 meV, suggesting that it could have applications in ultrahigh density magnetic memories and hard disk. 

We present an open-source software package, Vegas, for the atomistic simulation of magnetic materials. Using the classical Heisenberg model and the Monte Carlo Metropolis algorithm, Vegas provides the required tools to simulate and analyze magnetic phenomena of a great variety of systems. Vegas stores the history of the simulation, i.e. the magnetization and energy of the system at every time step, allowing to analyze static and dynamic magnetic phenomena from results obtained in a single simulation. Also, standardized input and output file formats are employed to facilitate the simulation process and the exchange and archiving of data. We include results from simulations performed using Vegas, showing its applicability to study different magnetic phenomena.

Differential and integral cross sections for elastic scattering of electron by NH₃ molecule are investigated for the energy ranging from 10 eV to 20 keV.  The calculations are carried out in the framework of partial wave formalism describing the target molecule by means of one center molecular Hartree-Fock functions.  A spherical complex optical potential used includes a static part – obtained here numerically from quantum calculation – and fine effects like correlation, polarization and exchange potentials. The results obtained in this model point out clearly the role played by the exchange and the correlation-polarization contributions in particular at lower scattering angles and lower incident energies. Both differential and integral cross sections obtained are compared with a large set of experimental data available in the literature and well agreement is found throughout the scattering angles and whole energy range investigated here.

In this work, we describe the application of a micro-spatial thermal lens spectroscopy setup (thermal lens microscope, TLM) with coaxial counter-propagating pump and probe laser beams and an integrated passive optical Fabry-Perot to quantify the Cr-VI concentration in water during a photocatalytic reaction in-situ. A series of test samples was analyzed using the 1,5 diphenil carbazide colorimetric method. A calibration curve was obtained by plotting of the TLM signal as a function of the concentration of Cr(VI) in a range between 0 and 10 μg/L (1 μg/L = 1 ppb, part per billion), with a detection limit of 53 ng/L (1 ng/L = 1 ppt, part per trillion). A solution of 10 μg/L Cr(VI) in distillated water was placed into a cell in contact with an iron-incorporated titanium dioxide film, which was previously grown onto a 1 mm thick glass microscope slide by the sol-gel dip-coating technique. The TLM signal was registered as a function of the photocatalysis time measured from the beginning of the process, radiating the film with UV-violet light. The Cr(VI) concentration was determined with the calibration curve and after the first 50 minutes a reduction of 95 % of Cr(VI) was observed, being the chemical reaction kinetic described by a potential time decreasing function.

We present a compact source of sub-nanosecond infrared laser pulses. The system is a diode-pumped, passively Q-switched Nd:YLF laser that emits 1.047µm pulses with a FWHM duration of 750 ps, an energy of 2.3 mJ per pulse at a repetition rate of over 40 Hz. We also provide a simple rate-equation model that adequately describes the experimental results. This laser is ideal for several applications that require energetic sub-nanosecond pulses that cannot be obtained easily with actively Q-switched lasers or mode-locked lasers.

Numerical study of three dimensional Carreau liquid flow with heat and mass transport features over a variable thickness sheet filled with microorganisms is analyzed. We considered the non-uniform heat sink or source and multiple slip effects. The governing non-linear partially differential expressions are developed into ordinary differential systems by using variable transformations. These expressions are solved numerically by using Runge-Kutta fourth order method connected with shooting methodology. A Parametric study is implemented to demonstrate the effects of Hartmann number, Prandtl number, Weissenberg number, Peclet number, chemical reaction and heat sink/source parameters on liquid velocity, temperature and concentration profiles. The quantities of physical interest are described within the boundary layer. From this analysis, we found that the magnetic parameter decrease the local Sherwood and local Nusselt numbers for both and cases. The constraint of chemical reaction enhances the mass transfer rate and decelerates the density of motile mass transfer rate. The space dependent and temperature dependent heat source/sink suppress the local Nusselt number.

This paper discusses experimentally the advance and distribution of preformed,
wet aqueous foam ow when it is injected into a horizontal rectangular
cell with dierent transversal opening channels. Speeds of the simultaneous
foam fronts in each channel were obtained experimentally. The
relationship between bubble size and the opening of the channel, the apparent
viscosity and the friction factor were obtained with experimental data.
Experiments were performed under three dierent injection pressures. The
diversion of the foam ow into the transversal channel was also studied. With
these experiments we show that the foam ow stream is aected by sudden
changes in the geometry.

In this paper, we have generalized the fractional cubic isothermal auto-catalytic chemical system (FCIACS) with Liouville-Caputo, Caputo-Fabrizio-Caputo, and Atangana-Baleanu-Caputo fractional time derivatives, respectively. We apply the Homotopy Analysis Transform Method (HATM) to compute the approximate solutions of FCIACS using these fractional derivatives. We study the convergence analysis of HATM by computing the residual error function. Also, we find the optimal values of h so we assure the convergence of the approximate solutions. Finally we show the behavior of the approximate solutions graphically. The results obtained are very effectiveness and accuracy.