Double giant dipole resonances in time-dependent density-matrix theory

The strength functions of the double giant dipole resonances (DGDR) in 16O and 40Ca are calculated with the use of an extended version of the time-dependent Hartree-Fock theory known as the time-dependent density-matrix theory. The calculations are done in a self-consistent manner, in which the same Skyrme force as that used for a mean-field potential is used as an effective interaction for a two-body correlation function. It is found that the DGDR in 16O has a large width due to the Landau damping, although the centroid energy of the strength distribution is close to twice the energy of the giant dipole resonance (GDR) calculated in RPA. The DGDR in 40Ca is found more harmonic than that in 16O: the strength function of the DGDR in 40Ca is similar to what is predicted from the strength function of the GDR in RPA.