Effects of ground-state correlations on dipole and quadrupole excitations of ⁴⁰Ca and ⁴⁸Ca

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The effects of ground-state correlations on the dipole and quadrupole excitations are studied for ⁴⁰Ca and ⁴⁸Ca using the extended random phase approximation (ERPA) derived from the time-dependent density-matrix theory. Large effects of the ground-state correlations are found in the fragmentation of the giant quadrupole resonance in ⁴⁰Ca and in the low-lying dipole strength in ⁴⁸Ca. It is discussed that the former is due to a mixing of different configurations in the ground state and the latter is from the partial occupation of the neutron single-particle states. The dipole and quadrupole strength distributions below 10 MeV calculated in ERPA are in qualitatively agreement with experiment.