Structure of giant quadrupole resonances in neutron drip line nuclei

We study the giant quadrupole resonances in the neutron drip line nucleus, ${}_8^{28}{\mathrm{O}}_{20},$ using the self-consistent Hartree-Fock calculation plus the random-phase approximation (RPA) with Skyrme interactions. Including simultaneously both the isoscalar and the isovector correlation, the RPA response function is calculated in the coordinate space so as to take into account properly the continuum effect. Though in neutron drip line nuclei the distribution of the quadrupole strength is much affected by the presence of the low-energy threshold strength, the isoscalar and the isovector correlation structure as well as the transition densities of the isoscalar giant resonance are in good agreement with those given by the collective model. In contrast, the transition density of the isovector giant resonance shows a clear deviation from that of the liquid-drop polarization mode.