Pygmy and giant dipole resonances in Ni isotopes

The isovector giant and pygmy dipole resonances in even-even Ni isotopes are studied within the framework of a fully consistent relativistic random-phase approximation built on the relativistic mean field ground state. An additional isoscalar-isovector nonlinear coupling term is adopted in the standard effective mean field Lagrangian, which could modify the density dependence of the symmetry energy and soften the symmetry energy at the saturation density without changing the agreement with experimentally existing data of ground state properties. We found that the centroid energy of the isovector giant dipole resonance is tightly correlated to the neutron skin thickness. In contrast, the centroid energy of the isovector pygmy resonance is insensitive to the density dependence of the symmetry energy by tuning the isoscalar-isovector nonlinear coupling.