Norberto Majlis

Brazil's nuclear shakeup: military still in control

Despite four years of civilian rule and a recent reorganization, democracy has not reached Brazils expensive, dangerous nuclear programs.

A simplified model of impurity induced temper embrittlement

Abstract A simplified model of impurity induced temper embrittlement is developed assuming that the metal can be represented by a one dimensional chain of atoms where a tight-binding first nearest neighbour basis is used. The grain boundary is represented by a point occupied by an impurity. The model predicts that as a consequence of the strengthening of the impurity-metal bond the cohesion energy corresponding to the metal-metal bond in the immediate vicinity of the impurity decreases. This result confirms the main feature of a qualitative model proposed to explain grain boundary failure.

Self-consistent theory of the screened impurity band in heavily doped semi-conductors, with an overlapping tight-binding basis

We have developed an analytic treatment of a model of the impurity band in doped semi-conductors, using a tight-binding basis and a self-consistent treatment to handle the configurational averages. New ingredients in the present calculation are: a) overlap between different impurity orbitals; b) simultaneous treatment of diagonal and off-diagonal disorder; c) inclusion of electron-electron interactions through the substitution of the bare Coulomb impurity potential by a Yukawa potential. The Greens functions, the locator self-energy, the density of states and the position of the Fermi level (for T = 0) are calculated for different impurity concentrations.

Self-consistent calculation of the surface magnetization profile and the localized magnons in a Heisenberg ferromagnet

Abstract Static and dynamic surface properties of a Heisenberg ferromagnet are studied in RPA over the whole temperature range. Magnetization at the 3rd plane and the bulk are taken equal. Self-consistent results for magnetization and surface magnon energies are reported. The bulk drives the surface critical behaviour. Dimensional cross-over appears as the surface becomes decoupled from the bulk.

Paramagnetic phase boundary of uniaxial antiferromagnets at low temperatures

Abstract In this work we determine the paramagnetic phase boundary of an uniaxial antiferromagnetic model in the region of very low temperatures. We have applied the method outlined by Devlin to treat exactly the contribution of single ion anisotropy. We show that the canted-paramagnetic field depends explicitly on the ratio between the magnitude of the single-ion term and the exchange coupling. We show that if this ratio is very small this contribution reduces to that determined by Anderson and Callen, within the Callen decoupling scheme. We have also applied our results to the antiferromagnet NiCl 2 .4 H 2 O , which has a large anisotropy. We have shown that our results fit the experimental data better than the Anderson and Callen calculations.

Spin-wave resonance in semi-infinite anisotropic ferromagnets

Abstract We apply the Greens function formalism in the Random Phase Approximation to calculate the power absorbed by a semi-infinite Heisenberg ferromagnet, upon the excitation of surface spin waves by an external radiation field (spin wave resonance) at finite temperatures. The system consists of magnetic layers in the (1 1 1) direction of an f.c.c. lattice, with nearest neighbour isotropic exchange interactions in the bulk and between the surface and the bulk. We suppose that in the surface plane there is a uniaxial anisotropy in the exchange interaction. A uniform magnetic field is applied in the easy magnetic axis, which is parallel to the surface plane. The magnetic component of the radiation field is polarized perpendicularly to the static field and to the magnetization. We determined the self-consistent magnetization profile and the renormalized magnon energy spectrum at finite T. We assume that at the third plane the magnetization attains its bulk value. We show that for reasonable values of the exchange anisotropy in the surface, absorption obtained at elevated temperatures by excitation of “optical” spin-wave modes, with energies (for k ‖ = 0 ) above the bulk continuum spectrum. From the temperature dependence of the power absorption we can obtain estimates of the exchange anisotropy at the surface.

An RPA approximation for a Heisenberg model of a spin-glass

Abstract A numerical model was proposed to obtain the configurational and thermal average of the spin propagator for a spin glass described by a Heisenberg Hamiltonian with RKKY spin-spin interactions. The dependence of the order parameter and the susceptibility on temperature was obtained using the parameter appropriated to Cu0.988Mn0.012 alloy.