Charge-exchange dipole excitations in neutron-rich nuclei: $-1 \hbar \omega_0$, anti-analog pygmy, and anti-analog giant resonances

Abstract

The occurrence of the low-lying charge-exchange non spin-flip dipole modes below the giant resonance in neutron-rich nuclei is predicted on the basis of nuclear density functional theory. The ground and excited states are described in the framework of the self-consistent Hartree-Fock-Bogoliubov and the proton-neutron quasiparticle-random-phase approximation employing a Skyrme-type energy density functional. The model calculations are performed for the spherical neutron-rich Ca, Ni, and Sn isotopes. It is found that the low-lying states appear sensitively to the shell structure associated with the $-1 \hbar \omega_0$ excitation below the Gamow-Teller states. Furthermore, the pygmy resonance emerges below the giant resonance when the neutrons occupy the low-$\ell (\ell \leq 2 -3)$ orbitals analogous to the pygmy resonance seen in the electric dipole response.