M. T. Thomaz

High Temperature Expansion for a Chain Model

We consider an arbitrary translationally invariant chain model with nearest neighbors interaction and satisfying periodic boundary condition. The approach developed here allows a thermodynamic description of the chain model directly in terms of grand potential per site. This thermodynamic function is derived from an auxiliary function constructed only from open connected subchains. In order to exemplify its application and how this approach works we consider the Heisenberg XXZ model. We obtain the coefficients of the high temperature expansion of the free energy per site of the model up to third order.

Berry's phase in the presence of a dissipative medium

We consider the spin 12 model coupled to a slowly varying magnetic field in the presence of a weak damping represented by a Lindblad-form operators. We show that Berrys geometrical phase remains unaltered by the two dissipation mechanism considered. Dissipation effects are twofold: a shrinking in the modulus of the Blochs vector, which characterizes coherence loss and a time-dependent (dissipation related) precession angle. We show that the line broadening of the Fourier transformation of the components of magnetization is only due to the presence of dissipation.

Comment on the Adiabatic Condition

The experimental observation of effects due to Berry’s phase in quantum systems is certainly one of the most impressive demonstrations of the correctness of the superposition principle in quantum mechanics. Since Berry’s original paper in 1984, the spin 12 coupled with a rotating external magnetic field has been one of the most studied models where those phases appear. We also consider a special case of this soluble model. A detailed analysis of the coupled differential equations and comparison with exact results teach us why the usual procedure (of neglecting nondiagonal terms) is mathematically sound.

High-temperature series expansion of the Helmholtz free energy of the quantum spin- S X Y Z chain

We consider the

Remarks on the convergence of the β-expansion of s=12 and s=1 Ising models

XYZ

Imaginary phases in two-level model with spontaneous decay

chain model of arbitrary spin

Unidimensional generalized Hubbard model in the high temperature limit

S

Beyond Gaussian integrals in Grassman algebra

in the high temperature region, with external magnetic field and single-ion anisotropy term. Our high-temperature expansion of the Helmholtz free energy is analytic in the parameters of the model for

Application of non-commutative algebra to a soluble fermionic model

S

The magnetization plateaus of the ferro and anti-ferro spin-1 classical models with Sz2 term

, which may range from 1/2 to the classical limit of infinite spin (