arXiv subjects

F. Gulminelli

Publications and source records attributed to F. Gulminelli.

At least 91 records · Page 5Linked to original sources

arXiv2007

Ising analogue to compact-star matter

By constructing an Ising analogue of compact-star matter at sub-saturation density we explored the effect of Coulomb frustration on the nuclear liquid-gas phase transition. Our conclusions is twofold. First, the range of temperatures where inhomogeneous phases form expands with increasing Coulomb-field strength. Second, within the approximation of uniform electron distribution, the limiting point upon which the phase-coexistence region ends does not exhibit any critical behaviour. Possible astrophysics consequences and thermodynamical connections are discussed.

nucl-th↗

arXiv2007

Extended Gibbs ensembles with flow

A statistical treatment of finite unbound systems in the presence of collective motions is presented and applied to a classical Lennard-Jones Hamiltonian, numerically simulated through molecular dynamics. In the ideal gas limit, the flow dynamics can be exactly re-casted into effective time-dependent Lagrange parameters acting on a standard Gibbs ensemble with an extra total energy conservation constraint. Using this same ansatz for the low density freeze-out configurations of an interacting expanding system, we show that the presence of flow can have a sizeable effect on the microstate distribution.

cond-mat.stat-mech↗

arXiv2007

Transition from participant to spectator fragmentation in Au+Au reaction between 60 AMeV and 150 AMeV

Using the quantum molecular dynamics approach, we analyze the results of the recent INDRA Au+Au experiments at GSI in the energy range between 60 AMeV and 150 AMeV. It turns out that in this energy region the transition toward a participant-spectator scenario takes place. The large Au+Au system displays in the simulations as in the experiment simultaneously dynamical and statistical behavior which we analyze in detail: The composition of fragments close to midrapidity follows statistical laws and the system shows bi-modality, i.e. a sudden transition between different fragmentation pattern as a function of the centrality as expected for a phase transition. The fragment spectra at small and large rapidities, on the other hand, are determined by dynamics and the system as a whole does not come to equilibrium, an observation which is confirmed by FOPI experiments for the same system.

nucl-th↗

arXiv2007

Looking for bimodal distributions in multi-fragmentation reactions

The presence of a phase transition in a finite system can be deduced, together with its order, from the shape of the distribution of the order parameter. This issue has been extensively studied in multifragmentation experiments, with results that do not appear fully consistent. In this paper we discuss the effect of the statistical ensemble or sorting conditions on the shape of fragment distributions, and propose a new method, which can be easily implemented experimentally, to discriminate between different fragmentation scenarii. This method, based on a reweighting of the measured distribution to account for the experimental constraints linked to the energy deposit, is tested on different simple models, and appears to provide a powerful discrimination.

nucl-th↗

arXiv2006

Bimodal pattern 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 present several signals characteristic of a first order, liquid-gas-like phase transition. For the present analysis, events are sorted in bins of transverse energy of light charged particles emitted by the quasi-target source. A sudden change in the fragmentation pattern is observed from the distributions of the asymmetry of the two largest fragments, and the charge of the largest fragment. This latter distribution shows a bimodal behavior. The interpretation of this signal is discussed.

nucl-ex↗

arXiv2006

Fluctuations of fragment observables

This contribution presents a review of our present theoretical as well as experimental knowledge of different fluctuation observables relevant to nuclear multifragmentation. The possible connection between the presence of a fluctuation peak and the occurrence of a phase transition or a critical phenomenon is critically analyzed. Many different phenomena can lead both to the creation and to the suppression of a fluctuation peak. In particular, the role of constraints due to conservation laws and to data sorting is shown to be essential. From the experimental point of view, a comparison of the available fragmentation data reveals that there is a good agreement between different data sets of basic fluctuation observables, if the fragmenting source is of comparable size. This compatibility suggests that the fragmentation process is largely independent of the reaction mechanism (central versus peripheral collisions, symmetric versus asymmetric systems, light ions versus heavy ion induced reactions). Configurational energy fluctuations, that may give important information on the heat capacity of the fragmenting system at the freeze out stage, are not fully compatible among different data sets and require further analysis to properly account for Coulomb effects and secondary decays. Some basic theoretical questions, concerning the interplay between the dynamics of the collision and the fragmentation process, and the cluster definition in dense and hot media, are still open and are addressed at the end of the paper. A comparison with realistic models and/or a quantitative analysis of the fluctuation properties will be needed to clarify in the next future the nature of the transition observed from compound nucleus evaporation to multi-fragment production.

nucl-ex↗

arXiv2006

Comparisons of Statistical Multifragmentation and Evaporation Models for Heavy Ion Collisions

The results from ten statistical multifragmentation models have been compared with each other using selected experimental observables. Even though details in any single observable may differ, the general trends among models are similar. Thus these models and similar ones are very good in providing important physics insights especially for general properties of the primary fragments and the multifragmentation process. Mean values and ratios of observables are also less sensitive to individual differences in the models. In addition to multifragmentation models, we have compared results from five commonly used evaporation codes. The fluctuations in isotope yield ratios are found to be a good indicator to evaluate the sequential decay implementation in the code. The systems and the observables studied here can be used as benchmarks for the development of statistical multifragmentation models and evaporation codes.

nucl-ex↗

arXiv2006

Thermodynamics of compact-star matter within an ising approach

In the formation and evolution of compact stars, nuclear matter explores high thermal excursions and is the site of intense neutrino emission. Neutrino transport as well as structural properties of this matter depend on the presence of inhomogeneous phases (named "pasta" phases), which are the result of Coulomb frustration of the Liquid-Gas phase transition. We take into account charge fluctuations by employing a frustrated lattice-gas model to which we impose a neutrality constraint by the addition of an homogeneous background of charge, representing delocalised electrons. Within this schematic model we highlight a generaic feature of the phase-transition phenomenology: the temperature interval where pasta phases are formed is enhanced by Coulomb-frustration effects. This result is at variance with the behaviour of frustrated ferromagnetic systems as well as hot nuclei and mean-field approaches. Moreover, the region of phase coexistence is not found to end upon a critical point, indicating that no critical opalescence can occur in compact-star matter.

nucl-th↗

arXiv2006

Multifragmentation and the symmetry term of the nuclear equation of state

We investigate the possibility to extract the symmetry energy from multifragmentation data. The applicability of the grandcanonical formula earlier proposed by Ono {\it et al.} [Phys. Rev. C {\bf 68}, 051601(R)] in the case of finite excited nuclei is tested within a microcanonical framework. Relatively good results are obtained except for large residual nuclei, especially when large sources are highly excited. Effects of secondary particle emission and the extent in which relevant information may be inferred from experimental observables are finally discussed.

nucl-th↗

arXiv2006

From multifragmentation to supernovae and neutron stars

The thermodynamics properties of globally neutral dense stellar matter are analyzed both in terms of mean field instabilities and structures beyond the mean field. The mean field response to finite wavelenght fluctuations is calculated with the realistic Sly230a effective interaction. A Monte Carlo simulation of a schematic lattice Hamiltonian shows the importance of calculations beyond the mean field to calculate the phase diagram of stellar matter. The analogies and differences respect to the thermodynamics of nuclear matter and finite nuclei are stressed.

nucl-th↗

arXiv2006

Role of isospin in the nuclear liquid-gas phase transition

We study the thermodynamics of asymmetric nuclear matter using a mean field approximation with a Skyrme effective interaction, in order to establish its phase diagram and more particularly the influence of isospin on the order of the transition. A new statistical method is introduced to study the thermodynamics of a multifluid system, keeping only one density fixed the others being replaced by their intensive conjugated variables. In this ensemble phase coexistence reduces to a simple one dimensional Maxwell construction. For a fixed temperature under a critical value, a coexistence line is obtained in the plane of neutron and proton chemical potentials. Along this line the grand potential presents a discontinuous slope showing that the transition is first order except at the two ending points where it becomes second order. This result is not in contradiction with the already reported occurrence of a continuous transformation when a constant proton fraction is imposed. Indeed, the proton fraction being an order parameter in asymmetric matter, the constraint can only be fulfilled by gradual phase mixing along the first-order phase transition line leading to a continuous pressure.

nucl-th↗

arXiv2006

Bimodality: a possible experimental signature of the liquid-gas phase transition of nuclear matter

We have observed a bimodal behaviour of the distribution of the asymmetry between the charges of the two heaviest products resulting from the decay of the quasi-projectile released in binary Xe+Sn and Au+Au collisions from 60 to 100 MeV/u. Event sorting has been achieved through the transverse energy of light charged particles emitted on the quasi-target side, thus avoiding artificial correlations between the bimodality signal and the sorting variable. Bimodality is observed for intermediate impact parameters for which the quasi-projectile is identified. A simulation shows that the deexcitation step rather than the geometry of the collision appears responsible for the bimodal behaviour. The influence of mid-rapidity emission has been verified. The two bumps of the bimodal distribution correspond to different excitation energies and similar temperatures. It is also shown that it is possible to correlate the bimodality signal with a change in the distribution of the heaviest fragment charge and a peak in potential energy fluctuations. All together, this set of data is coherent with what would be expected in a finite system if the corresponding system in the thermodynamic limit exhibits a first order phase transition.

nucl-ex↗

arXiv2005

Tracking energy fluctuations from fragment partitions in the Lattice Gas model

Partial energy fluctuations are known tools to reconstruct microcanonical heat capacities. For experimental applications, approximations have been developed to infer fluctuations at freeze out from the observed fragment partitions. The accuracy of this procedure as well as the underlying independent fragment approximation is under debate already at the level of equilibrated systems. Using a well controlled computer experiment, the Lattice Gas model, we critically discuss the thermodynamic conditions under which fragment partitions can be used to reconstruct the thermodynamics of an equilibrated system.

nucl-th↗

arXiv2005

Information theory of open fragmenting systems

An information theory description of finite systems explicitly evolving in time is presented. We impose a MaxEnt variational principle on the Shannon entropy at a given time while the constraints are set at a former time. The resulting density matrix contains explicit time odd components in the form of collective flows. As a specific application we consider the dynamics of the expansion in connection with heavy ion experiments. Lattice gas and classical molecular dynamics simulations are shown.

nucl-th↗

arXiv2004

Correlations in mesoscopic magnetic systems

The purpose of this proposal is to study the ferro/para phase transition in a mesoscopic Ising-like lattice and in particular demonstrate the existence of a negative magnetic susceptibility in the fixed magnetization ensemble. To this aim we will use the correlation < S(r) S(r+Dr) > = < M2>/N2 where N is the total number of spins for a single cluster, M the total magnetization of the cluster, and the equality holds if we choose r0<Dr<R where r0 is the linear size of a spin site and R is the linear size of a cluster.

cond-mat.stat-mech↗

arXiv2004

Phase Transition in Small System

Everybody knows that when a liquid is heated, its temperature increases until the moment when it starts to boil. The increase in temperature then stops, all heat being used to transform the liquid into vapor. What is the microscopic origin of such a strange behavior? Does a liquid drop containing only few molecules behave the same? Recent experimental and theoretical developments seem to indicate that at the elementary level of very small systems, this anomaly appears in an even more astonishing way: during the change of state - for example from liquid to gas - the system cools whereas it is heated, i.e. its temperature decreases while its energy increases. This paper presents a review of our understanding of the negative specific heat phenomenon.

cond-mat.stat-mech↗

arXiv2004

Comment on "Partial energies fluctuations and negative heat capacities" by X. Campi et al

Studying the energy partioning published in nucl-th/0406056v2 we show that the presented results do not fulfill the sum rule due to energy conservation. The observed fluctuations of the energy conservation test point to a numerical problem. Moreover, analysis of the binding energies show that the fragment recognition algorithm adopted by Campi et al. leads with a sizeable probability to fragments containing up to the total mass even for excitation energies as large as 3/4 of the total binding. This surprising result points to another problem since the published inter-fragment energy is not zero while a unique fragment is present. This problem may be due to either the fragment recognition algorithm or to the definition of the inter and intra-fragment energy. These numerical inconsistencies should be settled before any conclusion on the physics can be drawn.

nucl-th↗

arXiv2004

Enhancement of kinetic energy fluctuations due to expansion

Global equilibrium fragmentation inside a freeze out constraining volume is a working hypothesis widely used in nuclear fragmentation statistical models. In the framework of classical Lennard Jones molecular dynamics, we study how the relaxation of the fixed volume constraint affects the posterior evolution of microscopic correlations, and how a non-confined fragmentation scenario is established. A study of the dynamical evolution of the relative kinetic energy fluctuations was also performed. We found that asymptotic measurements of such observable can be related to the number of decaying channels available to the system at fragmentation time.

nucl-th↗