Vol. 48 No. 4 (2002): Revista Mexicana de Física.

Published: 2002-01-01

Articles

  • Nanocrystalline hard magnetic alloys

    J. I
    283-0
    Abstract:
    The recent progress on the understanding of the physical properties of nanocrystalline hard magnetic alloys, based on Rare earth-Iron-Boron (REFeB) compositions, are reviewed. The discussion includes: fundamental properties, crystal structure, coercivity mechanism, micromagnetic studies and recent applications of these functional alloys. Prospective of future developments are also included.
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  • Shift of saddle-node bifurcation points in modulated Hénon map

    J. M, A. N, V. Aboites
    290-0
    Abstract:
    We study the influence of a harmonic parametric modulation on the positions of critical points in the low-dissipative Hénon map with coexisting period-1 and period-3 attractors. The shift of the saddle-node bifurcation and crisis points depends strongly on the modulation frequency and amplitude. Resonance phenomena play a significant role in the displacement of the attractor boundaries as well as in attractor annihilation due to boundary crisis.
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  • Capture of carriers by quantum wells via optical-phonon deformation potential

    J. Hernández-Rosas, L. Villegas-Lelovsk, G. González de la Cruz, .
    295-0
    Abstract:
    A calculation is reported for the probability of electron capture by a single quantum well that takes into account the quasi-two-dimensional electron system in the quantum well. The influence of continuum resonances, which are localized in the vicinity of the quantum well at energies above the barrier, were considered in the calculations. The scattering rate by optical-phonon deformation potentials as function of the incident electron energy and the well width are discussed.
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  • Dimensional crossover in the non-linear sigma model

    Denjoe O' Connor., C. R, J. A
    300-0
    Abstract:
    We consider dimensional crossover for an $O(N)$ model on a $d$-dimensional layered geometry of thickness $L$, in the $\sigma$-model limit, using ``environmentally friendly'' renormalization. We show how to derive critical temperature shifts, giving explicit results to one loop. We also obtain expressions for the effective critical exponents $\delta_{\rm eff}$ and $\beta_{\rm eff}$ that interpolate between their characteristic fixed point values associated with a $d$ and $(d-1)$-dimensional system in the limits $T\rightarrow T_c(L)$, with $L(T-T_c(L))^{\nu}\rightarrow\infty$, and $T\rightarrow T_c(L)$ for $L$ fixed respectively, where $T_c(L)$ is the $L$-dependent critical temperature of the system.
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  • Polarization in the non-leptonic weak decays of spin-3/2 hyperons

    J. García Ravelo, A. Queijeiro Fontana
    307-0
    Abstract:
    We compute the transition rate for polarized states in the decay of a spin-3/2 particle hyperon into a spin-$1/2$ baryon and a spin-0 meson.
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  • Stationary processes and equilibrium states in non-symmetric neural networks

    A. Castellanos, L. Viana
    310-0
    Abstract:
    Stationary processes and equilibrium states are discussed in finite separable recurrent neural networks with sequential dynamics and away from saturation. We describe thermal fluctuations of the dynamical order parameters originated as finite size effects of order ${\mathcal{O}}\left( N^{-1/2}\right)$ by means of their corresponding Fokker-Planck equation, and find their time dependent probability distribution. We introduce the concept of extended entropy of fluctuations in order to find a general condition to characterize stationary states in Neural Networks with non symmetric interactions. Divergence and rotational of the probability current in the space of fluctuations are also used to differentiate between stationary and equilibrium states. Besides, algebraic conditions are found to know when stationary states can exist. The results are illustrated by analyzing a neural network with a macroscopic dynamical fixed point but not satisfying detailed balance at microscopic level.
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  • On the SU(2) Wigner function dynamics

    A. B, S. M
    317-0
    Abstract:
    We study the quantum dynamics of the $SU(2)$ quasiprobability distribution (``Wigner function'') for the simple nonlinear Hamiltonian (finite analog of the Kerr medium, $H=S_{z}^{2}$). The quasiclassical approximation for the Wigner function and the corresponding evolution of mean values are considered and compared with the exact and classical solutions.\newline
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  • Química primordial en un modelo no estándar del Big Bang

    A. Lipovka, J. Saucedo, J. Campos
    325-0
    Abstract:
    In the present paper the formation of primordial molecules in pregalactic epochs characterized by redshifts $10
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  • Universal relations for three-dimensional thermal, electric and magnetic properties

    F. J, C. E
    335-0
    Abstract:
    A periodic fibre-reinforced two phase material is considered here. The material properties of the constituents are anisotropic and may be specified by either the thermal, electrical conductivity, dielectric or magnetic permeability tensor. The orientation of each of the characteristic directions of the material is not parallel to the axis of geometric symmetry. The asymptotic homogenization method is used to study this doubly periodic composite. The interface between fibres and matrix must conform with the periodicity, but otherwise is arbitrary and may be disjoint in the periodic cell. A connection is found among the solutions of the three local problems that appear in the implementation of the method. This relation is basic for the elementary derivation of three universal relations for this three dimensional scalar problem. Several examples show how to relate the structure of the material tensor considered in this paper to that provided by the material data.
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  • On Casimir forces for media with arbitrary dielectric properties

    W. L, C. Villarreal, R.Esquivel-Sirvent.
    339-0
    Abstract:
    We derive an expression for the Casimir force between slabs with arbitrary dielectric properties characterized by their reflection coefficients. The formalism presented here is applicable to media with a local or a non-local dielectric response, an infinite or a finite width, inhomogeneous dissipative, etc. Our results reduce to the Lifshitz formula for the force between semi-infinite dielectric slabs by replacing the reflection coefficients by the Fresnel amplitudes.
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  • Debye potentials adapted to cylindrical coordinates

    G. F, R. Rosas Rodríguez
    343-0
    Abstract:
    Using the method of adjoint operators, the solution to the source-free Maxwell equations is expressed in terms of two real Debye potentials adapted to circular, parabolic or elliptic cylindrical coordinates. Analogous expressions are obtained for the solutions of the Einstein vacuum field equations linearized about the Minkowski space-time.
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  • Multipole expansion of electrostatic and magnetostatic fields

    G. F
    348-0
    Abstract:
    The multipole moments of an electrostatic or magnetostatic field and the components of the field itself are expressed in terms of two-component spinor and of spin-weighted spherical harmonics, obtaining the correspondence between the Cartesian and spherical components. It is shown that the $2^{l}$-pole moment of a charge or current distribution defines $l$, not necessarily distinct, directions which determine the angular dependence of the multipole field.
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  • Complete description of weakly coupled chaotic subsystems

    Lev Glebsk., Antonio Morante., .
    355-0
    Abstract:
    We study the dynamics of one--dimensional lattices of weakly coupled maps of $\R$. The local dynamics has an invariant hyperbolic set. Moreover, the trajectories from non expanding (and weakly expanding) points go to infinity (for local dynamical system). Under these assumptions we show that, if the coupling is weak enough, the extended system has similar dynamics.
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  • Función de onda hidrogenoide: nueva fórmula para una vieja integral

    Antonio Ortiz Castro.
    360-0
    Abstract:
    We present a general formula that allows us to calculate the mean value $\langle r^n \rangle, n \in \mathbf{Z}$, for the radial wave functions of the hidrogen-like atoms.
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  • Introducción a las oscilaciones de neutrinos

    Luis G. Cabral-Rosetti
    366-0
    Abstract:
    Recently the Solar Neutrino Observatory (SNO) group in Sudbury Canda, showed the first experimental evidence that neutrino produced in the Sun's core, change their flavour when they propagate. The most probably explanation of such phenomenon are the neutrino oscillation. In this work we give a pedagogical introduction to neutrino oscillation, both in the vacum and in the material medium, emphathizing the resonant MSW effect in the medium. This work is formulated for two neutrino generations however, we give a brief summary of three neutrino generations.
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  • On Bell's theorem\footnote{Based mainly in Ref. 1.}

    M. Socolovsk, .
    384-0
    Abstract:
    Bell's theorem is a crucial tool to solve the contradiction between local realism conceptions $\grave{a$ la} Einstein-Podolsky-Rosen and quantum mechanics. In this note we present three versions of the theorem: the original one of Bell of 1964, the Clauser et al stronger version of 1969, and the Wigner's version of 1970, and show how the simultaneous validity of EPR ideas and quantum theory leads to a contradiction with number theory. The quantum mechanical formula for the correlation between two spin 1/2 particles is derived in an appendix.
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