Arturo López

Dynamical susceptibility of intermediate valence systems

Abstract The transverse susceptibility if magnetic systems described by the Anderson model is calculated evaluating the self energy for the two particle Green function. The results are applied to semiconductors and metals. In the first case a shift in the resonance frequency is obtained as well as a continuum of excitations corresponding to the promotion of electrons from the localized state to the conduction band states. In the metallic case a Korringa relation for the relaxation time in the magnetic limit is obtained. The real and imaginary parts of the self energy are given as functions of the distance from the localized level to the Fermi level.

Valence, electronic specific heat and magnetic susceptibility of alpha-cerium

Abstract A sixfold degenerate Anderson Hamiltonian, which includes the Coulomb repulsion between localized and band states in the mean field approximation following Ramirez and Falicov, is treated in the limit of infinitely correlated 4ƒ states and to second order in the mixing parameter. Assuming a line width Γ ≈ 0.02eV we obtain at low temperatures and 11 kbar, a valency of 3.76 and sizeable contributions from the 4ƒ levels to the electronic specific heat and magnetic susceptibility of alpha Ce. Thus we show, that the highly correlated picture necessary to the explanation of the alpha—gamma transition by Ramirez and Falicov needs not to be abandoned in order to explain the anomalies in alpha Ce.

A model for the phase diagram of (V1−xCrx)2O3

Abstract A simple extension of the model proposed by Falicov and Kimball allows for the description of the phase diagram of the mixed oxides (V1−xCrx)2O3, in which the equivalence between decreasing pressure and increasing Cr concentration is easily obtained.

Possibility of a magnetic transition in Ce alloys

Abstract The fact that divalent rare earth impurities have a strong effect in lowering the α-γ transition temperature in Ce, allows the γ phase to become stable at low temperature without much change in its physical properties. The assumption that in this way an ordering (Neel) temperature can be reached, plus a thermodynamical analysis of the electronic transformation that takes place at the α-γ transition, leads to the prediction of a phase diagram for these alloys similar to that found in Cr doped V 2 O 3 .

The de Gennes-Alexander Theory of Superconducting Micronetworks

The technology of the second half of the XX century was largely based on the applications of the quantum physics of the solid state, which explains the electronic properties of matter. Technology of the XXI century will surely be based on the macroscopic quantum properties of matter, that is to say, on the coherent macroscopic states of physical systems, some of their best known examples are the laser, superfluidity and superconductivity. The present review is dedicated to a presentation of the bibliography and the more relevant results referred to superconducting micronetworks, that is to say, circuits made of superconducting material, fabricated by photolitography or carved using particle beams, electrons or ions, on insulating substrates. The circuits to which we refer have connections with characteristic lengths of the order of 1 µm and transverse sections of about 0.01 µm 2. This allows to identify them as mesoscopic, since, from the microscopic point of view, they are large relative to the superconducting properties characterized by the coherence length of the Cooper pairs, but small relative to the characteristic dimensions of macroscopic superconductivity. It was de Gennes [9,10] who first introduced these structures in 1981 as a model for inhomogeneous superconductors. His interest was centered in the analysis of the magnetic susceptibility of a kind of “spaghetti and sauce” superconductor, where the spaghetti were made up of superconducting material, and the sauce was some insulating material.

Narrow band limit of the equivalence between Anderson and Kondo Hamiltonians

Abstract It is shown that for a narrow band version of the Anderson Hamiltonian, the transformation of Schrieffer and Wolff can be carried out to all orders in the mixing parameter. The resulting Kondo-like Hamiltonian is in diagonal form. The results can be used to derive a systematic approximation scheme for the Anderson Hamiltonian, through use of the level shift operator of Primas. The resulting interaction parameters remain regular when the energy of the localized level approaches the Fermi level.