arXiv subjects

R. S. Mendes

Publications and source records attributed to R. S. Mendes.

43 records · Page 3Linked to original sources

arXiv1999

Nonlinear differential equations based on nonextensive Tsallis entropy and physical applications

A family of nonlinear ordinary differential equations with arbitrary order is obtained by using nonextensive concepts related to the Tsallis entropy. Applications of these equations are given here. In particular, a connection between Tsallis entropy and the one-dimensional correlated anomalous diffusion equation is established. It is also developed explicitly a WKB-like method for second order equations and it is applied to solve approximately a class of equations that contains as a special case the Thomas-Fermi equation for an atom. It is expected that the present ideas can be useful in the discussion of other nonlinear contexts.

cond-mat.stat-mech↗

arXiv1998

The Contribution of the Smectic-Nematic Interface to the Surface Energy

The contribution of the smectic-nematic interface to the surface energy of a nematic liquid crystal sample is analyzed. By means of a simple model it is shown that the surface energy depends on the thickness of the region over which the transition smectic-nematic takes place. For perfectly flat substrates this thickness is of the order of the correlation length entering in the transition. An estimate of this contribution shows that it is greater than the one arising from the nematic-substrate interaction. Moreover, it is also shown that the surface energy determined in this way presents a non-monotonic behavior with the temperature.

cond-mat↗

arXiv1998

Quantum Corrections for Generalized Partition Functions

The classical limit for generalized partition functions is obtained using coherent states. In this framework it is presented a general procedure to obtain all the corrections to the classical limit. In particular, the first and second order quantum corrections are worked out explicitly, and the classical limit for the Tsallis thermostatistics is determined. The results of this work generalize the ones obtained by E. Wigner (Phys. Rev. 40 (1932) 749) for usual statistical mechanics.

cond-mat↗

arXiv1998

Perturbation and Variational Methods in Nonextensive Tsallis Statistics

A unified presentation of the perturbation and variational methods for the generalized statistical mechanics based on Tsallis entropy is given here. In the case of the variational method, the Bogoliubov inequality is generalized in a very natural way following the Feynman proof for the usual statistical mechanics. The inequality turns out to be form-invariant with respect to the entropic index $q$. The method is illustrated with a simple example in classical mechanics. The formalisms developed here are expected to be useful in the discussion of nonextensive systems.

cond-mat.stat-mech↗

arXiv1998

Connection between energy-spectrum self-similarity and specific heat log-periodicity

As a first step towards the understanding of the thermodynamical properties of quasiperiodic structures, we have performed both analytical and numerical calculations associated with succesive hierarchical approximations to multiscale fractal energy spectra. We show that, in a certain range of temperatures, the specific heat displays log-periodic oscillations as a function of the temperature. We exhibit scaling arguments that allow for relating the mean value as well as the amplitude and period of the oscillations to the characteristic scales of the spectrum.

cond-mat.stat-mech↗

arXiv1992

Electron--Electron Bound States in QED$_{3}$

This paper is dedicated to the study of the existence and the properties of electron-electron bound states in QED$_3$. A detailed analysis of the infrared structure of the perturbative series of the theory is presented. We start by analyzing the two-point Green's function, in the Bloch-Nordsieck approximation. The theory appears to be plagued by severe infrared divergencies, which nevertheless disappear when vacuum-polarization effects are non-perturbatively taken into account. The dynamical induction of a Chern-Simons term is at the root of this mechanism. {}From the inspection of the electron-electron non-relativistic potential it then follows that equally charged fermions may either repel or attract and, moreover, that bound states do in fact exist in the theory. We calculate numerically the binding energies and average radius of the bound states. We find an accidental quasi-degeneracy of the ground state of the system, between the lowest-energy $l=-3$ and $l=-5$ states, which could be related to a radio-frequency resonance in high-$T_c$ superconductors.

hep-th↗

arXiv1992

Attractive Forces Between Electrons in QED$_{3}$

Vacuum polarization effects are non-perturbatively incorporated into the photon propagator to eliminate the severe infrared problems characteristic of QED$_3$. The theory is thus rephrased in terms of a massive vector boson whose mass is $e^2/(8π)$. Subsequently, it is shown that electron-electron bound states are possible in QED$_3$.

hep-th↗