Nuclear quantum shape-phase transitions in odd-mass systems

TL;DR

This paper investigates nuclear shape-phase transitions in odd-mass Eu isotopes, revealing enhanced first-order quantum phase transition signatures due to unpaired proton shape polarization effects.

Contribution

It introduces a microscopic approach using a core-quasiparticle Hamiltonian to analyze shape-phase transitions in odd-mass nuclei, highlighting the role of unpaired protons.

Findings

Accurate reproduction of experimental data for odd-mass Eu isotopes.

More pronounced shape transition discontinuities at neutron number N=90.

Identification of unpaired proton polarization as key to transition enhancement.

Abstract

Microscopic signatures of nuclear ground-state shape phase transitions in odd-mass Eu isotopes are explored starting from excitation spectra and collective wave functions obtained by diagonalization of a core-quasiparticle coupling Hamiltonian based on energy density functionals. As functions of the physical control parameter -- the number of nucleons -- theoretical low-energy spectra, two-neutron separation energies, charge isotope shifts, spectroscopic quadrupole moments, and $E2$ reduced transition matrix elements accurately reproduce available data, and exhibit more pronounced discontinuities at neutron number $N=90$, compared to the adjacent even-even Sm and Gd isotopes. The enhancement of the first-order quantum phase transition in odd-mass systems can be attributed to a shape polarization effect of the unpaired proton which, at the critical neutron number, starts predominantly coupling to Gd core nuclei that are characterized by larger quadrupole deformation and weaker proton pairing correlations compared to the corresponding Sm isotopes.