arXiv subjects

G. Colo`

Publications and source records attributed to G. Colo`.

16 recordsLinked to original sources

arXiv2005

The fully self-consistent quasiparticle random phase approximation and its application to the isobaric analog resonances

A microscopic model aimed at the description of charge-exchange nuclear excitations along isotopic chains which include open-shell systems, is developed. It consists of quasiparticle random phase approximation (QRPA) made on top of Hartree-Fock-Bardeen-Cooper-Schrieffer (HF-BCS). The calculations are performed by using the Skyrme interaction in the particle-hole channel and a zero-range, density-dependent pairing force in the particle-particle channel. At variance with the (many) versions of QRPA which are available in literature, in our work special emphasis is put on the full self-consistency. Its importance, as well as the role played by the charge-breaking terms of the nuclear Hamiltonian, like the Coulomb interaction, the charge symmetry and charge independence breaking (CSB-CIB) forces and the electromagnetic spin-orbit, are elucidated by means of numerical calculations of the isobaric analog resonances (IAR). The theoretical energies of these states along the chain of the Sn isotopes agree well with the experimental data in the stable isotopes. Predictions for unstable systems are presented.

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arXiv2004

Dipole states in stable and unstable nuclei

A nuclear structure model based on linear response theory (i.e., Random Phase Approximation) and which includes pairing correlations and anharmonicities (coupling with collective vibrations), has been implemented in such a way that it can be applied on the same footing to magic as well as open-shell nuclei. As applications, we have chosen to study the dipole excitations both in well-known, stable isotopes like $^{208}$Pb and $^{120}$Sn as well as in the neutron-rich, unstable $^{132}$Sn nucleus, by addressing in the latter case the question about the nature of the low-lying strength. Our results suggest that the model is reliable and predicts in all cases low-lying strength of non collective nature.

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arXiv2004

Microscopic determination of the nuclear incompressibility within the non-relativistic framework

The nuclear incompressibility $K_\infty$ is deduced from measurements of the Isoscalar Giant Monopole Resonance (ISGMR) in medium-heavy nuclei, and the resulting value turns out to be model dependent. Since the considered nuclei have neutron excess, it has been suggested that the model dependence is due to the different behaviour of the symmetry energy in different models. To clarify this issue, we make a systematic and careful analysis based on new Skyrme forces which span a wide range of values for $K_\infty$, for the value of the symmetry energy at saturation and for its density dependence. By calculating, in a fully self-consistent fashion, the ISGMR centroid energy in $^{208}$Pb we reach, for the first time within the non-relativistic framework, three important conclusions: (i) the monopole energy, and consequently the deduced value of $K_\infty$, depend on a well defined parameter related to the shape of the symmetry energy curve and called $K_{sym}$; (ii) Skyrme forces of the type of SLy4 predict $K_\infty$ around 230 MeV, in agreement with the Gogny force (previous estimates using Skyrme interactions having been plagued by lack of full self-consistency); (iii) it is possible to build forces which predict $K_\infty$ around 250 MeV, although part of this increase is due to our poor knowledge of the density dependence and effective mass.

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arXiv2003

Theoretical understanding of the nuclear incompressibility: where do we stand ?

The status of the theoretical research on the compressional modes of finite nuclei and the incompressibility $K_\infty$ of nuclear matter, is reviewed. It is argued that the recent experimental data on the Isoscalar Giant Monopole Resonance (ISGMR) allow extracting the value of $K_\infty$ with an uncertainity of about $\pm$ 12 MeV. Non-relativistic (Skyrme, Gogny) and relativistic mean field models predict for $K_\infty$ values which are significantly different from one another, namely $\approx$ 220-235 and $\approx$ 250-270 MeV respectively. It is shown that the solution of this puzzle requires a better determination of the symmetry energy at, and around, saturation. The role played by the experimental data of the Isoscalar Giant Dipole Resonance (ISGDR) is also discussed.

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arXiv2003

Cross sections for the excitation of isovector charge-exchange resonances in 208Tl

The Glauber approximation for the treatment of heavy-ion scattering, has already been shown to give reliable predictions for the reaction cross section in the particular case of intermediate energy charge-exchange processes. In the present work, we couple a Glauber-type model to microscopic Random Phase Approximation calculations of the charge-exchange excitations of $^{208}$Pb. The aim is to solve the longstanding question whether the very elusive charge-exchange isovector monopole has been really identified in the past experiments, or other multipoles were prevalent in the observed spectra.

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arXiv2001

QRPA plus Phonon Coupling Model and the Photoabsorbtion Cross Section for $^{18,20,22}$O

We have calculated the electric dipole strength distributions in the unstable neutron rich oxygen isotopes $^{18,20,22}$O, in a model which include up to four quasi-particle-type configurations. The model is the extension, to include the effect of the pairing correlations, of a previous model very successful around closed shell nuclei, and it is based on the quasi-particle-phonon coupling. Low-lying dipole strength is found, which exhausts between 5 and 10% of the Thomas-Reiche-Kuhn (TRK) energy-weighted-sum-rule (EWSR) below 15 MeV excitation energy, in rather good agreement with recent experimental data. The role of the phonon coupling is shown to be crucial in order to obtain this result.

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arXiv2000

On dipole compression modes in nuclei

Isoscalar dipole strength distributions in spherical medium- and heavy-mass nuclei are calculated within random phase approximation (RPA) or quasiparticle RPA. Different Skyrme-type interactions corresponding to incompressibilities in the range 200 - 250 MeV are used. The results are discussed in comparison with existing data on isoscalar giant dipole resonances. Two main issues are raised, firstly the calculated giant resonance energies are somewhat higher than the observed ones, and secondly a sizable fraction of strength is predicted below 20 MeV which needs to be experimentally confirmed.

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arXiv2000

Room Temperature Organic Superconductor?

The electron--phonon coupling in fullerene C28 has been calculated from first principles. The value of the associated coupling constant lambda/N(0) is found to be a factor three larger than that associated with C60. Assuming similar values of the density of levels at the Fermi surface N(0) and of the Coulomb pseudopotential for C28-based solids as those associated with alkali-doped fullerides A3C60, one obtains Tc(C28) \approx 8 Tc(C60).

cond-mat.supr-con↗

arXiv2000

Electron-phonon interaction in C70

The matrix elements of the deformation potential of C$_{70}$ are calculated by means of a simple, yet accurate solution of the electron-phonon coupling problem in fullerenes, based on a parametrization of the ground state electronic density of the system in terms of $sp^{2+x}$ hybridized orbitals. The value of the calculated dimensionless total electron-phonon coupling constant is $λ\approx0.1$, an order of magnitude smaller than in C$_{60}$, consistent with the lack of a superconducting phase transition in C$_{70}$A$_3$ fullerite, and in overall agreement with measurements of the broadening of Raman peaks in C$_{70}$K$_4$. We also calculate the photoemission cross section of C$_{70}^-$, which is found to display less structure than that associated with C$_{60}^-$, in overall agreement with the experimental findings.

cond-mat↗

arXiv1999

Compression modes in nuclei: microscopic models with Skyrme interactions

The isoscalar giant monopole resonances (ISGMR) and giant dipole resonances (ISGDR) in medium-heavy nuclei are investigated in the framework of HF+RPA and HF-BCS+QRPA with Skyrme effective interactions. It is found that pairing has little effect on these modes. It is also found that the coupling of the RPA states to 2p-2h configurations results in about (or less than) 1 MeV shifts of the resonance energies and at the same time gives the correct total widths. For the ISGMR, comparison with recent data leads to a value of nuclear matter compression modulus close to 215 MeV. However, a discrepancy between calculated and measured energies of the ISGDR in $^{208}$Pb is found and remains an open problem.

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arXiv1998

Restoration of Isospin Symmetry in Highly Excited Nuclei

Explicit relations between the isospin mixing probability, the spreading width $Γ_{IAS}^{\downarrow}$ of the Isobaric Analog State (IAS) and the statistical decay width $Γ_c$ of the compound nucleus at finite excitation energy, are derived by using the Feshbach projection formalism. The temperature dependence of the isospin mixing probability is discussed quantitatively for the first time by using the values of $Γ_{IAS}^{\downarrow}$ and of $Γ_c$ calculated by means of microscopic models. It is shown that the mixing probability remains essentially constant up to a temperature of the order of 1 MeV and then decreases to about 1/4 of its zero temperature value, at higher temperature than $\approx$ 3 MeV, due to the short decay time of the compound system.

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arXiv1998

Response function beyond mean field of neutron-rich nuclei

The damping of single-particle and collective motion in exotic isotopes is a new topic and its study may shed light on basic problems of nuclear dynamics. For instance, it is known that nuclear structure calculations are not able, as a rule, to account completely for the empirical single-particle damping. In this contribution, we present calculations of the single-particle self-energy in the case of the neutron-rich light nucleus $^{28}$O, by taking proper care of the continuum, and we show that there are important differences with the case of nuclei along the valley of stability.

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arXiv1998

Widths of Isobaric Analog Resonances: a microscopic approach

A self-consistent particle-phonon coupling model is used to investigate the properties of the isobaric analog resonance in $^{208}$Bi. It is shown that quantitative agreement with experimental data for the energy and the width can be obtained if the effects of isospin-breaking nuclear forces are included, in addition to the Coulomb force effects. A connection between microscopic model predictions and doorway state approaches which make use of the isovector monopole resonance, is established via a phenomenological ansatz for the optical potential.

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arXiv1998

Electron-Phonon Coupling in Charged Buckminsterfullerene

A simple, yet accurate solution of the electron-phonon coupling problem in C_{60} is presented. The basic idea behind it is to be found in the parametrization of the ground state electronic density of the system calculated making use of ab-initio methods, in term of sp$^{2+x}$ hybridized orbitals. This parametrization allows for an economic determination of the deformation potential associated with the fullerene's normal modes. The resulting electron-phonon coupling constants are used to calculate Jahn-Teller effects in C_{60}^-, and multiple satellite peaks in the corresponding photoemission reaction. Theory provides an accurate account of the experimental findings.

cond-mat↗

arXiv1997

Competition Between Particle-Hole and Particle-Particle Correlations in Forbidden Electron Capture: the Case of $^{123}$Te

The K-electron capture half-life of $^{123}$Te has been recently measured to be $t^K_{exp}=2.4\times 10^{19}$ yr, and constitutes the longest half-life ever measured in a single $β$-transition of any nuclear species. We have calculated this second unique forbidden transition within the framework of the proton-neutron quasi-particle random phase approximation, making use of Skyrme-type effective interactions. A strong cancellation effect between particle-hole and particle-particle correlations is found. The model, without any renormalization of the force, provides a lower limit for the K-electron capture half-life of $\sim 10^{17}$ yr, which unambiguously rules out the old experimental values of $10^{13} - 10^{14}$ yr. A few percent increase of the particle-particle matrix elements of the Skyrme interaction allows to reproduce the experimental findings.

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arXiv1994

Escape and Spreading Properties of Charge-Exchange Resonances in Bi 208

The properties of charge-exchange excitations of ${}^ {208}$Pb with $ΔL = 0$, i.e., the isobaric analog and Gamow-Teller resonances, are studied within a self-consistent model making use of an effective force of the Skyrme type. The well-known isobaric analog case is used to assess the reliability of the model. The calculated properties of the Gamow-Teller resonance are compared with recent experimental measurements with the aim of better understanding the microscopic structure of this mode.

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