Marco A. De Moura

Remarks on the traversal time in a tunneling process

Abstract We present the calculation of the traversal time of a flux of particles through a rectangular barrier. We do this using the definition of the duration of a collision from the kinetic point of view in which we use only the current velocity concept. Our result leads to a more transparent physical picture of the problem as compared with others presented in the literature.

Lifshitz points and Landau theory in the presence of a magnetic field

Abstract We analyse the effect of a magnetic field on the phase diagram near the Lifshitz point using the Landau theory. In particular, we have determined the value of the tricritical field above which the transition parasinusoidal becomes first order.

A new pulse-like soliton solution for the uniaxial 1-D Heisenberg ferromagnet

Abstract Nonlinear excitations in a Heisenberg chain with uniaxial anisotropy in the presence of an external field are studied within a semi-classical approach. In addition to the solitary-wave solution obtained by Long and Bishop, a new soliton solution, energetically more favorable than magnons, is also obtained.

Propagation of Gaussian beams in a nonlinear medium

We investigate the propagation of cw (continuous wave) circularly symmetric Gaussian beams in a nonlinear saturable medium using a modified variational approach. We find the equations that describe the characteristics of the beam, solving them analytically in various regimes. We also determine the conditions under which these solutions may be stable in two transverse dimensions. Finally, we solve these equations numerically in the case of loss, comparing them with the lossless analytical solutions in the limit of small losses.

Solitary waves in an isotropic ferromagnetic chain

Abstract It is shown that due to high-order terms in the Holstein-Primakoff boson-like spin operator transformation we can have a solitary wave solution even in an isotropic ferromagnetic chain.

The effect of the biquadratic exchange on the existence of solitary waves in 1D ferromagnets

Abstract The existence of solitary waves in the isotropic ferromagnetic chain with biquadratic exchange is investigated. It is shown that such excitations appear as solutions of a nonlinear Schrodinger equation with nonlinearity of third and fifth powers.

The interaction of solitons and electronic spins in 1D magnetic systems

Abstract The coupling of nonlinear excitations and electronic spins in 1D magnetic systems with planar symmetry is studied. Through a simple mechanism we calculate the longitudinal ( T l ) relaxation time which can be used to probe magnetic solitons in a system like CsNiF 3 .

Magnetic transitions in europium selenide

Abstract It is suggested that the varius spin configurations in EuSe are stablized by lattice distortions. In the NNSS phase the distance between (111) planes alternately contract and expand, producing no macroscopic lattice dilatation but converting the phase transition to first order. In the NNS phase the distortion consists of two inter-plane expansions and one contraction, leading to a net macroscopic dilatation. The NSNS phase is accopanied by expansion of all inter-plane distance and thence by a net dilatation (three times larger than in NNS). These proposals are in qualitative agreement with available experimental data.