Giant quadrupole excitation in nuclei with neutron skin

The electric quadrupole excitation of unstable nuclei with the neutron skin is studied in terms of a Hartree-Fock RPA calculation, taking ${}_{}{}^{28}{}_{}{}^{}\mathrm{O}$ as an example. It is found that there are giant excitation peaks, referred to as giant neutron modes, below the isoscalar giant resonance and these peaks consist of neutron excitations only. Their transient densities are shown to be extended radially in comparison to the isoscalar giant resonance. A strong correlation between B(E2) sum of the giant neutron modes and the numbers of neutrons in the neutron skin is found.