Examinando por Autor "Lopez, A."
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Ítem Acceso Abierto A note on the equation of state for systems that underqo valence changes.(Pergamon Press, 1974) Alascio, Blas Rafael; Lopez, A.In this paper we want to discuss in a simple way the equation of state of a system that undergoes valence changes. This requires consideration of the cohesive energy of the compounds which is, in general, a major problem. For this reason we shall consider only those compounds which are mainly ionic in character, so that their cohesive energy can be reasonably described by the Madelung and repulsive core contributions. In fact, the collapsed phases form rather poor metals and there should still be a high degree of ionicity left, as is indicated by the stability of the NaCl structure.Ítem Acceso Abierto Effect of finite f level linewidth on the theory of the x-y transition in Ce.(Institute of Physics and Physical Society, 1973) Alascio, Blas Rafael; Olmedo, C. F. E.; Lopez, A.We have extended the Ramirez-Falicov theory of the a-y transition in Ce to include the effect of hybridization of the localized 4f states with the band. The physical properties of the x and y phases are calculated using thermodynamic Green functions which are determined by a decoupling procedure that accounts for the highly correlated character of the f levels. The decoupling procedure is exact to second order in the hybridization parameter. We find satisfactory valúes for the noninteger valency, electronic specific heat and magnetic susceptibility of the a phase. A Curie law for the y phase is also obtained.Ítem Acceso Abierto Phase diagram of samarium Monochalcogenides(Pergamon Press, 1974) Wio, Horacio Sergio; Alascio, Blas Rafael; Lopez, A.By using a simple ionic model for the cohesive energy and electronic terms similar to those of the Falicov—Kimball model for metal—insulator transitions we obtain a phase diagram for SmS with a critical point at Tk = 554°K. The model reduces to the thermodynamic theory proposed by Aptekar and Ponyatovskiy and allows to calculate the p —V isotherms for SmS, SmSe and SmTe, and the concentration of holes in the 4/shell as functions of pressure and temperature.Ítem Acceso Abierto Series Development of tlie Solution of an Eigenvalue Problem.(Österreichische Akademie der Wissenschaften, 1966) Lopez, A.In view of the divergencies of the field theory, the convergence of series developments of bound eigenfunctions in powers of the coupling constant is studied. Born approximation represents a suitable development in cases where an unperturbated state can be defined and in which the energy shift due to the interaction is finite. Another series development, at fixed energy, can be deviced. This latter development leads necessarily to coefficients which represent non-regular functions in configuration space, nevertheless, those series converge to a regular function if the fixed energy coincides with an eigenvalue of the considered problem or, in cases where the energy of the systems is given, furnish an eigenvalue problem for the coupling constant.Ítem Acceso Abierto The use of coherent states in the theory of superfluidity.(American Chemical Society, 1967) Lopez, A.It is shown how the matrix elements of the pseudo-unitary operator producing the Bogoliubov transformation for bosons and the corresponding states with sharp particle number. can be rather simply evaluated using eigenstates of the destruction operators.Ítem Acceso Abierto Theory of Mössbauer effect in intermediate-valence compounds.(American Chemical Society, 1979) Balseiro, Carlos Antonio; Alascio, Blas Rafael; Lopez, A.Using a simple microscopic model, we study the response function corresponding to the Mössbauer effect in an intermediate-valence atom. We derive expressions for the spectra which can be used to analyze experimental results at all temperatures, as well as for different hosts in which the intermediate-valence ion maybe embedded. For metallic hosts at high temperatures our theory justifies the use of the expressions previously proposed by Bauminger, Nowik, Ofer, Felner, and Mayer, based on Kubo ’s stochastic theory of line shape. A t low temperatures, where the latter cannot be applied due to the non-Markovian character of the fluctuations, our results allow us to interpret experimental data and to obtain from them the parameters characterizing the intermediate-valence condition. For semiconducting hosts, where the phenomenological stochastic theory does not apply either, we can predict spectra which are consistent with experimental resultsÍtem Acceso Abierto Thermodynamic theory of the phase diagram of mixed transition-metal oxides.(American Chemical Society's, 1972) Alascio, Blas Rafael; Grunfeld, Ana María Verónica; Lopez, A.The model of Falicov and Kim ball for metal-insulator transitions has been extended to include two types of localized states and the effect of spin -spin interactions. This allows for the description of the phase diagram s of mixed transition -metal oxides of the type (V 1-X Crx)2 O3.