Beyond-mean-field models for correlated nucleons: Applications of second random-phase approximation to ~(16)O and ~(48)Ca

摘要

Second RPA calculations with a Skyrme force are performed to describe both high- and low-lying excited states in ~(16)O and ~(48)Ca. The coupling between 1 particle-1 hole and 2 particle-2 hole as well as that between 2 particle-2 hole configurations are fully taken into account and the residual interaction is considered with the proper rearrangement terms. A several-MeV shift to lower energies is systematically found with respect to the RPA distributions for the strength distributions of giant resonances. The nature of the low-energy dipole excitations is analyzed in ~(48)Ca: it is found that in general these excitations cannot be in interpreted in terms of oscillations of the neutron skin against the core, with the exception of the peak with the largest B(E1) value, which is located at 9.09 MeV. For this peak the neutron transition density dominates and the neutron and proton transition densities oscillate out of phase in the internal part of the nucleus leading to a strong mixing of isoscalar and isovector components. Therefore, this state shows some features usually associated to pygmy resonances.