Islands of shape coexistence from single-particle spectra in covariant density functional theory

TL;DR

This study uses covariant density functional theory to identify regions in the nuclear chart where shape coexistence occurs, revealing islands near magic numbers and explaining them through particle-hole excitations involving protons and neutrons.

Contribution

It provides a microscopic interpretation of shape coexistence in nuclei based on single-particle spectra and particle-hole excitations, highlighting regions of proton- and neutron-induced shape coexistence.

Findings

Islands of shape coexistence near Z=82 and Z=50 consistent with p-h interpretation.

Smaller islands near N=90 and N=60 related to excitations across 3D-HO magic numbers.

Identification of a region with both neutron and proton p-h excitations at N=40, Z=40.

Abstract

Using covariant density functional theory with the DDME2 functional and labeling single-particle energy orbitals by Nilsson quantum numbers, a search for particle-hole (p-h) excitations connected to the appearance of shape coexistence is performed for Z=38 to 84. Islands of shape coexistence are found near the magic numbers Z=82 and Z=50, restricted in regions around the relevant neutron midshells N=104 and N=66 respectively, in accordance to the well accepted p-h interpretation of shape coexistence in these regions, which we call neutron-induced shape coexistence, since the neutrons act as elevators creating holes in the proton orbitals. Similar but smaller islands of shape coexistence are found near N=90 and N=60, restricted in regions around the relevant proton midshells Z=66 and Z=39 respectively, related to p-h excitations across the 3-dimensional isotropic harmonic oscillator (3D-HO) magic numbers N=112 and N=70, which correspond to the beginning of the participation of the opposite parity orbitals 1i13/2 and 1h11/2 respectively to the onset of deformation. We call this case proton-induced shape coexistence, since the protons act as elevators creating holes in the neutron orbitals, thus offering a possible microscopic mechanism for the appearance of shape coexistence in these regions. In the region around N=40, Z=40, an island is located on which both neutron p-h excitations and proton p-h excitations are present.