Triaxial rigidity of $^{166}$Er and its Bohr-model realization

TL;DR

This paper investigates the triaxial shapes of low-lying rotational bands in $^{166}$Er using the Bohr Hamiltonian and many-fermion models, revealing a rigid triaxial structure contrary to traditional views.

Contribution

It introduces a novel interpretation of the triaxial nature of $^{166}$Er bands, supported by both collective and shell-model calculations showing rigidity and implications of softness.

Findings

Good description of excitation energies and E2 transitions by models

Evidence of rigid triaxiality in the nuclear shape

Discussion of double $b3$-phonon excitation

Abstract

The triaxial nature of low-lying rotational bands of $^{166}$Er is presented from the viewpoint of the Bohr Hamiltonian and from that of many-fermion calculations by the Monte Carlo shell model and the constrained Hartree-Fock method with projections. A recently proposed novel picture of those bands suggests definite triaxial shapes of those bands, in contrast to the traditional view with the prolate ground-state band and the $\gamma$-vibrational excited band. Excitation level energies and E2 transitions can be described well by the Bohr Hamiltonian and by the many-fermion approaches, where rather rigid triaxiality plays vital roles, although certain fluctuations occur in shell-model wave functions. Based on the potential energy surfaces with the projections, we show how the triaxial rigidity appears and what the softness of the triaxiality implies. The excitation to the so-called double $\gamma$-phonon state is discussed briefly.