arXiv subjects

M. T. Thomaz

Publications and source records attributed to M. T. Thomaz.

27 records · Page 2Linked to original sources

arXiv2001

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 sub-chains. 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.

hep-ph↗

arXiv2000

Analytical Results of the One-Dimensional Hubbard Model in the High Temperature Limit

We investigate the grand potential of the one-dimensional Hubbard model in the high temperature limit, calculating the coefficients of the high temperature expansion ($β$-expansion) of this function up to order $β^4$ by an alternative method. The results derived are analytical and do not involve any perturbation expansion in the hopping constant, being valid for arbitrary density of electrons in the one-dimensional model. In the half-filled case, we compare our analytical results for the specific heat and the magnetic susceptibility, in the high-temperature limit, with the ones obtained by Beni {\it et al.} and Takahashi's integral equations, showing that the latter result does not take into account the complete energy spectrum of the one-dimensional Hubbard model. The exact integral solution by Jüttner {\it et al}. is applied to the determination of the range of validity of our expansion in $β$ in the half-filled case, for several different values of $U$.

cond-mat↗

arXiv2000

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 1/2 coupled with 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.

quant-ph↗

arXiv1999

Free Fermions at Finite Temperature: An Application of the Non-Commutative Algebra

Charret et. al. applied the properties of the Grassmann generators to develop a new method to calculate the coefficients of the high temperature expansion of the grand canonical partition function of self-interacting fermionic models in any d-dimensions (d>=1). The method explores the anti-commuting nature of fermionic fields and avoids the calculation of the fermionic path integral. We apply this new method to the relativistic free Dirac fermions and recover the known results in the literature.

hep-th↗

arXiv1998

Grassmann Algebra and Fermions at Finite Temperature

For any d-dimensional self-interacting fermionic model, all coefficients in the high-temperature expansion of its grand canonical partition function can be put in terms of multivariable Grassmann integrals. A new approach to calculate such coefficients, based on direct exploitation of the grassmannian nature of fermionic operators, is presented. We apply the method to the soluble Hatsugai-Kohmoto model, reobtaining well-known results.

cond-mat.stat-mech↗

arXiv1997

Analytical Results for the Grand-Canonical Partition Function for Unidimensional Hubbard Model up to Order $β^5$

We calculate the exact analytical coefficients of the $β$ expansion of the grand-canonical partition function of the unidimensional Hubbard model up to order $β^5$, using an alternative method, based on properties of the Grassmann algebra. The results derived are non-perturbative and no restrictions on the set of parameters that characterize the model are required. By applying this method we obtain analytical results for the thermodynamical quantities, in the high-temeprature limit, for arbitrary density of electrons in the unidimensional chain.

cond-mat.str-el↗

arXiv1996

Dynamics of the Two-Site Hubbard Model

The dynamical evolution of a two-site Hubbard model is derived in the presence of an uniform external magnetic field for a general initial state. The time evolution of the half-filled (two-particle) case has a complex behaviour. Under certain initial conditions, the average number of fermions in one-particle state, in the half-filled case, is not necessarily periodic, even though the magnetization remains periodic. The results obtained may be applied to study the magnetization and the transition dipole moment of the organic charge-transfer salts in the in-phase mode.

cond-mat↗

arXiv1996

Grand Canonical Partition Function for Unidimensional Systems: Application to Hubbard Model up to Order beta^3

We exploit the grassmannian nature of the variables involved in the path integral expression of the grand canonical partition function for self--interacting fermionic models to show, in one-space dimension, a general relation among the terms of it expansion in the high temperature limit and a combination of co-factors of a suitable matrix with commuting entries. As an application, we apply this framework to calculate the exact coefficients, up to order β^3, of the expansion of the grand canonical partition function for the Hubbard model in d=(1+1) in the high temperature limit. The results are valid for any set of parameters that characterize the model.

cond-mat↗

arXiv1996

Solitons in polymeric chains with periodic interactions

In this paper we follow the lines of recent works to investigate systems of two coupled real scalar fields defined by potentials that describe periodic interactions between the scalar fields. We work with polymeric chains containing periodic interactions between the coupled fields, and we investigate the topological sectors to obtain explicit soliton solutions and their corresponding energy. In particular, we offer an example that considers deoxyribonucleic acid (DNA) as a system of coupled fields, and we present the main steps to describe DNA as a polymeric chain belonging to the class of systems of two coupled real scalar fields.

cond-mat.soft↗