Scielo RSS <![CDATA[Revista mexicana de física]]> http://www.scielo.org.mx/rss.php?pid=0035-001X20160001&lang=es vol. 62 num. 1 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Effect of nitrogen gas in the agglomeration and photoluminescence of Zn-ZnO nanowires after high-temperature annealing</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100001&lng=es&nrm=iso&tlng=es The effect of anti-agglomeration and enhanced photoluminescence after high-temperature annealing of Zn-ZnO nanowires in nitrogen atmosphere is reported. The Zn-ZnO nanowires were deposited by the hot filament chemical vapor deposition technique and subsequently annealed at 1100°C in oxygen or nitrogen atmospheres. It was found that under both annealing atmospheres, the structure of the nanowires was completely oxidized. Morphological studies suggest that annealing under oxygen-rich atmosphere, grain growth occurs, resulting in a continuous surface with a micrograin-shaped structure. However, it seems that nitrogen-rich annealing partially prevents complete agglomeration and longitudinal structures composed by nanometric grains were observed. Although photoluminescence properties of the annealed nanowires are improved in both annealing atmospheres, it was observed that the PL spectrum of nanowires annealed in nitrogen showed a stronger UV emission than that of the oxygen annealed nanowires. <![CDATA[<b>Fabrication and characterization of ZnO</b>: <b>Zn(n<sup>+</sup>)/Porous-Silicon/Si(p) heterojunctions for white light emitting diodes</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100002&lng=es&nrm=iso&tlng=es The fabrication and characterization of electroluminescent ZnO:Zn(n+)/Porous Silicon/Si(p) heterojunctions is presented. Highly conductive ZnO films (ZnO:Zn(n+)) were produced by applying a temperature annealing at 400°C by 5 min to the ZnO/Zn/ZnO arrange formed by DC sputtering, and the Porous Silicon (PS) films were prepared on p-type (100) Si wafers by anodic etching. The ZnO:Zn(n+)/PS/Si(p) heterojunction is accomplished by applying a brief temperature annealing stage to the entire ZnO/Zn/ZnO/PS/Si structure to preserve the PS luminescent characteristics. The ZnO:Zn(n+) films were characterized by X-ray diffraction and Hall-van der Pauw measurements. The PS and ZnO:Zn(n+) films were also studied by photoluminescence (PL) measurements. The current-voltage characteristics of the heterojunctions showed well defined rectifying behavior with a turn-on voltage of 1.5 V and ideality factor of 5.4. The high ideality factor is explained by the presence of electron tunneling transport aided by energy levels related to the defects at the heterojunction interface and into the PS film. The saturation current and the series resistance of the heterostructure were 4 x 10-7 A/cm² and 16 Ω-cm², respectively. White color electroluminescence is easily observed at the naked eye when excited with square wave pulses of 8 V and 1 KHz.<hr/>Se reportan los resultados de la fabricación y caracterización de heterouniones ZnO:Zn(n+)/Silicio Poroso/Si(p). Las películas conductivas de ZnO (ZnO:Zn(n+)) se obtuvieron aplicando un breve tratamiento térmico a 400°C por 5 min al arreglo ZnO/Zn/ZnO formado por DC sputtering, y las películas de Silicio Poroso (PS) se obtuvieron por anodizacion electroquímica de obleas de silicio tipo-p con orientacion (100). La preparation de las heterouniones de ZnO:Zn(n+)/PS/Si(p) se hizo aplicando el tratamiento termico a la estructura completa de ZnO/Zn/ZnO/PS/Si para conservar las propiedades luminiscentes del PS. Las características de las películas de ZnO:Zn se determinaron por Difraccion de Rayos X y por mediciones electricas con el metodo de Hall-van der Pauw. Las películas de SP y ZnO:Zn(n+) se caracterizaron tambien por mediciones de fotoluminiscencia. Las características corriente-voltaje de las heterouniones presentan buen comportamiento rectificante, con voltaje de encendido de 1.5 V y factor de idealidad de 5.4. El factor de idealidad indica que el transporte de carga esta controlado por el mecanismo de corriente tmiel auxiliado por estados de defecto situados en la interface y en la película de PS. Los valores de la corriente de saturation y la resistencia serie fueron de 4 x 10-7 A/cm² y 16 Ω-cm², respectivamente. Se produce electroluminiscencia observable a simple vista de color blanco, cuando las heteroestructuras se polarizan con una señal pulsada de 8V de amplitud y frecuencia de 1 KHz. <![CDATA[<b>Efectos del solvente en la respuesta óptica de un sistema de dos niveles</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100003&lng=es&nrm=iso&tlng=es En este trabajo fueron modelados los efectos del reservorio térmico sobre las propiedades (ópticas no lineales absortivas y dispersivas de un sistema molecular de dos niveles en presencia de campos electromagnéticos clásicos. Los efectos colectivos del reservorio térmico son modelados como una frecuencia dependiente del tiempo, manifestada en el ensanchamiento del nivel superior, de acuerdo a una función aleatoria prescrita. Expresiones analíticas fueron obtenidas para las susceptibilidades no lineales inducidas y las propiedades ópticas, usando ecuaciones de Bloch ópticas estocásticas. Cálculos numéricos se llevaron a cabo para construir las superficies correspondientes a estas propiedades ópticas como una función de la desintonización de frecuencias bombeo-prueba, las relaciones entre los tiempos de relajación longitudinal y transversal, y la concentración molecular de soluto. Finalmente, se observa una atenuación de estas respuestas ópticas por los efectos de solvente y la alta intensidad del bombeo.<hr/>In this work, the thermal reservoir effects over the absorptive and dispersive nonlinear optical properties of a two-level molecular system in presence of classical electromagnetic fields, were modeled. The collective effects proper of the thermal reservoir are modeled as a time dependent frequency, whose manifestation is the broadening of the upper level according to a prescribed random function. Using the stochastic optical Bloch equations, analytical expression for the nonlinear induced susceptibilities and absorptive and dispersive optical properties, were obtained. Numerical calculations were carried out to construct surfaces corresponding to these optical properties as a function of the pump-probe frequency detuning, relationships between the longitudinal and transversal relaxation times, and molecular concentration of solute. Finally, we see an attenuation of these optical responses by the solvent effects and the high pump-intensity <![CDATA[<b>Analysis of the adiabatical pulsation of Cepheids</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100004&lng=es&nrm=iso&tlng=es A model for the adiabatical pulsation of a Cepheid based on primary physical principles such as energy conservation, mechanical adiabaticity and hydrostatic equilibrium is presented. The adiabatical treatment of the pulsation has been already studied by other authors but the novelty of our model is that it allows to obtain physical parameters of the star, namely the amplitude of the oscillation, the mass, maximal velocity of the pulsation and luminosity. <![CDATA[<b>Hamiltonian dynamics</b>: <b>four dimensional BF-like theories with a compact dimension</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100005&lng=es&nrm=iso&tlng=es A detailed Dirac's canonical analysis for a topological four dimensional BF-like theory with a compact dimension is developed. By performing the compactification process we find out the relevant symmetries of the theory, namely, the full structure of the constraints and the extended action. We show that the extended Hamiltonian is a linear combination of first class constraints, which means that the general covariance of the theory is not affected by the compactification process. Furthermore, in order to carry out the correct counting of physical degrees of freedom, we show that must be taken into account reducibility conditions among the first class constraints associated with the excited KK modes. Moreover, we perform the Hamiltonian analysis of Maxwell theory written as a BF-like theory with a compact dimension, we analyze the constraints of the theory and we calculate the fundamental Dirac's brackets, finally the results obtained are compared with those found in the literature. <![CDATA[<b>Chaotic synchronization of irregular complex network with hysteretic circuit-like oscillators in hamiltonian form and its application in private communications</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100006&lng=es&nrm=iso&tlng=es In this paper, a study on chaotic synchronization of an irregular network is made. Synchronization is achieved by using a modified Hamiltonian approach in a bidirectional irregular arrayed network made of 20 chaotic oscillators. The chaotic oscillator used as example is the Hysteretic circuit. Afterwards the concept is used in chaotic encryption to send secured confidential analog information. As a result, an image is encrypted using additive chaotic encrytion with two channels.<hr/>En este trabajo, un estudio de la sincronización caótica de una red irregular es realizada. La sincronización es alcanzada al usar una modificación al enfoque Hamiltoniano sobre una red bidireccional irregular creada por 20 osciladores caóticos. El oscilador caótico usado como ejemplo es el circuito de Histeresis. Después, este concepto es usado en encriptamiento caótico para enviar información análoga confidencial de manera segura. Como resultado, una imagen es encriptada usando encriptamiento caótico aditivo de dos canales. <![CDATA[<b>Transporte de electrones en anillos cuánticos de confinamiento variable</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100007&lng=es&nrm=iso&tlng=es Se analiza las influencias que tiene sobre el transporte de electrones en un anillo cuántico de Aharonov-Bohm el modelar las entradas en forma de constricciones cuadráticas tipo contacto de punto cuántico (QPCs), con presencia de interacción espín-orbita tipo Rashba. Se obtienen comportamientos ondulatorios de la conductancia consistentes con las oscilaciones de Aharonov-Bohm (AB) y Aharonov-Casher (AC) y se estudian las modificaciones de las mismas ante variaciones de la opacidad de los QPCs. Se reproducen comportamientos anteriormente analizados y se aprecian marcadas diferencias en los períodos de los armónicos de la conductancia con respecto a trabajos anteriores del tema. Además, se discuten los formalismos teoricos, obteniendose toda la fenomenología del anillo como solucion directa de la ecuación de Pauli para un anillo unidimensional. Se plantean algunos de los posibles efectos de las asimetrías en los confinamientos en los QPCs sobre los comportamientos antes discutidos.<hr/>Influences on electronic transport in a Aharonov-Bohm quantum ring of modeling the entrances as quadratic quantum point contact (QPC) are taken into account in the presence of Rashba spin-orbit interaction. Oscillations in the conductance are obtained, consistent with Aharonov-Bohm and Aharonov-Casher oscillations and their behaviour is analyzed as the opacity in the quantum point contacts changes. We are able to reproduce previous phenomena and obtain differences with other results for the periods between the conductance harmonics. We discuss the theoretical formalism, and besides have studied all the physics of interest by solving the one-dimensional Pauli equation for the ring solely. Some possible effects of the asymmetrical confinements in the QPCs are studied as well as their influence on the previous results. <![CDATA[<b>Electrical characterization of GaN/ALN heterostructures grown by molecular beam epitaxy on silicon substrates</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100008&lng=es&nrm=iso&tlng=es In this paper, n-type gallium nitride thin films were grown on p-type and n-type silicon substrates by Molecular Beam Epitaxy technique, employing an aluminum nitride layer as an insulator buffer coating. The samples constitute primary semiconductor-insulator-semiconductor or SIS heterostructures, on which silver electrical contacts were deposited superficially by sputtering for electrical characterization purposes. The current intensity vs. voltage curves, charge carriers density, diffusion coefficient and mobility of charge carriers were determined in darkness conditions. Under AM1.5 homogenous illumination, the samples exhibited relatively large open-circuit voltage values, but low values for energy conversion efficiencies. Additionally, the dependency in photon energy of the open-circuit voltage was determined, in the range 1.77 eV to 4.13 eV, showing a minimum in the energy corresponding to the GaN energy band gap. Finally we discuss the prevailing charge transfer mechanism. <![CDATA[<b>Simulation of cylindrical Poiseuille flow in multiparticle collision dynamics using explicit fluid-wall confining forces</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100009&lng=es&nrm=iso&tlng=es Multiparticle collision dynamics (MPC) is a numerical technique that has been extensively used in recent years to simulate fluids supporting hydrodynamic interactions and thermal fluctuations. In this paper, we describe a method that allows MPC fluids to be confined in cavities with a complex geometry. This method is based on the introduction of an explicit repulsive interaction between the particles of the MPC fluid and the walls of the confining cavities. We apply the proposed technique in simulations of MPC fluids confined in cylindrical channels and subjected to uniform pressure gradients. We show that our method yields the correct hydrodynamic cylindrical Poiseuille flow for stick boundary conditions. We conduct an extensive numerical analysis of the method to determine the kinematic viscosity of the simulated fluid, to study finite size effects and to establish the limits for its applicability. We conclude that this technique is reliable to simulate cylindrical Poiseuille flow for a wide range of system sizes, applied pressure gradients, and viscosities and densities of the simulated fluids. <![CDATA[<b>Conversion of zero point energy into high-energy photons</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100010&lng=es&nrm=iso&tlng=es An unusual phenomenon, observed in experiments, is studied. X-ray laser bursts of keV energy are emitted from a met al where long-living states, resulting in population inversion, are totally unexpected. Anomalous electron-photon states are revealed to be formed inside the met al. These states are associated with narrow, 10-11 cm, potential well created by the local reduction of zero point electromagnetic energy. In contrast to analogous van der Waals potential well, leading to attraction of two hydrogen atoms, the depth of the anomalous well is on the order of 1 MeV . The states in that well are long-living which results in population inversion and subsequent laser generation observed. The X-ray emission, occurring in transitions to lower levels, is due to the conversion of zero point electromagnetic energy. <![CDATA[<b>Automatización de un microscopio de barrido por efecto túnel utilizando una tarjeta OMB-DaqBoard/2000 y LabVIEW</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0035-001X2016000100011&lng=es&nrm=iso&tlng=es El artículo muestra el trabajo para la automatización y control de un microscopio de barrido por efecto túnel (STM) construido por los autores. La interfase entre la computadora y el microscopio ha sido implementada por medio de una tarjeta de adquisicion de datos OMB-DaqBoard/2000. Un programa desarrollado en LabVIEW genera las seriales requeridas para el barrido X-Y, y simultáneamente adquiere los voltajes de Z relacionados con la corriente de tunel entre la punta y la muestra. El programa construye la imagen de microscopia de la superficie estudiada a partir de los voltajes Z. El proceso para la calibration del instrumento utilice) imágenes de resolución atómica de superficies conocidas.<hr/>This paper shows the work for the automation and control of a scanning tunneling microscope (STM) built by the authors. The interface between the computer and the microscope has been implemented by mean of the data acquisition board OMB-DaqBoard/2000. A developed software in LabVIEW generates the signals required for the X-Y scanning, and it simultaneously acquires the Z voltages, related to the tunneling current between the tip and the sample. The program constructs the microscopy images of the studied surface from the Z voltages. The process to calibrate the instrument using atomic resolution images of known samples is also presented.