Low energy strength in low-multipole response function of nuclei near the neutron drip line

Abstract

The very low-energy transition strength unique in neutron drip line neclei is studied, taking an example of ${}_8{}^{28}{}_{}{}^{}\mathrm{O}_{20}^{}{}_{}{}^{}$ and performing the Hartree-Fock plus RPA calculation with Skyrme interaction. The most dramatic example is monopole modes, however, an appreciable amount of isovector dipole strength may appear also in the very low excitation energy. Unperturbed response functions are carefully studied, which contain all basic information on the exotic behavior of the RPA strength function. The low-energy transition strength is induced by the excitations of neutrons, which have smaller binding energies and smaller angular momenta. The neutrons with a few MeV binding energies are sufficient for obtaining this strength, and the phenomena are differentiated from the so-called soft multipole excitations in halo nuclei.