Isovector spin susceptibility: Isotopic evolution of collectivity in spin response

TL;DR

This paper investigates how the collectivity of isovector spin responses evolves with isotopic changes in Ca and Ni nuclei using a nuclear energy-density functional approach, revealing the influence of shell structure and pairing effects.

Contribution

It provides a unified analysis of neutral and charge-exchange spin responses and highlights the role of shell occupation and pairing in the isotopic evolution of collectivity.

Findings

Collective shift in neutral channel linked to neutron occupation in specific orbitals.

Coherent particle-hole excitations form giant resonances in neutron-rich nuclei.

Isotopic evolution of IV spin susceptibility reflects shell structure and pairing effects.

Abstract

Background: Response to spin-dependent operators has been investigated in the magnetic dipole and Gamow-Teller transitions, which provides magnetic properties of a nuclear system. Purpose: I investigate an isotopic dependence of the collectivity generated by the spin-dependent interactions in the Ca and Ni isotopes through the isovector (IV) spin-flip excitations. The responses in the neutral $(t_z)$ and charge-exchange $(t_{\pm})$ channels are considered in a unified way. Method: A nuclear energy-density functional approach is employed for calculating the response functions based on the Skyrme-Kohn-Sham-Bogoliubov method and the quasiparticle-random-phase approximation (QRPA). I adopt the like-particle QRPA and the proton-neutron QRPA for the neutral and charge-exchange channels, respectively. I consider the fluctuation of the proton-neutron pair fields. Results: The collective shift due to RPA correlations for the response in the neutral channel is explained by the occupation probability of neutrons in the $j_>=\ell+1/2$ orbital. Many particle-hole or two-quasiparticle excitations have a coherent contribution to form a giant resonance in neutron-rich nuclei for the charge-exchange channel. The IV spin susceptibility displays the isotopic evolution of the collectivity and the underlying shell structure. Conclusions: A repulsive character of the residual interaction in the spin-isospin channel diminishes the IV spin susceptibility due to the collectivity, while the dynamic ${}^{3}S$ pairing appearing in the charge-exchange channel opposes the reduction.