Fernández Álvarez-Estrada, Ramón

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First Name
Last Name
Fernández Álvarez-Estrada
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Física Teórica
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Now showing 1 - 10 of 28
  • Publication
    Erratum: Effective chiral lagrangian from QCD at nonzero chemical potential (vol 355, pg 288, 1995)
    (Elsevier Science, 1996-07-11) Fernández Álvarez-Estrada, Ramón; Gómez Nicola, Ángel
  • Publication
    Proton decay in a nucleus: nonrelativistic treatment of nuclear effects
    (Amer Physical Soc, 1983) Fernández Pérez, Luis Antonio; Fernández Álvarez-Estrada, Ramón; Sánchez Gómez, J. L.
    In this paper, proton decay in a large nucleus is studied in the framework of SU(5) grand unification theory (GUT). By using a method based upon the Green s-function technique of many-body physics, nuclear effects on spectator and pole terms are computed. The decay width in the nucleus is found to be practically the same as in free space. However, nuclear effects are of considerable importance concerning the positron spectrum. A densitycorrelation expansion is introduced which is useful for carrying out a systematic study of nuclear effects in proton decay in a large nucleus. The method presented here can be easily extended to other GUT's or supersymmetric GUT's.
  • Publication
    Single-mode Anisotropic Cylindrical Dielectric Waveguides
    (Taylor and Francis Ltd., 1983) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María Luisa
    We study anisotropic cylindrical dielectric waveguides under the following conditions: (1) their cross section Ω has arbitrary shape; (2) the dielectric permittivity tensor ε has discontinuities at the boundaries, as well as a graded spatial variation; (3) Ω is small, and ε minus the unit 3 × 3 matrix (I) is small and smooth, so that only one pair of propagation modes is allowed. A general and exact dispersion relation for the propagation modes is derived, using integral equation techniques and the long wavelength singularity of the free-space Green's function in two dimensions. We present approximate explicit solutions of the dispersion relation to first order in (ε− I), which exhibit a logarithmic dependence and birefringence, in general. Some numerical estimates are also given. Approximate dispersion relations up to and including second order in (ε − I) are also discussed. The scattering problem is treated briefly.
  • Publication
    Electromagnetic scattering by an infinite inhomogeneous dielectric cylinder: New Green’s function and integral equations
    (American Institute of Physics, 1980) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María Luisa
    The scattering of electromagnetic waves by an infinite dielectric cylinder with variable dielectric permeability presents, in general, certain mathematical difficulties regarding the construction of rigorous solutions. We give a new divergenceless tensorGreen’s function, specially appropriate for cylindrical symmetry, and, in terms of it, present new scattering integral equations. We prove that the series for med by all successive iterations of those scattering integral equations converge under certain conditions. Suitable transformations lead to new integral equations with Hilbert–Schmidt kernels, which imply further rigorous results.
  • Publication
    Flow-gauge Slavnov-Taylor identities for Zwanziger's gauge fixing
    (American Physical Society, 1988-04-15) Fernández Álvarez-Estrada, Ramón; Muñoz Sudupe, Antonio
    The generalization of the Slavnov-Taylor identities for the stochastically quantized Yang-Mills field theory with either Zwanziger gauge fixing or, equivalently, Faddeev-Popov Bow-gauge fixing in one higher dimension is presented. Those exact relationships among Green’s functions in the stochastically quantized theory are derived by extending suitably Slavnov's method. As a consequence there is no renormalization of the longitudinal part of Green’s functions in α=0, to all perturbative orders. Based on the general identities, the divergent longitudinal part of the two-point Green’s function is calculated to second order for α= 1, and it is found to agree with other independent calculations.
  • Publication
    Diffraction of a classical electromagnetic wave by a thin periodic slab: a rigorous approach
    (IOP Publishing, 1978) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María Luisa
    The interaction and diffraction of a classical electromagnetic wave by a thin slab characterised by a periodic dielectric permeability is studied rigorously, thereby completing previously existing studies. New integral equations for the induction vector in terms of a suitable Green function, are presented, and a condition which ensures the convergence of their iterations is obtained. A bound for the error of the usual Debye-Born-Rayleigh-Gans approximation to the diffraction amplitudes is given.
  • Publication
    Inhomogeneous Slab and Cylindrical Dielectric Waveguides with Discontinuous Permittivities: Propagation Modes
    (Taylor and Francis Ltd., 1982) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María Luisa
    We study monomode slab and cylindrical waveguides, in which the dielectric permittivity l has both a graded spatial variation and finite discontinuities at the boundaries. New integral equations for the H-type propagation mode in the slab and for the one in the cylinder are obtained, which display separate contributions due to both the graded variations and the discontinuities, and in which the boundary value problem arising from the latter is solved. Based on the specific long wavelength singularities of the free-space Green's functions in one and two dimensions, exact dispersion relations are presented for the slab and cylinder propagation modes, respectively. We give approximate explicit solutions of those dispersion relations, which are correct up to and including second order (in some parameter measuring the deviation of l from unity), and present applications for parabolic permittivity profiles in both geometries. The discontinuities of l at the boundary give rise to contributions of second and higher orders in the dispersion relations, but never at first order. Electromagnetic scattering by the cylinder in general and, in particular, its long wavelength behaviour, are studied briefly.
  • Publication
    Neutron fibres. II. Some improving alternatives and analysis of bending losses
    (IOP Publishing Ltd, 1986-06-14) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María Luisa
    For pt.I see ibid., vol.17, p.475 (1984). The possible confined propagation of slow (thermal) neutrons along cladded waveguides of small cross section (fibres) is studied, in order to improve a previously published analysis. Quite suitable possibilities for the core of the waveguide could be either Al or Si (or vacuum), so as to allow for confined propagation of neutrons along reasonable lengths, without large scattering and absorption losses. Tentative possibilities for the cladding could be either Ni or Fe. The bending losses are analysed for a curved neutron fibre in two spatial dimensions. Such bending losses turn out to be completely negligible, except in the case of a small radius of curvature Rc. To quote some typical values, for any fibre diameter between 0.1 and 1 mu m and reasonable lengths for it (between 5 and 10 cm), bending losses can be neglected other than for Rc
  • Publication
    Three-dimensional analysis of bending losses in dielectric optical waveguides with arbitrary refractive-index profile
    (Optical Society of America, 1987-04) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María Luisa
    A three-dimensional analysis of bending losses in dielectric optical waveguides is presented. It constitutes a nontrivial generalization of previous two- and three-dimensional studies by other authors. Our analysis is based on homogeneous integral equations for the total radiation field and suitable asymptotic approximations for Green’s functions. A key role is played by a new three-dimensional approximation for a relevant Bessel function with large order and argument (the former being larger than the latter). A nontrivial check of the consistency of all those approximations is given. General formulas are presented for the radiated field and the energy flow and for a bending-loss coefficient in three dimensions. Numerical results are also given, in order to assess the difference between the results of other authors and ours. Such a difference is rather small for monomode behavior near cutoff, increases as the behavior of the waveguide changes from monomode to multimode, and decreases as the parameter V increases for a given core radius and propagation mode.
  • Publication
    Classical systems: moments, continued fractions, long-time approximations and irreversibility
    (American Institute of Physics, 2011) Fernández Álvarez-Estrada, Ramón