Magnetic Dipole Excitations in Magic Nuclei with Subtracted Second Random-Phase Approximation

Magnetic dipole (M1) excitations of magic nuclei 48Ca, 90Zr, and 132Sn are investigated by a self-consistent Hartree-Fock (HF) plus subtracted second random-phase approximation (SSRPA) with Skyrme energy density functions (EDFs). We clarify the effects of the mixing of two-particle two-hole (2p-2h) configurations and tensor correlations in SSRPA on the excitation energies and the quenching of the M1 strength. It is shown that the 2p-2h configurations shift the M1 peak energies downwards by about 1.5 MeV in both 48Ca and 90Zr. In contrast, the tensor correlations move the peaks upwards and improve the description of experimental data. We also show that the 2p-2h configurations largely fragment the M1 strength, and reduce the cumulative strengths around the main M1 peaks by about 25% and 27% in 48Ca and 90Zr, respectively, compared with those of random-phase approximation (RPA), as an outcome of the combined effect with the tensor force. The M1 transitions in 132Sn are calculated by the same SSRPA model with the Skyrme EDF including the tensor terms.