Vol. 64 No. 2 Mar-Apr (2018): Revista Mexicana de Física

We determined the thermal effusivity of liquids using an optical sensor based on the laser beam deflection technique, without directly heat the samples and thus minimally altering them. Applying a heat pulse in a thermo-optical slab we generate a unidimensional temperature distribution. This temperature distribution modifies the refractive index in the slab that finally causes the deflection of a laser beam that propagates perpendicularly to the direction of the heat propagation. The deflection of the laser beam depends on the interaction of the thermal energy with the sample at the slab interface. The exchange of thermal energy between the thermo-optical slab and the sample depends, on the thermal properties of both of them, being the thermal effusivity of our particular interest. Utilizing a theoretical model, we estimate the thermal effusivity of liquids using tridistilled-water and glycerine as reference.
We present a simplified version of a past sensor proposal as well as the theoretical analysis of the sensor response. We obtain the thermal effusivity of tridistilled water and glycerine samples with a maximum error of 3%. Finally, we estimate the thermal effusivity of dissolutions of NaCl in tridistilled-water with maximum error of 7.3%.

We designed and implemented an electronic oscillatory system consisting of two mutually coupled oscillators (each one based on the 555-
timer IC working in astable mode), and investigated its synchronization dynamics. For that purpose, we characterized the phase-locking
rhythms achieved in a large set of values of the uncoupled frequency ratio and of the coupling strength between both oscillators. We further
developed an ODE mathematical model that qualitative reproduces the obtained experimental results and provides further insight into the
system behavior.

In this work, the preliminary design of an electron linear particle accelerator (eLINAC) capable of
delivering a beam within an energy range of 5 to 100 MeV using the S-band acceleration frequency
is presented. At its peak energy, it can be used as an injector for a synchrotron light source,
independent free electron laser, or to generate neutrons. The eLinac will be separated into three
beam lines to deliver electron beams at dierent energies permitting the possibility of industrial
applications such as food irradiation, medical use, and national security.

Se llevó a cabo la caracterización microestructural de la aleación Co-20% en peso de Cr solidificada en un molde metálico de cobre con una velocidad de enfriamiento de 450 K/s, la cual fue seleccionada de un grupo de aleaciones de Co-20 a 44% en peso de Cr, solidificadas sistemáticamente a diferentes velocidades de enfriamiento (130 a 450 K/s), previamente reportadas [1], con el propósito de presentar la microestructura resultante, la cual consistió de dendritas columnares sin la presencia de fase eutéctica o compuestos intermetálicos en regiones interdendríticas. Una velocidad de enfriamiento de 450 K/s fue suficiente para obtener dendritas columnares las cuales estuvieron compuestas por más de 90% de martensita HCP ε-Co, siendo el remanente la fase metaestable FCC γ-Co. Esta velocidad de enfriamiento permitió la generación de defectos tales como vacancias, dislocaciones y fallas de apilamiento. Estas últimas sirvieron como sitios de nucleación para el crecimiento de placas de martensita HCP γ-Co.

We present a preliminary study on the effects of alpha - Al_2O_3 nanoparticles (NPs) in the transport properties of Bi_1.65 Pb_0.35 Sr_2Ca_2 Cu_3O_y ((Bi,Pb)-2223) ceramic samples. The (Bi,Pb)-2223 samples were synthesized by the solid state reaction method and alpha - Al_2O_3 NPs, with concentrations ranging from 0.1 to 0.8 wt. percent (% wt.), were added before the last heat treatment. All samples were characterized by means of X-rays diraction patterns, and temperature and magnetic eld dependence of the electrical resistivity, rho(T;Ba). From the experimental (T;Ba) data we extracted the critical temperature dependence of the % wt. of alpha - Al_2O_3 NPs, and by using the Arrhenius
plot the eective pinning energy has been also determined. The results indicate that mostly of the transport properties are optimized for the sample B03.

We study the generation of an electromagnetic current in monolayer graphene immersed
in a weak perpendicular magnetic eld and radiated with linearly polarized monochromatic light.
Such a current emits Brehemstrahlung radiation with the same amplitude above and below the
plane of the sample, in the latter case consistent with the small amount of light absorption in the
material. This mechanism could be an important contribution for the re exion of light phenomenon
in graphene.

We investigate the structural, elastic, electronic and magnetic properties of the Heusler compounds Cu₂MnSi, Cu₂MnAl and Cu₂MnSi_1-xAl_x quaternary alloys, using the full-potential linear-augmented plane-wave method (FP-LAPW) in the framework of the density functional theory (DFT) using the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE). Our results provide predictions for the quaternary alloy Cu₂MnSi_1-xAl_x            (x = 0.125, 0.25, 0.375, 0.5) in which no experimental or theoretical data are currently available. We calculate the ground state’s properties of Cu₂MnSi_1-xAl_x alloys for both nonmagnetic and ferromagnetic configurations, which lead to ferromagnetic and metallic compounds. Also, the calculations of the elastic constants and the elastic moduli parameters show that these quaternary Heusler alloys are ductile and anisotropic.

In this work, we present and discuss results concerning the hard magnetic behavior of rare earth-free MnBi alloys obtained by suction casting technique. The physics of coercivity for these type of alloys is based on the nucleation process of reverse domains, which in turn is determined by the alloy microstructure features such as phase distribution, morphology, grain size and in particular, defects, which are characteristic of
real materials. The microstructure of the as-cast alloy presented here comprises the formation of the Low Temperature Intermetallic Phase (LTIP)-MnBi, interspersed within Bi- and Mn-rich areas. A considerable intrinsic coercivity field of 238 kA/m together with a saturation magnetization of 0.04 T were observed. The nucleation controlled mechanism of this alloy was described in terms of the Kronm¨uller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces and local demagnetizing fields. A notorious switching of coercivity mechanism associated with domain wall pinning was found to be produced upon annealing of the alloy at 583 K for 24 hrs, yielding a drastic reduction of coercivity (down to 16 kA/m). The key microstructural feature determining the switching of coercivity mechanism is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of LTIP.

The double perovskite compound Sr2FeMoO6 has a half-metallic ferromagnetic character and a
high Curie temperature (420 K). Fe-Mo usually present some degree of disorder in either Fe or
Mo ions, it is therefore fundamental to understand the role of electronic and structural parameters
controlling the half-metallic character together with a Curie temperature as high as possible. We replaced
divalent Sr2+ by trivalent La3+ ions in Sr2FeMoO6 system to observe the Curie temperature
behavior. We present an electronic approach using the Green's functions and the renormalization
perturbation expansion method, with localized Fe-spins and conduction Mo-electrons interacting
with the local spins via a double-exchange-type mechanism. We also include the electronic correlations
among the conduction electrons within a mean-eld approximation. Our results show
the density of states and the Curie temperature behavior when La concentration increases for the
Sr2y-yLayFeMoO6 system

The application for the integrals of the motion of a quantum system in deriving Green function or propagator is presented. The Green function is shown to be the eigenfunction of the integrals of the motion which described initial points of the system trajectory in the phase space. The exact expressions for the Green functions of the dual damped oscillators and the coupled harmonic oscillators are evaluated in co-ordinate representations. The relation between the integrals of the motion method and other methods such as Feynman path integral and Schwinger method are also presented.

A covariant generalization of a non-relativistic stochastic quantum mechanics introduced by de la Peña and Cetto is formulated. The analysis is done in space-time and avoids the use of a non-covariant time evolution parameter in order to search for Lorentz invariance. The covariant form of the set of iterative equations for the joint coordinate and momentum distribution function Q(x; p) is derived and expanded in power series of the coupling of the particle with the stochastic forces. Then, particular solutions of the zeroth order in the charge of the iterative equations for Q(x; p) are considered. For them, it follows that the space-time probability density ρ(x) and the function S(x) which gradient defines the mean value of the momentum at the space time point x, define a complex function ψ(x) which exactly satisfies the Klein-Gordon (KG) equation. These results for the zeroth order solution reproduce the ones formerly and independently derived in the literature. It is also
argued that when the KG solution is either of positive or negative energy, the total number of particles conserves in the random motion. Other solutions for the joint distribution function in lowest order, satisfying the positive condition are also presented here. The are consistent with the assumed lack of stochastic forces implied by the zeroth order equations. It is also argued that such joint distributions, after considering the action of the stochastic forces, might furnish an explanation of the quantum mechanical properties, as associated to ensembles of particles in which the vacuum makes such particles behave in a similar way as Couder’s droplets moving over oscillating liquid surfaces. Some remarks on the solutions of the positive joint distribution problem proposed in the Olavos’s analysis are also presented.

The fractional conformable derivative and its properties have been introduced recently. Using this derivative we obtain a new class of smooth solutions for the Newton’s law of cooling in terms of a stretched exponential function depending on the fractional order parameter 0 < γ ≤ 1. In addition, the convection coefficient of fractional order k(γ) can be calculated easily. Also, it is shown, that in the particular case γ = 1 these solutions become the ordi- nary ones.

We discuss the geometric characterization of the trajectory
of a moving charged particle, for the case of a homogeneous electromagnetic
eld, in De-Sitter space when the motion is governed by the Lorentz equa-
tion. We employ totally relativistic approach during the discussion and it
is based on a systematic use of the four-dimensional Frenet-Serret formulae,
which is adapted to the De-Sitter space to determine the worldline geometry
of the electromagnetic eld acting on the particle in De-Sitter space, and of
the Faraday antisymmetric tensor properties.

We aim to derive the most general f(R) gravity theory, including the matter, so that it be Weyl invariant. Making use of the mathematical equivalence of these theories with an type of scalar-tensor theory, and by imposing the Weyl invariance for the pure gravity as well as for the matter sector, we obtain the fundamental equation that restricts the form of V (phi) (and, accordingly, of f(R)) so that the resulting action to be Weyl invariant in the Jordan frame. We show that this action is not other than the so-called dilaton gravity action with one scalar eld,, which eective mass is R and Phi dependent. In the Einstein frame, the action becomes the Einstein-Hilbert action with the Ricci scalar being constant due to that the eective mass of scalar eld in this frame vanish. So, we can assume that the Ricci scalar, in the Einstein frame, is the true Cosmological Constant. Therefore, is not preposterous to guess that, at least mathematically, all Weyl invariant metric f(R) theory in the Jordan frame is equivalent, at classical level, to the Einstein gravity, in the Einstein frame, with a constant Ricci scalar. At quantum level, as it is known, both theories are not equivalent due to the presence of anomalies in one of the frames.

The classical central field is analyzed within the Hammond theory of radiation reaction force. For the attractive Coulomb field, the trajectories deduced from Ford and Hammond equations are numerically obtained. Ford and Hammond equations are rewritten by using a recent correction to the non-relativistic equations for charged point particles which include a radiation reaction force term. Also, for the attractive Coulomb case, the trajectories are numerically obtained for both corrected equations. A comparison between all these trajectories is made. It is proved that Hammond equation satisfies the constraint proposed by Dirac of getting an equation of motion which should make the electron in the hydrogen atom spiralling inwards and ultimately falling into the nucleus. A further analysis of the applicability of such a theory is described for experiments particularly in Plasma Physics and some comments are made for the generalization of Hammond equation to General Relativity.

The particle and void sizes distributions of the soil are related with the assumption that a void
constant relationship at all di§erent scales and a relationship between pore and particle size of
a given packing. In the relationship between pore and particle sizes, a particle elongation factor
is applied based on the Arya and Paris model. A áow trajectory tortuosity factor based on the
Fuentes model can also be applied. Since the void size distribution is associated to the soil water
retention curve, the proposed relationship can be used to estimate this curve from particle size
distribution.