J. Ricardo de Sousa

Field-induced phase transition in the quantum Heisenberg antiferromagnetic model

Abstract The criticality of the quantum anisotropic spin- 1 2 Heisenberg antiferromagnetic model in the presence of a magnetic field is studied by mean field approximation. The critical temperature (TN) is obtained as a function of anisotropy parameter η (where η=1 and 0 correspond the isotropic Heisenberg and Ising models, respectively) and magnetic field on a three-dimensional lattice (d=3). Particular attention is draw to the influence of quantum effect in the phase diagram of the Heisenberg metamagnet (η=1). It is shown that quantum influence does not modify the critical field, where in the limit T=0 the results correspond the exact result hc=zJ, i.e., hc is independent of the anisotropy parameter η.

Phase diagrams of thin classical n-vector films

Abstract The Curie temperature T c of a classical n -vector ferromagnetic thin film was theoretically investigated in the framework of an effective-field theory. The effect of surface interaction and of film thickness l on the critical temperature was carefully investigated in the case of simple cubic films.

Quantum spin-12 two-dimensional XXZ model: an alternative quantum renormalization-group approach

We study the phase diagram of the two-dimensional quantum spin-12 anisotropic Heisenberg model. We adapt a renormalization-group approach to treat this quantum model, and an analysis on low temperatures is presented and compared with the exact results in the isotropic limit, obtained from the Mermin–Wagner theorem. We comment on extensions of the procedure to treat antiferromagnetic models, for S=12 and S>12.

Molecular dynamics simulations of two-dimensional clusters of charges

We study structural and spectral properties of finite classical systems of N two-dimensional charged particles, confined by a parabolic potential ∝ r n , and interacting via inverse power-law potentials ∝ 1/r n . Molecular dynamics simulations are performed for different cluster sizes (N =3 0to 230 )a ndn, nvalues 1, 2, 3 and 10. We also analyze the phase transition from a ring-like configuration to a Wigner structure as a function of param- eter nand anisotropy. We compare our results with Monte Carlo simulations of Bedanov and Peeters, obtaining good agreement. In addition, we determine the Voronoi structure of the cluster. Our work complements that of Candido, Rino and Studart, who analyzed confinement in a screened parabolic potential.

Análise da convergência na Teoria da Perturbação Estacionária

In this work, we take a revision of perturbation theory in Quantum Mechanics and we were shown that Rayleigh-Schrodinger perturbative series did not converge to exact numerical results of anharmonic oscillator or Stark effect in Hydrogen atom. We utilize Pade approximants, as a technique of renormalization of perturbative energies. We treat fourth order perturbation to anharmonic oscillator lx4 and verify the accuracy of Pade approximants in relation to numerical exact solution.

First-order phase transition in the quantum spin glass at T=0

Abstract The van Hemmen model with transverse and random longitudinal field is studied to analyze the tricritical behavior in the quantum Ising spin glass at T=0. The free energy and order parameter are calculated for two types of probability distributions: Gaussian and bimodal. We obtain the phase diagram in the Ω –H plane, where Ω and H are the transverse and random longitudinal fields, respectively. For the case of Gaussian distribution the phase transition is of second order, while the bimodal distribution we observe second-order transition for high-transverse field and first-order transition for small transverse field, with a tricritical point in the phase diagram.

The random anisotropic Heisenberg antiferromagnet: application to FepMn1−pF2

Abstract The criticality of a quantum spin- 1 2 Heisenberg antiferromagnetic model with random anisotropy is studied to simulate the phase diagram of the random system Fe p Mn 1− p F 2 . The critical temperature ( T N ) is obtained by the variational approach based on Bogoliubov inequality for the free energy in the pair approximation. The calculated phase diagram in the T – p plane is in good agreement with experimental data where the maximum point for T N ( p ) in p ≃0.75 is predicted here for the first time. The values of the anisotropy parameters are also estimated.