arXiv subjects

F. Guzman

Publications and source records attributed to F. Guzman.

34 records · Page 2Linked to original sources

arXiv2005

Quasi-stationary evolution of systems driven by particle evaporation

We study the quasi-stationary evolution of systems where an energetic confinement is unable to completely retain their constituents. It is performed an extensive numerical study of a gas whose dynamics is driven by binary encounters and its particles are able to escape from the container when their kinetic energies overcome a given cutou Uc .We use a parametric family of differential cross sections in order to modify the effectiveness of this equilibration mechanism. It is verified that when the binary encounters favor an effective exploration of all accessible velocities, the quasi-stationary evolution is reached when the detailed balance is imposed for all those binary collisions which do not provoke particle evaporation. However, the weakening of this effectiveness leads to energy distribution functions which could be very well fitted by using a Michie-King-like profile. We perform a theoretical analysis, in the context of Hamiltonian systems driven by a strong chaotic dynamics and particle evaporation, in order to take into account the effect of the nonhomogeneous character of the confining potential.

cond-mat.stat-mech↗

arXiv2003

Remarks about the thermodynamic limit in selfgravitating systems

The present effort addresses the question about the existence of a well-defined thermodynamic limit for the astrophysical systems with the following power law form: to tend the number of particles, N, the total energy, E, and the characteristic linear dimension of the system, L, to infinity, keeping constant E/N^{Λ_{E}} and L/N^{Λ_{L}}, being Λ_{E} and Λ_{L} certain scaling exponent constant. This study is carried out for a system constituted by a non-rotating fluid under the influence of its own Newtonian gravitational interaction. The analysis yields that a thermodynamic limit of the above form will only appear when the local pressure depends on the energy density of fluid as $p=γε$, being $γ$ certain constant. Therefore, a thermodynamic limit with a power law form can be only satisfied by a reduced set of models, such as the selfgravitating gas of fermions and the Antonov isothermal model.

astro-ph↗

arXiv2003

Alternative model of the Antonov problem

Astrophysical systems will never be in a real Thermodynamic equilibrium: they undergo an evaporation process due to the fact that the gravity is not able to confine the particles. Ordinarily, this difficulty is overcome by enclosing the system in a rigid container which avoids the evaporation. We proposed an energetic prescription which is able to confine the particles, leading in this way to an alternative version of the Antonov isothermal model which unifies the well-known isothermal and polytropic profiles. Besides of the main features of the isothermal sphere model: the existence of the gravitational collapse and the energetic region with a negative specific heat, this alternative model has the advantage that the system size naturally appears as a consequence of the particles evaporation.

cond-mat.stat-mech↗

arXiv2003

Remarks about the microcanonical description of astrophysical systems

We reconsider some general aspects about the mean field thermodynamical description of the astrophysical systems based on the microcanonical ensemble. Starting from these basis, we devote a special attention to the analysis of the scaling laws of the thermodynamical variables and potentials in the thermodynamic limit. Geometrical considerations motivate a way by means of which could be carried out a well-defined generalized canonical-like description for this kind of systems, even being nonextensive. This interesting possibility allows us to extend the applicability of the Standard Thermodynamic methods, even in the cases in which the system exhibits a negative specific heat. As example of application, we reconsider the classical Antonov problem of the isothermal spheres.

cond-mat.stat-mech↗

arXiv2003

On the dynamical anomalies in the Hamiltonian Mean Field model

We study the N-dependence of the thermodynamical variables and the dynamical behavior of the well-known Hamiltonian Mean Field model. Microcanonical analysis revealed a thermodynamic limit which defers from the a priory traditional assumption of the N-dependence of the coupling constant g as 1/N: to tend N?infinity keeping constant E/N^3 and g/N, prescription which guarantees the extensivity of the Boltzmann entropy. The analysis of dynamics leads to approximate the time evolution of the magnetization density m by means of a Langevin equation with multiplicative noise. This equation leads to a Fokker-Planck's equation which is N-independent when the time variable is scaled by the N-dependent time constant T_{mac}=sqrt(IN/g), which represents the characteristic time scale for the dynamical evolution of the macroscopic observables derived from the magnetization density. This results explains the origin of the slow relaxation regimen observed in microcanonical numerical computations of dynamics of this model system. Connection with the system self-similarity is suggested.

cond-mat.stat-mech↗

arXiv2003

On the dynamical anomalies in numerical simulations of selfgravitating systems

According to self-similarity hypothesis, the thermodynamic limit could be defined from the scaling laws for the system self-similarity by using the microcanonical ensemble. This analysis for selfgravitating systems yields the following thermodynamic limit: send N to infinity, keeping constant E/N^{(7/3)} and LN^{(1/3)}, in which is ensured the extensivity of the Boltzmann entropy S_{B}=lnW(E,N). It is shown how the consideration of this thermodynamic limit allows us to explain the origin of dynamical anomalies in numerical simulations of selfgravitating systems.

cond-mat.stat-mech↗

arXiv2002

Astrophysical Systems: A model based on the Self-similarity Scaling Postulates

In the present work, it is developed a formalism to deal with the macroscopic study of the astrophysical systems, which is based on the consideration of the exponential self-similarity scaling laws that these systems exhibit during the realization of the thermodynamic limit. Due to their scaling laws, these systems are pseudoextensive, since although they are nonextensive in the usual sense, they can be studied by the Boltzmann-Gibbs Statistics if an appropriate representation of the integrals of motion of the macroscopic description is chosen. As example of application, it is analyzed the system of classical identical particles interacting via Newtonian interaction. A renormalization procedure is used in order to perform a well-defined macroscopic description of this system in quasi-stationary states, since it can not be in a real thermodynamic equilibrium. Our analysis showed that the astrophysical systems exhibit self-similarity under the following thermodynamic limit: $E\to \infty ,$ $L\to 0,$ $N\to \infty ,$ keeping $E/N^{7/3}=$const, $LN^{1/3}=$const, where $L$ is the characteristic linear dimension of the system. It is discussed the effect of these scaling laws in the dynamical properties of the system. In a general way, our solution exhibits the same features of the Antonov problem: the existence of the gravitational collapse at low energies as well as a region with a negative heat capacity.

cond-mat.stat-mech↗

arXiv2002

Microcanonical Aproach for the OLA Model

In the present paper it is analyzed a very simple example of pseudoextensive system, the tridimensional system of Linear Coupled Oscillators (OLA Model). The same one constitutes a classical tridimensional system of identical interacting particles by means of harmonic oscillators. This academic problem possesses a complete analytical solution allowing this way that it can find application in modeling some properties of the self-gravitating systems. It is shown that although this is a nonextensive system in the usual sense, it can be dealt in the thermodynamic limit with the usual Boltzmann-Gibbs' Statistics with an appropriate selection of the representation of the space of the integrals of motion.

cond-mat.stat-mech↗

arXiv2002

A non extensive approach for DNA breaking by ionizing radiation

Tsallis entropy and a maximum entropy principle allows to reproduce experimental data of DNA double strand breaking by electron and neutron radiation. Analytic results for the probability of finding a DNA segment of length l are obtained reproducing quite well the fragment distribution function experimentally obtained.

cond-mat↗

arXiv2002

Remarks on Tsallis' Satistics

In the present paper the conditions for the validity of the Tsallis' Statistics are analyzed. The same has been done following the analogy with the traditional case: starting from the microcanonical description of the systems and taking into account their self-similarity scaling properties in the thermodynamic limit, it is analyzed the necessary conditions for the equivalence of microcanonical ensemble with the Tsallis' generalization of the canonical ensemble. It is shown that the Tsallis' Statistics is appropriate for the macroscopic description of systems with potential scaling laws of the asymptotic accessible states density of the microcanonical ensemble. Our analysis shows many details of the Tsallis' formalism: the q-expectation values, the generalized Legendre's transformations between the thermodynamic potentials, as well as the conditions for its validity, having a priori the possibility to estimate the value of the entropic index without the necessity of appealing to the computational simulations or the experiment. On the other hand, the definition of physical temperature received a modification which differs from the Toral's result. For the case of finite systems, we have generalized the microcanonical thermostatistics of D. H. E. Gross with the generalization of the curvature tensor for this kind of description.

cond-mat.stat-mech↗

arXiv2001

The microcanonical theory and pseudoextensive systems

In the present paper are considered the self-similarity scaling postulates in order to extend the Thermodynamics to the study of one special class of nonextensive systems: the pseudoextensive, those with exponential behavior for the asymptotical states density of the microcanonical ensemble. It is shown that this kind of systems could be described with the usual Boltzmann-Gibbs' Distribution with an appropriate selection of the representation of the movement integrals. It is shown that the pseudoextensive systems are the natural frame for the application of the microcanonical thermostatistics theory of D. H. E. Gross.

cond-mat.stat-mech↗

arXiv2001

Generalizing the Extensive Postulates

We addressed the problem of generalizing the extensive postulates of the standard thermodynamics in order to extend it to the study of nonextensive systems. We did it in analogy with the traditional analysis, starting from the microcanonical ensemble, but this time, considering its equivalence with some generalized canonical ensemble in the thermodynamic limit by means of the scaling properties of the fundamental physical observables.

cond-mat.stat-mech↗

arXiv2001

Microcanonical Thermostatistical Investigation of the Blackbody Radiation

In this work is presented the microcanonical analysis of the blackbody radiation. In our model the electromagnetic radiation is confined in an isolated container with volume V in which the radiation can not escape, conserving this way its total energy, E. Our goal is to precise the meaning of the Thermodynamic Limit for this system as well as the description of the nonextensive effects of the generalized Planck formula for the spectral density of energy. Our analysis shows the sterility of the intents of finding nonnextensive effects in normal conditions: the traditional description of the blackbody radiation is extraordinarily exact. The nonextensive effects only appear in the low temperature region, however, they are extremely difficult to detect.

cond-mat.stat-mech↗

arXiv2001

Where the Tsallis Statistic is valid?

In the present paper are analysed the conditions for the validity of the Tsallis Statistics. The same have been done following the analogy with the traditional case: starting from the microcanonical description of the systems and analysing the scaling properties of the fundamental macroscopic observables in the Thermodynamic Limit. It is shown that the Generalized Legendre Formalism in the Tsallis Statistic only could be applied for one special class of the bordering systems, those with non exponential growth of the accessible states density in the thermodynamic limit and zero-order divergency behavior for the fundamental macroscopic observables, systems located in the chaos threshold.

cond-mat↗

arXiv2001

Some geometrical aspects of the Microcanonical Distribution

In the present work is presented some considerations for a possible generalization of the Microcanonical thermoestatics (M.Th) of D.H.E. Gross.The same reveals a geometric aspect that commonly it has been disregarded so far: the local reparametrization invariance . This new characteristic leads to the needing of generalizing the methods of M.Th to be consequent with this property.

cond-mat↗

arXiv1999

Particle-hole level densities in deformed nuclei

Microscopic Combinatorial approach is used to calculate the state and level densities with fixed exciton numbers, in some actinide nuclei. Deformed Saxon-Woods shell model was used as a basis from which all posible configurations were generated. The pairing interaction was taken into account by applying the BCS theory to each configurations. Both the spin and parity distributions were obtained,considering the deformation of the nucleus. Relevance of the result to parity nonconservation studies involving epithermal neutrons on $^{238}$U and $^{232}$Th is discussed.

nucl-th↗