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F. Gulminelli

Publications and source records attributed to F. Gulminelli.

At least 73 records · Page 4Linked to original sources
arXiv2011

Ensemble in-equivalence in supernova matter within a simple model

A simple, exactly solvable statistical model is presented for the description of baryonic matter in the thermodynamic conditions associated to the evolution of core-collapsing supernova. It is shown that the model presents a first order phase transition in the grandcanonical ensemble which is not observed in the canonical ensemble. Similar to other model systems studied in condensed matter physics, this ensemble in-equivalence is accompanied by negative susceptibility and discontinuities in the intensive observables conjugated to the order parameter. This peculiar behavior originates from the fact that baryonic matter is subject to attractive short range strong forces as well as repulsive long range electromagnetic interactions, partially screened by a background of electrons. As such, it is expected in any theoretical treatment of nuclear matter in the stellar environment. Consequences for the phenomenology of supernova dynamics are drawn.

nucl-th↗
arXiv2010

Statistical description of complex nuclear phases in supernovae and proto-neutron stars

We develop a phenomenological statistical model for dilute star matter at finite temperature, in which free nucleons are treated within a mean-field approximation and nuclei are considered to form a loosely interacting cluster gas. Its domain of applicability, that is baryonic densities ranging from about $ρ>10^8$ g $\cdot$ cm$^{-3}$ to normal nuclear density, temperatures between 1 and 20 MeV and proton fractions between 0.5 and 0, make it suitable for the description of baryonic matter produced in supernovae explosions and proto-neutron stars. The first finding is that, contrary to the common belief, the crust-core transition is not first order, and for all subsaturation densities matter can be viewed as a continuous fluid mixture between free nucleons and massive nuclei. As a consequence, the equations of state and the associated observables do not present any discontinuity over the whole thermodynamic range. We further investigate the nuclear matter composition over a wide range of densities and temperatures. At high density and temperature our model accounts for a much larger mass fraction bound in medium nuclei with respect to traditional approaches as Lattimer-Swesty, with sizeable consequences on the thermodynamic quantities. The equations of state agree well with the presently used EOS only at low temperatures and in the homogeneous matter phase, while important differences are present in the crust-core transition region. The correlation among the composition of baryonic matter and neutrino opacity is finally discussed, and we show that the two problems can be effectively decoupled.

nucl-th↗
arXiv2010

Monopole oscillations in light nuclei with a molecular dynamics approach

Collective monopole vibrations are studied in the framework of an antisymmetrized version of molecular dynamics as a function of the vibration amplitude. The giant monopole resonance energy in ${}^{40}Ca$ is sensitive to the incompressibility of the effective interaction, in good agreement with complete time-dependent Hartree-Fock calculations. The collective response of ${}^{12}C$, ${}^{16}O$ and ${}^{24}Mg$ is also studied. For these lighter nuclei that have an important contribution of an $α$-clustered component, different frequencies are observed, corresponding to two different types of vibrations associated with breathing and moving of the underlying clusters. Possible connections with direct breakup into $α$ clusters at high excitation energy are discussed.

nucl-th↗
arXiv2010

Multifragmentation and phase transition for hot nuclei

Recent important progress on the knowledge of multifragmentation and phase transition for hot nuclei, thanks to the high detection quality of the INDRA array, is reported. It concerns i) the radial collective energies involved in hot fragmenting nuclei/sources produced in central and semi- peripheral collisions and their influence on the observed fragment partitions, ii) a better knowledge of freeze-out properties obtained by means of a simulation based on all the available experimental information and iii) the quantitative study of the bimodal behaviour of the heaviest fragment distribution for fragmenting hot heavy quasi-projectiles which allows the extraction, for the first time, of an estimate of the latent heat of the phase transition.

nucl-ex↗
arXiv2010

Advancement in the understanding of multifragmentation and phase transition for hot nuclei

Recent advancement on the knowledge of multifragmentation and phase transition for hot nuclei is reported. It concerns i) the influence of radial collective energy on fragment partitions and the derivation of general properties of partitions in presence of such a collective energy, ii) a better knowledge of freeze-out properties obtained by means of a simulation based on all the available experimental information and iii) the quantitative study of the bimodal behaviour of the heaviest fragment charge distribution for fragmenting hot heavy quasi-projectiles which allows, for the first time, to estimate the latent heat of the phase transition.

nucl-ex↗
arXiv2009

Symmetry energy from fragment observables in the canonical thermodynamic model

Different formulas relying measurable fragment isotopic observables to the symmetry energy of excited nuclei have been proposed and applied to the analysis of heavy ion collision data in the recent literature. In this paper we examine the quality of the different expressions in the framework of the McGill Canonical Thermodynamic Model. We show that even in the idealized situation of canonical equilibrium and in the absence of secondary decay, these formulas do not give a precise reconstruction of the symmetry energy of the fragmenting source. However, both isotopic widths and isoscaling appear very well correlated to the physical symmetry energy.

nucl-th↗
arXiv2009

Thermodynamics of clusterized matter

Thermodynamics of clusterized matter is studied in the framework of statistical models with non-interacting cluster degrees of freedom. At variance with the analytical Fisher model, exact Metropolis simulation results indicate that the transition from homogeneous to clusterized matter lies along the $ρ=ρ_0$ axis at all temperatures and the limiting point of the phase diagram is not a critical point even if the surface energy vanishes at this point. Sensitivity of the inferred phase diagram to the employed statistical framework in the case of finite systems is discussed by considering the grand-canonical and constant-pressure canonical ensembles. A Wigner-Seitz formalism in which the fragment charge is neutralized by an uniform electron distribution allows to build the phase diagram of neutron star matter.

nucl-th↗
arXiv2008

Isoscaling as a measure of Symmetry Energy in the Lattice Gas Model

The energetic properties of nuclear clusters inside a low-density, finite-temperature medium are studied with a Lattice Gas Model including isospin dependence and Coulomb forces. Important deviations are observed respect to the Fisher approximation of an ideal gas of non-interacting clusters, but the global energetics can still be approximately expressed in terms of a simple modified energy-density functional. The multi-fragmentation regime appears dominated by combinatorial effects in this model, but the isoscaling of the largest fragment in low energy collisions appears a promising observable for the experimental measurement of the symmetry energy.

nucl-th↗
arXiv2008

Bimodality and Coulomb effects with a canonical thermodynamic model

The effect of the Coulomb interaction on the phase diagram of finite nuclei is studied within the Canonical Thermodynamic Model. If Coulomb effects are artificially switched off, this model shows a phenomenology consistent with the liquid-gas phase transition. The inclusion of Coulomb does not significantly affect the phase diagram but it drastically modifies the nature and order parameter of the transition. A clear understanding of the phenomenon can be achieved looking at the distribution of the largest fragment produced in each fragmentation event. Possible connections with experimental observations are outlined.

nucl-th↗
arXiv2008

Isospin dependent thermodynamics of fragmentation

The thermal and phase properties of a multifragmentation model which uses clusters as degrees of freedom, are explored as a function of isospin. A good qualitative agreement is found with the phase diagram of asymmetric nuclear matter as established by different mean-field models. In particular, from the convexity properties of the nuclear entropy, we show that uncharged finite nuclei display first and second order liquid-gas-like phase transitions. Different quantities are examined to connect the thermal properties of the system to cluster observables. In particular we show that fractionation is only a loose indication of phase coexistence. A simple analytical formula is proposed and tested to evaluate the symmetry (free) energy from the widths of isotopic distributions. Assuming that one may restore the isotopic composition of break-up fragments, it is found that some selected isotopic observables can allow to quantitatively access the freeze-out symmetry energy in multifragmentation experiments.

nucl-th↗
arXiv2008

Signals of bimodality in the fragmentation of Au quasi-projectiles

Signals of bimodality have been investigated in experimental data of quasi-projectile decay produced in Au+Au collisions at 35 AMeV. This same data set was already shown to provide several signals characteristic of a first order, liquid-gas-like phase transition. Different event sortings proposed in the recent literature are analyzed. A sudden change in the fragmentation pattern is revealed by the distribution of the charge of the largest fragment, compatible with a bimodal behavior.

nucl-ex↗
arXiv2007

Phase Transitions in Finite Systems using Information Theory

(abridged) In this paper, we present the issues we consider as essential as far as the statistical mechanics of finite systems is concerned. In particular, we emphasis our present understanding of phase transitions in the framework of information theory. Information theory provides a thermodynamically-consistent treatment of finite, open, transient and expanding systems which are difficult problems in approaches using standard statistical ensembles. As an example, we analyze is the problem of boundary conditions, which in the framework of information theory must also be treated statistically. We recall that out of the thermodynamical limit the different ensembles are not equivalent and in particular they may lead to dramatically different equation of states, in the region of a first order phase transition. We recall the recent progresses achieved in the understanding of first-order phase transition in finite systems: the equivalence between the Yang-Lee theorem and the occurrence of bimodalities in the intensive ensemble and the presence of inverted curvatures of the thermodynamic potential of the associated extensive ensemble.

cond-mat.stat-mech↗
arXiv2007

Expansion dynamics of Lennard-Jones systems

The dynamics of the expansion of a Lennard-Jones system, initially confined at high density and subsequently expanding freely in the vacuum, is confronted to an expanding statistical ensemble, derived in the diluted quasi-ideal Boltzmann approximation. The description proves to be fairly accurate at predicting average one-body global observables, but important deviations are observed in the configuration-space structure of the events. Possible implications for finite expanding physical systems are outlined.

cond-mat.stat-mech↗
arXiv2007

Yield scaling, size hierarchy and fluctuations of observables in fragmentation of excited heavy nuclei

Multifragmentation properties measured with INDRA are studied for single sources produced in Xe+Sn reactions in the incident energy range 32-50 A MeV and quasiprojectiles from Au+Au collisions at 80 A MeV. A comparison for both types of sources is presented concerning Fisher scaling, Zipf law, fragment size and fluctuation observables. A Fisher scaling is observed for all the data. The pseudo-critical energies extracted from the Fisher scaling are consistent between Xe+Sn central collisions and Au quasi-projectiles. In the latter case it also corresponds to the energy region at which fluctuations are maximal. The critical energies deduced from the Zipf analysis are higher than those from the Fisher analysis.

nucl-ex↗
arXiv2007

Anomalous thermodynamics and phase transitions of neutron-star matter

In this letter we show that the presence of the long-range Coulomb force in dense stellar matter implies that the total charge cannot be associated with a chemical potential, even if it is a conserved quantity. As a further consequence, the analytical properties of the partition sum are modified, changing the order of the phase transitions and affecting the possible occurrence of critical behaviours. The peculiar thermodynamic properties of the system can be understood introducing a model hamiltonian in which each charge is independently neutralized by a uniform background of opposite charge.

astro-ph↗