Showing papers by "National University of Cuyo published in 1985"

Thermodynamics of a model for valence fluctuations between two magnetic configurations

Abstract: The thermodynamic Bethe-Ansatz equations of a model describing valence fluctuations between two magnetic configurations are derived. The model includes a spin-1/2 and a spin-1 configurations hybridized through the promotion of an electron to a conduction band. Thes−d like limits are reproduced. An expression for the specific heat at low temperatures is obtained.

Behavioral responses of high and low active male rats to the chronic ingestion of desipramine

Abstract: Male rats arbitrarily selected for high and low motor activity (HA and LA-rats) were submitted to the chronic ingestion (30 days) of desipramine (DSP) in doses of about 1.5, 3 and 6 mg/kg/24 hr. Their motor activity was assessed in an animal activity monitor providing a measure of total horizontal movements and vertical movements and in a le-board a measure of locomotion, head-dipping and grooming. There were significant differences between HA and LA-rats in their behavioral response to DSP treatment. At the doses used DSP did not affect horizontal and vertical movements and hole-board locomotion or exploration in HA-rats (Experiment 1). In LA-rats, however (Experiment 2), these motor activities were significantly stimulated by DSP. Such effect was dose dependent; 1.5 mg/kg/24 hr was ineffective while 6 mg/kg/24 hr produced a clear cut reversion of hypoactivity. It is speculated that DSP treatment increased resistance of LA-rats to the mild stress caused by testing.

Contributions from off-energy-shell states to inner-shell electron capture.

Abstract: It is shown that contributions from off-energy-shell intermediate states are small for inner-shell electron capture in highly asymmetric ion-atom collisions. We analyze the influence of the on-energy-shell discontinuities of the electron wave function by introducing a cut-off Coulomb potential. We found that these discontinuities, which are mainly responsible for large corrections introduced by the strong-potential Born approximation, do not have physical meaning.

Derivation of model Hamiltonians: Exchange Hamiltonian for H2.

Abstract: The derivation of model Hamiltonians such as crystal-field and spin Hamiltonians requires the decoupling of electrons, which may be done by defining an appropriate equivalent Hamiltonian ${H}_{\mathrm{eq}}$. The connection between ${H}_{\mathrm{eq}}$ and the original Hamiltonian, and between the respective eigenfunctions, is explicitly given. The eigenfunctions of ${H}_{\mathrm{eq}}$ are linear combinations of simple products of atomic spin orbitals, the latter being chosen in such a way that its antisymmetrization leads to a linearly independent set. The problem is then reformulated in a chosen subspace (effective space) via the effective-Hamiltonian formalism. The model Hamiltonian ${H}_{\mathrm{mod}}$ is then obtained by projecting out the undesired degrees of freedom. If in the two-atom two-electron case one keeps in ${H}_{\mathrm{mod}}$ only first-order terms in the interatomic interaction, one gets the usual Heitler-London approximation. For this reason the formulation has been called the extended Heitler-London method. As an illustration the formalism is applied to the ${\mathrm{H}}_{2}$ molecule, where the usual Heisenberg Hamiltonian is rigorously derived and the values of the energy baricenter and magnetic exchange constant are given, up to the second order of perturbation theory.

Neutron scattering theory of intermediate-valence-Tm impurities

Abstract: We calculate the correlation function 〈${S}^{+}$${S}^{\mathrm{\ensuremath{-}}}$${〉}_{\ensuremath{\omega}}$ for a model that contains the most essential features of intermediate-valence Tm: fluctuations between two magnetic configurations. These calculations are an extension to finite temperatures of a previous work related to the zero-temperature inelastic neutron spectrum of intermediate-valence-Tm compounds. The results can reproduce the behavior of the neutron spectrum of TmSe, ${\mathrm{Tm}}_{\mathrm{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se, and ${\mathrm{Tm}}_{\mathrm{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se at different temperatures.

Gaussian corrections around the mean-field approximation for the mixed SU(2) lattice gauge theory

Abstract: We analyze the mixed fundamental-adjoint SU(2) lattice gauge theory in four dimensions by the mean-field method including Gaussian corrections in the axial gauge. In previous papers we have shown, with the help of an Abelian model, that in order to apply this technique it is necessary to introduce different unconstrained variables for the various characters which appear in the action. The present calculation is an explicit example of this idea applied to a non-Abelian case. The phase diagram given by Monte Carlo simulations is accurately reproduced.

Bound states between protons and magnetic poles: Influence on the Rubakov effect

Abstract: Taking into account the structure of the proton in a very simple way, we find the energy levels and the wave functions for the bound states of a proton in the field of an Abelian magnetic pole, confirming the enhancement of the Rubakov effect