Evolution of low-lying M1 modes in germanium isotopes

TL;DR

This study investigates how low-lying magnetic dipole (M1) modes evolve in germanium isotopes, revealing a bimodal structure with a low-energy enhancement near closed shells and a scissors mode resonance in the middle of the shell.

Contribution

It provides a comprehensive shell model analysis of M1 strength evolution across germanium isotopes, highlighting the correlation between low-energy enhancement and scissors mode resonance.

Findings

Low-energy M1 enhancement is strongest near closed shells.

Scissors mode resonance peaks in the middle of the open shell.

Results align with previous iron isotope studies.

Abstract

Magnetic dipole strength functions are determined for the series of germanium isotopes from $N = Z = 32$ to $N$ = 48 on the basis of a large number of transition strengths calculated within the shell model. The evolution of the strength with increasing neutron number in the $1g_{9/2}$ orbital is analyzed. A bimodal structure comprising an enhancement toward low transition enery and a resonance in the region of the scissors mode is identified. The low-energy enhancement is strongest near closed shells, in particular at the almost completely filled $1g_{9/2}$ orbital, while the scissorslike resonance is most pronounced in the middle of the open shell, which correlates with the magnitude of the also deduced electric quadrupole transition strengths. The results are consistent with previous findings for the shorter series of iron isotopes and proves the occurrence and correlation of the two low-lying magnetic dipole modes as a global structural feature.