arXiv subjects

Philippe Chomaz

Publications and source records attributed to Philippe Chomaz.

At least 19 recordsLinked to original sources

arXiv2009

Couplings between dipole and quadrupole vibrations in tin isotopes

We study the couplings between collective vibrations such as the isovector giant dipole and isoscalar giant quadrupole resonances in tin isotopes in the framework of the time-dependent Hartree-Fock theory with a Skyrme energy density functional. These couplings are a source of anharmonicity in the multiphonon spectrum. In particular, the residual interaction is known to couple the isovector giant dipole resonance with the isoscalar giant quadrupole resonance built on top of it, inducing a nonlinear evolution of the quadrupole moment after a dipole boost. This coupling also affects the dipole motion in a nucleus with a static or dynamical deformation induced by a quadrupole constraint or boost respectively. Three methods associated with these different manifestations of the coupling are proposed to extract the corresponding matrix elements of the residual interaction. Numerical applications of the different methods to 132Sn are in good agreement with each other. Finally, several tin isotopes are considered to investigate the role of isospin and mass number on this coupling. A simple 1/A dependence of the residual matrix elements is found with no noticeable contribution from the isospin. This result is interpreted within the Goldhaber-Teller model.

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arXiv2008

Cluster formation in compact stars: relativistic versus Skyrme models

We present various properties of nuclear and compact-star matter, comparing the predictions from two kinds of phenomenological approaches: relativistic models (both with constant and density-dependent couplings) and non-relativistic Skyrme-type interactions. We mainly focus on the liquid-gas instabilities that occur at sub-saturation densities, leading to the decomposition of the homogeneous matter into a clusterized phase. Such study is related to the description of neutron-star crust (at zero temperature) and of supernova dynamics (at finite temperature).

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arXiv2008

Pairing Vibrations Study with the Time-Dependent Hartree-Fock-Bogoliubov theory

We study pairing vibrations in $^{18,20,22}$O and $^{42,44,46}$Ca nuclei solving the time-dependent Hartree-Fock-Bogoliubov equation in coordinate space with spherical symmetry. We use the SLy4 Skyrme functional in the normal part of the energy density functional and a local density dependent functional in its pairing part. Pairing vibrations are excited by two-neutron transfer operators. Strength distributions are obtained using the Fourier transform of the time-dependent response of two-neutron pair-transfer observables in the linear regime. Results are in overall agreement with quasiparticle random phase approximation calculations for Oxygen isotopes, though differences appear when increasing the neutron number. Both low lying pairing modes and giant pairing vibrations (GPV) are discussed. The GPV is observed in the Oxygen but not in the Calcium isotopes.

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arXiv2007

Fusion process studied with preequilibrium giant dipole resonance in time dependent Hartree-Fock theory

The equilibration of macroscopic degrees of freedom during the fusion of heavy nuclei, like the charge and the shape, are studied in the Time-Dependent Hartree-Fock theory. The pre-equilibrium Giant Dipole Resonance (GDR) is used to probe the fusion path. It is shown that such isovector collective state is excited in N/Z asymmetric fusion and to a less extent in mass asymmetric systems. The characteristics of this GDR are governed by the structure of the fused system in its preequilibrium phase, like its deformation, rotation and vibration. In particular, we show that a lowering of the pre-equilibrium GDR energy is expected as compared to the statistical one. Revisiting experimental data, we extract an evidence of this lowering for the first time. We also quantify the fusion-evaporation enhancement due to gamma-ray emission from the pre-equilibrium GDR. This cooling mechanism along the fusion path may be suitable to synthesize in the future super heavy elements using radioactive beams with strong N/Z asymmetries in the entrance channel.

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arXiv2007

Isospin-dependent clusterization of Neutron-Star Matter

Because of the presence of a liquid-gas phase transition in nuclear matter, compact-star matter can present a region of instability against the formation of clusters. We investigate this phase separation in a matter composed of neutrons, protons and electrons, within a Skyrme-Lyon mean-field approach. Matter instability and phase properties are characterized through the study of the free-energy curvature. The effect of beta-equilibrium is also analyzed in detail, and we show that the opacity to neutrinos has an influence on the presence of clusterized matter in finite-temperature proto-neutron stars.

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arXiv2006

Isospin fractionation : equilibrium versus spinodal decomposition

This paper focuses on the isospin properties of the asymmetric nuclear-matter liquid-gas phase transition analyzed in a mean-field approach, using Skyrme effective interactions. We compare two different mechanisms of phase separation for low-density matter: equilibrium and spinodal decomposition. The isospin properties of the phases are deduced from the free-energy curvature, which contains information both on the average isospin content and on the system fluctuations. Some implications on experimentally accessible isospin observables are presented.

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arXiv2006

Quantum calculation of Coulomb reorientation and near-barrier fusion

We investigate the role of deformation on the fusion probability around the barrier using the Time-Dependent Hartree-Fock theory with a full Skyrme force. We obtain a distribution of fusion probabilities around the nominal barrier due to the different contributions of the various orientations of the deformed nucleus at the touching point. It is also shown that the long range Coulomb reorientation reduces the fusion probability around the barrier.

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arXiv2004

Generalized Gibbs ensembles for time dependent processes

An information theory description of finite systems explicitly evolving in time is presented for classical as well as quantum mechanics. We impose a variational principle on the Shannon entropy at a given time while the constraints are set at a former time. The resulting density matrix deviates from the Boltzmann kernel and contains explicit time odd components which can be interpreted as collective flows. Applications include quantum brownian motion, linear response theory, out of equilibrium situations for which the relevant information is collected within different time scales before entropy saturation, and the dynamics of the expansion.

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arXiv2004

Quantum calculations of Coulomb reorientation for sub-barrier fusion

Classical mechanics and Time Dependent Hartree-Fock (TDHF) calculations of heavy ions collisions are performed to study the rotation of a deformed nucleus in the Coulomb field of its partner. This reorientation is shown to be independent on charges and relative energy of the partners. It only depends upon the deformations and inertias. TDHF calculations predict an increase by 30% of the induced rotation due to quantum effects while the nuclear contribution seems negligible. This reorientation modifies strongly the fusion cross-section around the barrier for light deformed nuclei on heavy collision partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.

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arXiv2004

Super-Symmetry transformation for excitation processes

Quantum Mechanics SUper-SYmmetry (QM-SUSY) provides a general framework for studies using phenomenological potentials for nucleons (or clusters) interacting with a core. The SUSY potentials result from the transformation of the mean field potential in order to account for the Pauli blocking of the core orbitals. In this article, we discuss how other potentials (like external probes or residual interactions between the valence nucleons) are affected by the SUSY transformation. We illustrate how the SUSY transformations induce off-diagonal terms in coordinate space that play the essential role on the induced transition probabilities on two examples: the electric operators and Gaussian external fields. We show that excitation operators, doorway states, strength and sum rules are modified.

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arXiv2003

Exact pairing correlations in one-dimensional trapped fermions with stochastic mean-field wave-functions

The canonical thermodynamic properties of a one-dimensional system of interacting spin-1/2 fermions with an attractive zero-range pseudo-potential are investigated within an exact approach. The density operator is evaluated as the statistical average of dyadics formed from a stochastic mean-field propagation of independent Slater determinants. For an harmonically trapped Fermi gas and for fermions confined in a 1D-like torus, we observe the transition to a quasi-BCS state with Cooper-like momentum correlations and an algebraic long-range order. For few trapped fermions in a rotating torus, a dominant superfluid component with quantized circulation can be isolated.

cond-mat.stat-mech↗

arXiv2003

Coupled collective motion in nuclear reactions

In this paper, we review the roles of collective modes in nuclear reactions. We emphaize the strong couplings of various collective states with the monopole and quadrupole motions. In inelastic excitation, these couplings can be seen as an important source of anharmonicity in the multiphonon spectrum. In fusion reaction, the breathing and quadrupole motions strongly affects the oscillation of protons against neutrons. Finally, the modification of the collective properties induced by a large amplitude dilution might be the origin of the nuclear multifragmentation, directly related to the nuclear liquid-gas phase transition. In the three cases we derive a coupling matrix element which appears to be in good agreement.

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arXiv2003

Spinodal in asymmetric nuclear matter

The phase diagram of nuclear matter is quite rich - it shows such phenomena as phase-transitions, formation of condensates, clustering, etc. From the analysis of the spinodal instability, one can learn about the region of liquid-gas coexistence in nuclear matter at low densities and finite isospin asymmetries. In a recent paper, we have shown that asymmetric nuclear matter at sub-nuclear densities should undergo only one type of instability. The associated order parameter is dominated by the isoscalar density and so the transition is of liquid-gas type. The instability goes in the direction of a restoration of the isospin symmetry leading to a fractionation phenomenon.

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arXiv2003

Chemical and mechanical spinodals a unique liquid-gas instability

We demonstrate that the instabilities of asymmetric nuclear matter at sub-saturation densities do not present two types of instabilities as usually discussed but a unique one. The associated order parameter is everywhere dominated by the isoscalar density and so the transition is of liquid-gas type even in the so-called chemical instability region.

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arXiv2001

Collective response of nuclei: Comparison between experiments and extended mean-field calculations

The giant monopole, dipole and quadrupole responses in $^{40}$Ca, $^{90}$Zr, $^{120}$Sn and $^{208}$Pb are investigated using linear response treatment based on a stochastic one-body transport theory. Effects of the coupling to low-lying surface modes (coherent mechanism) and the incoherent mechanism due to nucleon-nucleon collisions are included beyond the usual mean-field description. We emphasize the importance of both mechanism in the fragmentation and damping of giant resonance. Calculated spectra are compared with experiment in terms of percentage of Energy-Weighted Sum-Rules in various energy regions. We obtained reasonable agreement in all cases. A special attention as been given to the fragmentation of the Giant Quadrupole Resonance in calcium and lead. In particular, the equal splitting of the $2^{+}$ in $^{40}$Ca is correctly reproduced. In addition, the appearance of fine structure in the response $^{208}$Pb is partly described by the calculations in which the coherent mechanism play an important role.

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arXiv1999

Multiscale fluctuations in nuclear response

The nuclear collective response is investigated in the framework of a doorway picture in which the spreading width of the collective motion is described as a coupling to more and more complex configurations. It is shown that this coupling induces fluctuations of the observed strength. In the case of a hierarchy of overlapping decay channels, we observe Ericson fluctuations at different scales. Methods for extracting these scales and the related lifetimes are discussed. Finally, we show that the coupling of different states at one level of complexity to some common decay channels at the next level, may produce interference-like patterns in the nuclear response. This quantum effect leads to a new type of fluctuations with a typical width related to the level spacing.

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arXiv1999

On the Simulation of Extended TDHF Theory

A novel method is presented for implementation of the extended mean-field theory incorporating two-body collisions. At a given time, stochastic imaginary time propagation of occupied states are used to generate a convenient basis. The quantal collision terms, including memory effects, is then computed by a backward mean-field propagation of these single-particle states. The method is illustrated in an exactly solvable model. Whereas the usual TDHF fails to reproduce the long time evolution, a good agreement is found between the extended TDHF and the exact solution.

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arXiv1998

Beating of monopole modes in nuclear dynamics

Time-dependent Hartree-Fock simulations of the evolution of excited gold fragments have been performed. The observed dynamics appears more complex than the collective expansion picture. The minimum density is often not reached during the first density oscillation because of the beating of several collective compression modes.

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