Relativistic random-phase-approximation description of M1 excitations with the inclusion of π mesons

Based on the covariant density functional theory, the magnetic dipole (M1) resonance is described in the framework of relativistic random-phase approximation with density-dependent meson-nucleon coupling. The isovector-pseudovector interaction channel, represented by the exchange of pi meson, is included in the residual interaction to describe unnatural parity transitions. The strength distributions of M1 resonances are studied in doubly magic nuclei Ca-48, Zr-90, and Pb-208, in comparison with their analog Gamow-Teller (GT) excitations. It is found that the pi meson and its zero-range counterterm are responsible for almost all of the energy shift caused by residual interaction, which is similar to the case of GT excitation. However, the strength of the counterterm suggested by the GT study is not suitable to simultaneously reproduce the experimental M1 peak energies from Ca-48 to Pb-208. To improve the descriptions of M1, effects caused by adjusting the strength of pionic counterterm and introducing the density dependence of the pi meson channel are explored. Finally, from the analyses of dominant transition configurations of GT and M1 resonance, we find that the proper spin-orbit splitting is the key to simultaneously reproduce the M1 strength distributions from light to heavy nuclei.