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Register a new userPygmy dipole response of proton rich argon beyond the random phase approximation
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The occurrence of a pygmy dipole resonance in proton rich Ar-32 and Ar-34 is studied using the unitary correlator operator method interaction Vucom, based on Argonne V18. Predictions from the random phase approximation (RPA) and the shell model in a no-core basis are compared. It is found that the inclusion of configuration mixing up to two-particle--two-holes broadens the pygmy strength slightly and reduces sensibly its strength, as compared to the RPA predictions. For Ar-32 a clear peak associated with a pygmy resonance is found. For Ar-34, the pygmy states are obtained close to the giant dipole resonance and mix with it.
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In a recent article by C. Barbieri, E. Caurier, K. Langanke, and G. Martinez-Pinedo cite{Bar.08}, low-energy dipole excitations were studied in proton-rich $^{32,34}$Ar with random-phase approximation (RPA) and no-core shell model (NCSM) using correlated realistic nucleon-nucleon interactions obtained by the unitary correlation operator method (UCOM) cite{Fel.98}. The main objective of this Comment is to argue that the article cite{Bar.08} contains an inconsistency with respect to previous study of excitations in the same UCOM-RPA framework using identical correlated Argonne V18 interaction cite{Paa.06}, it does not provide any evidence that the low-lying state declared as pygmy dipole resonance in $^{32}$Ar indeed has the resonance-like structure, and that prior to studying exotic modes of excitation away from the valley of stability one should ensure that the model provides reliable description of available experimental data on nuclear ground state properties and excitations in nuclei. Although the authors aimed at testing the UCOM based theory at the proton drip line, available experimental data that are used as standard initial tests of theory frameworks at the proton drip line have not been considered in the UCOM case (e.g., binding energies, one-proton separation energies, two-proton separation energies).
We respond to a Comment on our recent paper (Phys.Rev.C77:024304,2008) by Paar (arXiv:0803.0274).
We study the nature of the low-lying dipole strength in neutron-rich nuclei, often associated to the Pygmy Dipole Resonance. The states are described within the Hartree-Fock plus RPA formalism, using different parametrizations of the Skyrme interaction. We show how the information from combined reactions processes involving the Coulomb and different mixtures of isoscalar and isovector nuclear interactions can provide a clue to reveal the characteristic features of these states.
Starting from the quasiparticle random phase approximation based on the Skyrme interaction SLy5, we study the effects of phonon-phonon coupling~(PPC) on the low-energy electric dipole response in $^{40-58}$Ca. Using the same set of parameters we describe available experimental data for $^{40,44,48}$Ca and give prediction for $^{50-58}$Ca. The inclusion of the PPC results in the formation of low-energy $1^-$ states. There is an impact of the PPC effect on low-energy $E1$~strength of $^{40,44,48}$Ca. The PPC effect on the electric dipole polarizability is discussed. We predict a strong increase of the summed $E1$~strength below 10MeV, with increasing neutron number from $^{48}$Ca till $^{58}$Ca.
We develop a scheme to exactly evaluate the correlation energy in the random-phase approximation, based on linear response theory. It is demonstrated that our formula is completely equivalent to a contour integral representation recently proposed by Donau et al. being numerically more efficient for realistic calculations. Numerical examples are presented for pairing correlations in rapidly rotating nuclei.
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