Gallagher-Moszkowski splitting in deformed odd-odd nuclei within a microscopic approach

Abstract

Low-lying bandhead states in axially prolate deformed odd-odd nuclei have long been described essentially within the rotor+two-quasiparticle picture. This approach allows one to explain the appearance of so-called Gallagher-Moszkowski doublets of bandheads with $K = Ω_n \pm Ω_p$, sum and difference of neutron and proton angular momentum projections on the symmetry axis. According to an empirical rule stated by Gallagher and Moszkowski the spin-aligned configuration lies lower in energy than the spin-anti-aligned configuration. A recent study by Robledo, Bernard and Bertsch in Phys. Rev. C 89, 021303(R) (2014) within the Gogny energy-density functional with selfconsistent blocking of the unpaired nucleons showed that calculations fail to reproduce this rule in about half of the cases and points to the density-dependent term of the functional as responsible of this failure. In this paper we aim at pushing further this analysis to exhibit the mechanism underlying the energy splitting in a Gallagher-Moszkowski doublet. We work in the framework of the Skyrme energy-density functional approach, including BCS pairing correlations with selfconsistent blocking. We use the SIII parametrization with time-odd terms and seniority pairing matrix elements extending a previous study of K-isomeric states in even-even nuclei [Phys. Rev. C 105, 044329 (2022)]. We find that the energy splitting results from a competition between the spin-spin, density-dependent and current-current terms of the Skyrme energy-density functional. In doublets where the larger K value is lower in energy the Gallagher-Moszkowski rule is always satisfied by the SIII Skyrme energy-density functional. In doublets, on the contrary, where the smaller K value lies lower, the energy splittings are calculated to be rather small and often a disagreement with the Gallagher-Moszkowski rule occurs.