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The main goal of this paper is to investigate the properties of the low-energy quadrupole strength in Ni isotopes, especially for the evolution of the pygmy quadrupole states with increasing neutron number. And the effect of shell evolution on the pygmy resonance is also discussed in detail. The Skyrme Hartree-Fock+Bardeen-Cooper-Schrieffer (HF+BCS) theory and the selfconsistent quasiparticle random phase approximation (RPA) method are employed on top of three effective Skyrme interactions named SGII, SLy5 and SKM*. In the calculations, a density-dependent zero-range type force are adopted for the pairing correlations. The properties of the first 2+ state in Ni isotopes are studied firstly. As Fig. (a) shown, a good description on the experimental excited energies of the first 2+ states are achieved, the SGII and SLy5 can give a good description on the reduced electric transition probabilities for 58-68Ni. It is found that the energies of the first 2+ state for 68Ni and 78Ni are obviously high than others, which reflects the obvious shell effect. In addition to the first 2+ states, pygmy quadrupole states between 3 to 5 MeV with relative large electric transition probabilities are evidently found for 70-76Ni in the isoscalar quadruple strength distribution [see Fig. (b)]. With the increasing of neutron number, the pygmy quadrupole states have decreasing energies but hold gradually increasing strengths, and it is more sensitive to the changes in the shell structure. This is due to the fact that the gradually filling of the neutron level 1g9/2 has an very important impact on the pygmy quadrupole states of 70-76Ni, and switch from proton-dominated excitations to neutron-dominated ones. Since the pygmy quadrupole states for 70-76Ni are sensitive to the proton and neutron shell gaps, which can provide information on the shell evolution in neutron-rich nuclei.
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Keywords:
- Skyrme energy density functional /
- pygmy /
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