M.E. Gouvêa

Dynamics of the one-dimensional quantum antiferromagnet with Dzyaloshinski–Moriya interactions

Abstract The in-plane dynamical correlation function for the quantum one-dimensional antiferromagnet with a Dzyaloshinski–Moriya antisymmetric exchange term is calculated asymptotically exactly at low temperatures using a projection operator technique proposed by G. Reiter. The static correlation functions needed for the dynamics are evaluated using the harmonic approximation. Apart from a broadening of the spin-wave peak, the two-magnon processes give rise to additional resonances in the spectrum when the anisotropy parameter associated to the Dzyaloshinski–Moriya exchange is greater than the exchange constant.

Dynamical correlation from topological solitons in two-dimensional anisotropic models

The structure and dynamics of soliton configurations are considered for a two-dimensional classical Heisenberg model in the XY-and Ising-like models. Using a phenomenology based on a dilute gas of mobile solitons we obtain a central peak (ω=0) for dynamical correlation functions.

Dynamics of the quantum one-dimensional Heisenberg antiferromagnet with spin S=1 at low temperature

Abstract We report low-temperature dynamic properties of the quantum one-dimensional antiferromagnetic Heisenberg model with spin S =1. The dynamic correlation function is calculated using a procedure that combines a projection operator technique proposed by Reiter and a modified spin wave theory proposed by Takahashi. This modified spin wave theory predicts a temperature-dependent gap whose behavior is compared to the behavior obtained experimentally and also from different approaches. Our calculation suggests that the gap has strong effects on the shape of the dynamical correlation function.

Vortex-magnon interaction in the 2d XY ferromagnetic model

Abstract In this paper we calculate the phase shift and the bound state energy of magnons in the presence of a single vortex for the classical two-dimensional XY ferromagnet using the Born approximation.

Planar rotator system in two dimensions with long-range interactions

Abstract We study the two-dimensional (2D) planar rotator model with ferromagnetic long-range interactions decaying as r − p using the self-consistent harmonic approximation. We show that the overall behavior of the model approaches the one expected for nearest-neighbor model as p → ∞. The results obtained for the transition temperature are in agreement with Monte Carlo results reported in the literature. We check some asymptotic expressions currently used in the literature and propose a different expression for the case p = 4.

Spin-wave scattering by solitons in a two-dimensional easy-axis ferromagnetic model

Abstract We have obtained the phase shift of spin waves scattered by a soliton in the easy-axis two-dimensional ferromagnetic model.

Quantum fluctuations of vortex-antivortex in two-dimensional ferromagnets

Abstract We present an investigation of quantum fluctuations about vortex-antivortex solutions in the two-dimensional easy-plane ferromagnetic model. We solve the small oscillations problem and compute the first quantum correction to the classical result. An interaction between vortex and antivortex arises due to quantum effects.