DSpace Angular :: Browsing by Author "KIWI, M"

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A GAUGE-THEORY APPROACH TO A GENERALIZED HUBBARD-MODEL

(1993) HUERTA, L; KIWI, M

We present a gauge-invariant reformulation of the Hubbard model, which assumes a set of arbitrarily chosen local spin quantization axes. This way, the macroscopic spin angular degrees of freedom are described by gauge fields. An effective interaction between fermions, with a long-range confining potential. is thus obtained. We compute the ground state energy of the gauged model in the one-dimensional version, for a classical configuration of the gauge sector. It is shown that gauge fields contribute to stabilize the antiferromagnetic configuration. A brief discussion on possibilities unveiled by this formulation is also given.

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ANISOTROPIC HEISENBERG-ANTIFERROMAGNET WITH ARBITRARY DIMENSIONALITY

(1988) CABRERA, GG; LAGOS, M; KIWI, M

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DYNAMIC SYMMETRY-BREAKING IN MIXED-VALENCE SYSTEMS

(1988) ALASCIO, B; BALSEIRO, C; ORTIZ, G; KIWI, M; LAGOS, M

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ELECTRON-PHONON COUPLING IN MIXED-VALENCE SYSTEMS

(1991) RIVAS, M; ROSSLER, J; KIWI, M

The electron-phonon interaction in mixed-valence systems is modeled on the basis of an Anderson-like Hamiltonian that describes a cluster of one metallic rare-earth cation surrounded by six anions. Coupling between the electronic and phononic variables is introduced, keeping two different phonon modes: a breathing and an asymmetric mode. The first, related to the ionic radius, is treated exactly. The asymmetric mode, which determines the sd-f hybridization, is dealt with in the Born-Oppenheimer approximation. A variety of experimental results are adequately accounted for by this simple model, like the anomalous thermal expansion, the Debye-Waller factor, the phonon softening and broadening, and the charge-distance correlation.

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HIGH CORRELATION REGIME OF THE HUBBARD-MODEL

(1988) LAGOS, M; KIWI, M; RAMIREZ, R

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IMPURITY-INDUCED GRAIN-BOUNDARY EMBRITTLEMENT - A SIMPLE 3-DIMENSIONAL MODEL

(1990) MARTINEZ, G; KIWI, M

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INTERFACE AMORPHIZATION - A MOLECULAR-DYNAMICS APPROACH

(1994) GONCALVES, S; RAMIREZ, R; KIWI, M

A two-dimensional molecular dynamics model is introduced order to obtain a better understanding of interface amorphization. We start from a monodisperse system of particles arranged on a perfect two-dimensional triangular lattice, which is divided into two separate regions. In one of these regions the particle size is uniformly reduced, while in the other it is enlarged, preserving the total system 'volume'. Using a whole wealth of diagnostic tools, and visualizations of the particle distributions, a clear distinction between the bulk and the interface disordering transition emerges. Several interesting phenomena, such as the formation of defects, hysteresis of the transition, single-layer displacement and grain boundary development in the vicinity of the interface, show up in our simulations.

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MAGNETIC METAL-FILMS ON PARAMAGNETIC SUBSTRATES - A THEORETICAL-STUDY

(1989) ALTBIR, D; KIWI, M; MARTINEZ, G; ZUCKERMANN, MJ

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MAGNETOSTRUCTURAL CHARACTERIZATION OF A MONOHYDROXO-BRIDGED, 1,1,2,2-TETRAKIS(2-PYRIDYL)ETHYLENE-BRIDGED COPPER(II) DIMER

(1991) SPODINE, E; MANZUR, J; GARLAND, MT; KIWI, M; PENA, O; GRANDJEAN, D; TOUPET, L

The molecular structure of [Cu2(dpm)2L(OH)][ClO4]3.2H2O [dpm = di(2-pyridyl)methane; L = 1,1,2,2-tetrakis(2-pyridyl)ethylene] has been determined by single-crystal X-ray diffraction: triclinic, space group P-1, a = 11.685(3), b = 14.569(2), c = 15.509(4) angstrom, alpha = 87.97(2), beta = 73.34(2), gamma = 74.15(1)-degrees and Z = 2. The copper co-ordination geometry can be described as a square-based pyramid with the two copper(ll) centres bridged by two groups, N-C-C=C-C-N from L and the hydroxide ion. The bridging angle Cu-O-Cu is 137.9(4)-degrees and the copper centres are separated by 3.663(3) angstrom. Magnetic susceptibility data collected as a function of temperature (5-400 K) are indicative of a relatively strong antiferromagnetic interaction (2J almost-equal-to -360 cm-1).