Low-energy M1 states in deformed nuclei: spin-scissors or spin-flip?

TL;DR

This study uses self-consistent QRPA calculations with Skyrme forces to analyze low-energy M1 states in deformed and spherical nuclei, questioning the existence of the spin-scissors resonance and exploring spin-flip excitations.

Contribution

It provides a detailed microscopic analysis of low-energy M1 excitations, challenging the predicted spin-scissors resonance and examining the evolution of spin-flip states with nuclear deformation.

Findings

Fragmentation of orbital M1 strength explains observed states in $^{164}$Dy.

Results do not support the existence of the spin-scissors resonance.

Tensor forces have a minor effect on the low-energy M1 states.

Abstract

The low-energy $M1$ states in deformed $^{164}$Dy and spherical $^{58}$Ni are explored in the framework of fully self-consistent Quasiparticle Random-Phase Approximation (QRPA) with various Skyrme forces. The main attention is paid to orbital and spin $M1$ excitations. The obtained results are compared with the prediction of the low-energy {\it spin-scissors} $M1$ resonance suggested within Wigner Function Moments (WFM) approach. A possible relation of this resonance to low-energy spin-flip excitations is analyzed. In connection with recent WFM studies, we consider evolution of the low-energy spin-flip states in $^{164}$Dy with deformation (from the equilibrium value to the spherical limit). The effect of tensor forces is briefly discussed. It is shown that two groups of $1^+$ states observed at 2.4-4 MeV in $^{164}$Dy are rather explained by fragmentation of the orbital $M1$ strength than by the occurrence of the collective spin-scissors resonance. In general, our calculations do not confirm the existence of this resonance.