Vol. 64 No. 1 Jan-Feb (2018): Revista Mexicana de Física

Two-photon transitions from ground state to excited and ionized states are studied. The energy levels and radial matrix elements of an
impenetrable spherically confined hydrogenic atom embedded in plasma environment are evaluated using accurate Bernstein-polynomial
(B-polynomial) method. Transition probability amplitudes, transparency frequencies and resonance enhancement frequencies for various
transitions, namely, $1s-2s$, $1s-3s$ and $1s-3d$ are evaluated for various values of confining Debye potential parameter. The effect of
spherical confinement is studied and explained.

Contact angle measurements were performed on Co$^{60}$ gamma-ray irradiated poly(lactic acid) (PLA) samples at several doses, using the sessile drop method. It was found that irradiation alters the wettability of PLA. In particular, PLA wetting behavior changes from moderately hydrophilic at low dose ($< 100$ kGy), to hydrophobic after the samples were exposed above a threshold dose ($\approx 200$ kGy). At low doses, wettability follows the Wenzel relation but beyond a threshold dose the Cassie-Baxter regime takes place. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show that surface roughness of the samples increases as the applied dose increases. PLA wettability changes are mainly a consequence of surface modifications as roughness promotes the formation of air pockets under a drop, which reinforces the hydrophobic nature of is surface. Co$^{60}$ gamma ray irradiation can be used to tailor wetting properties of PLA. The method might have also application to produce biphilic PLA. Finally it is very important to remark that reported contact angles values for PLA (by different authors) of untreated samples vary from 60 to 85 degrees. Reasons for this behavior are given in this article.

In this work, thehighorderperturbationformulaeoftheelectronparamagneticresonance (EPR) parameters, g \& A for a VO$^{2+}$3d$^1$ ion in tetragonal symmetry (compressedoctahedron) are adopted to theoreticallystudytheseparametersofthe tetragonal VO$^{2+}$ center in CaO-Al$_2$O$_3$-SiO$_2$ system. Becausethespin-orbit (SO) couplingparameter$\zeta_p$ ($\approx$151 cm$^{-1}$) ofligand O$^{2-}$ is close to that ($\approx$248 cm$^{-1}$) ofthe central V$^{4+}$ ion in thecluster [VO$_6$]$^{8-}$withstrongcovalence, theeffectofthe SO couplingparameter$\zeta_p$onEPRparameters are consideredonthebasisofclusterapproach. TheobtainedEPRparameters show goodagreementwiththe experimental values and needfeweradjustableparametersthanpreviousstudies. Andthe local structureofthe tetragonal VO$^{2+}$ center in CaO-Al$_2$O$_3$-SiO$_2$ system due to Jahn-Tellereffect is obtained from theanalysisoftheEPRparameters. Theresults are discussed.

Cadmium telluride (CdTe) nanoparticles were grown in the interior of a SnO2 transparent matrix by means of the r.f. sputtering technique.
X-ray diffraction (XRD) patterns reveal that the CdTe quantum dots (QD) grow in the hexagonal wurtzite (W) phase. Diffraction data show
that SnO2 is constituted by nanoparticles also, with size of the same order of the W-CdTe QD. The broad bands of the XRD patterns and the
Scherrer formula allowed, by assuming a spherical shape, the W-CdTe QD size calculation, which have average diameters in the range 4.8
- 14.0 nm. These data were confirmed by electron microscopy images. Optical absorbance gives information to calculate the energy of the
two lowest excitonic states (band gap). The Raman spectra show several broad bands in the range 100 - 200 cm-1, on which deconvolution
allows separate five modes in the 100 - 200 cm-1 interval. The bands correspond to CdTe and tetragonal Te. The transversal optic (TO) and
longitudinal optic (LO) modes of CdTe at the Γ point of the first Brillouin zone, for phonons in nanoparticles, follow the expected behavior
if the radius of crystal decreases, taking into account that the selection rules for momentum conservation are relaxed.

The authors successfully synthesized aluminum nitride (AlN) nanowires by chemical vapor deposition (CVD) using aluminum (Al) powder
and ammonia (NH3) as starting materials with nickel (Ni) as catalyst. The morphologies, the composition and structure information of
samples were characterized by scanning electron microscopies (SEM), transmission electron microscopies (TEM), X-ray diffraction (XRD)
and X-ray photoelectron spectroscopy (XPS). Nanostructures with manifold morphologies were obtained by tuning the growth temperature
and the growth time. The AlN nanowires are proved to have the wurtzite structure as demonstrated by XRD. The growth mechanism of AlN
nanowires assisted with Ni catalyst is proposed to explain the nucleation and growth of nanowires.

There are two main physical properties needed to fabricate 1D photonic structures and form perfect photonic bandgaps: the quality of the

thickness periodicity and the refractive index of their components. Porous silicon (PS) is a nano-structured material widely used to prepare 1D

photonic crystals due to the ease of tuning its porosity and its refractive index by changing the fabrication conditions. Since the morphology

of PS changes with porosity, the determination of PS’s refractive index is no easy task. To find the optical properties of PS we can use

different effective medium approximations (EMA). In this work we propose a method to evaluate the performance of the refractive index of

PS layers to build photonic Bragg reflectors. Through a quality factor we measure the agreement between theory and experiment and therein

propose a simple procedure to determine the usability of the refractive indices. We test the obtained refractive indices in more complicated

structures, such as a broadband Vis-NIR mirror, and by means of a Merit function we find a good agreement between theory and experiment.

With this study we have proposed quantitative parameters to evaluate the refractive index for PS Bragg reflectors. This procedure could have

an impact on the design and fabrication of 1D photonic structures for different applications.

We provide a statistical mechanics approach to study the linear microrheology of thermally equilibrated and homogeneous ferrofluids. The

expressions for the elastic and loss moduli depend on the bulk microstructure of the magnetic fluid determined by the structure factor of the

suspension of magnetic particles. The comparison of the predicted microrheology with computer simulations confirms that as a function of

relaxation frequency of thermal fluctuations of the particle concentration both theory and simulations have the same trends. At very short

frequencies the viscous modulus relates to the translational and rotational self-diffusion coefficients of a ferro-particle.

We report results of the first principles calculations of structural, electronic and optical properties of SrF2 under pressure, performed using full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory as implemented on WIEN2k code. The exchange-correlation energy functional has been treated with generalised gradient approximation (GGA) for structural optimization,while the Tran-Blaha modifed Becke-Johnson potential (TB-mBJ) has been employed for electronic and optical calculations. Our results show that the first transition from Fm3m to Pnam structure occurs at 5.8 GPa and the second transformation from Pnam to P63/mmc structure takes place at 24.8 GPa. Our electronic calculation indicates an indirect gap X-Γ of Fm3m structure, direct gap Γ-Γ of Pnam structure and indirect gap Γ-K of P63/mmc structure. We do not observe the metallization up to 210 GPa. The linear optical properties such as absorption coefficient, reflectivity, refraction index, conductivity and energy loss function have been derived from calculated complex dielectric function for a wide energy range of 0-50 eV and pressure up to 50 GPa, and analyzed in detail.

In this paper, the numerical solutions of the Maung-Norbury-Kahana equation which has the complicated form of the eigenvalues are presented. Taken as examples, the bound states $e^+e^-$, $\mu^+\mu^-$ and $\mu^+e^-$ are discussed by employing the Maung-Norbury-Kahana equation with the Coulomb potential.

Weintend to use the description oftheelectron orbital trajectory in the de Broglie-Bohm (dBB) theory to assimilate to a geodesiccorresponding to the General Relativity (GR) and get from itphysicalconclusions. ThedBBapproachindicatesustheexistenceof a non-local quantumfield (correspondingwiththequantumpotential), anelectromagneticfield and a comparativelyveryweakgravitatoryfield, togetherwith a translationkineticenergyofelectron. Ifweadmitthatthosefields and kineticenergy can deformthespace time, according to Einstein'sfield equations (and to avoidtheviolationoftheequivalenceprinciple as well), we can madethehypothesisthatthegeodesicsof this space-time deformation coincide withtheorbitsbelonging to thedBBapproach (hypothesisthat is coherentwiththestabilityofmatter). Fromit, we deduce a general equation that relates thecomponentsofthemetric tensor. Thenwe find anappropriatemetric for it, bymodificationofanexactsolutionofEinstein'sfield equations, whichcorresponds to dust in cylindricalsymmetry. Thefoundmodelproofs to be in agreementwiththebasicphysicalfeaturesofthehydrogenquantum system, particularlywiththeindependenceoftheelectronkineticmomentum in relationwiththeorbit radius. Moreover, themodel can be done Minkowski-like for a macroscopicshortdistancewith a convenientelectionof a constant. According to this approach, theguiding function ofthewaveontheparticlecould be identifiedwiththedeformationsofthespace-time and thestabilityofmatterwould be easilyjustifiedbythe null accelerationcorresponding to a geodesicorbit.

The application of the integrals of the motion of a quantum system in deriving Green function or propagator is established. 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 explicit expressions for the Green functions of the damped harmonic oscillator, the harmonic oscillator with strongly pulsating
mass, and the harmonic oscillator with mass growing with time are obtained in co-ordinate representations. The connection between the
integrals of the motion method and other method such as Feynman path integral and Schwinger method are also discussed.

We discuss some interesting aspects of the well known quantum equivalence between the O(3) - σ and CP1 models in 3D, working in
the canonical and in the path integral formulations. We show first that the canonical quantization in the hamiltonian formulation is free
of ordering ambiguities for both models. We use the canonical map between the fields and momenta of the two models and compute the
relevant functional determinant to verify the equivalence between the phase-space partition functions and the quantum equivalence in all
the topological sectors. We also use the explicit form of the map to construct the soliton operator of the O(3) - σ model starting from the
representation of the operator in the CP1 model, and discuss their properties.

Se reporta un estudio experimental de la t\'ecnica de fabricaci\'on de acopladores bic\'onicos de fibra \'optica y su aplicaci\'on al desarrollo de multiplexores por divisi\'on de longitud de onda (WDM, wavelengthdivisionmultiplexer). Los par\'ametros de fabricaci\'on requeridos para el WDM son obtenidos del an\'alisis te\'orico de la respuesta espectral de un acoplador d\'ebilmente fusionado. Como caso particular, se reporta el desarrollo de un WDM 1550/1850 nm, el cual es un dispositivo fundamental en el desarrollo de l\'aseres de fibra \'optica dopada con Tulio. Las dimensiones finales del dispositivo son una longitud total de 36.3 mm, una cintura uniforme de 6.5 mm de largo con una secci\'on transversal de 23 μm. El dispositivo exhibe bajas p\'erdidas por inserci\'on de 0.17 dB, un espaciamiento entre canales de 303.52 nm y ancho de banda de  ± 50 nm. Este trabajo establece una gu\'ia de fabricaci\'on para el desarrollo de acopladores y dispositivos WDM de fibra \'optica.

We impose an upper bound to the amount of agents wealth in binary exchanges, but when the upper bound is continuously lowered, then
the entropy S decreases because the system exhibits more order. Furthermore, negative temperatures can mathematically obtained when the slope of the entropy as a function of the total energy (T = $(dS/dE)^{-1}$) is negative.