Proton-neutron pair correlations in neutron-rich nuclei

TL;DR

This paper investigates the relative strength of spin-triplet versus spin-singlet proton-neutron pairing in neutron-rich nuclei using nuclear energy-density functional methods, revealing shell effects as key indicators.

Contribution

It introduces a novel analysis of pn pair transfer responses to compare spin-triplet and spin-singlet correlations in nuclei.

Findings

Spin-singlet pn pairing strongest at N=Z

Spin-triplet pn pairing varies non-monotonically with neutron number

Shell effects are key indicators of pn-pair correlation strength

Abstract

[Background] Nuclear pairing is a well-established many-body correlation, particularly among like particles in a spin-singlet state. However, the strength of spin-triplet proton-neutron (pn) pairing in nuclei has remained a long-standing and unresolved issue. [Purpose] The relative strength of spin-triplet pn pairing compared to spin-singlet one is investigated by introducing and analyzing the polarizability of the response to pn pair transfers. [Method] The nuclear energy-density functional method is employed. The ground state of the target nucleus is described using the Hartree-Fock-Bogoliubov approximation, which accounts for the conventional superfluidity of like-particle pairs. The response to pn pair transfers is then analyzed using the pn quasiparticle random-phase approximation. [Results] The spin-singlet pn-pair correlation is strongest at $N=Z$ and decreases monotonically with the increasing number of excess neutrons, whereas the spin-triplet pn-pair correlation is shown to depend non-monotonically on the neutron number and can be enhanced in cases where the pn-pair transfers involving the $\pi j_> \otimes \nu j_<$ configuration occur at low energy. [Conclusions] The shell effect, which uniquely appears in spin-triplet pn-pair correlation, serves as a key indicator of the strength of pn-pair correlations.