Proton–neutron pair correlations in neutron-rich nuclei: Analysis of the static polarizability for the response of proton–neutron pair transfers

Abstract

Nuclear pairing is a well-established many-body correlation, particularly among like particles in the spin-singlet channel. However, the strength of spin-triplet proton–neutron (pn) pairing in nuclei remains a long-standing, unresolved issue. In this work, we investigate the relative strength of spin-triplet pn pairing compared to its spin-singlet one by introducing and analyzing the polarizability of the response to pn pair transfers. To this end, we employ the nuclear energy-density functional method, where the ground state of the target nucleus is described within the Hartree–Fock–Bogoliubov approximation to account for the conventional superfluidity of like-particle pairs. The response to pn pair transfers is subsequently analyzed using the pn quasiparticle random-phase approximation. We find that the spin-singlet pn-pair correlation peaks at N = Z and decreases monotonically with increasing neutron excess. In contrast, the spin-triplet pn-pair correlation exhibits a non-monotonic dependence on the neutron number; it is enhanced when pn-pair transfers involving the πj>⊗νj< configuration occur at low energies. These results demonstrate that the shell effect, which uniquely manifests in the spin-triplet channel, serves as a key indicator of the overall pn-pair correlation strength.