Proton-neutron symplectic model description of $^{106}$Cd

Microscopic shell-model description of the low-lying collective states in the weakly deformed nucleus $^{106}$Cd within the recently proposed microscopic version of the Bohr-Mottelson model is given. A good description of the excitation energies of the lowest ground, $\gamma$, and $\beta$ quasibands is obtained without the involvement of adjustable kinetic energy term. The $\gamma$ degrees of freedom are shown to play a crucial role in the description of spectroscopy of this nucleus. A modified $SU(3)$ preserving high-order interaction is used to produce a $\gamma$-unstable type of the odd-even staggering, observed experimentally between the states of the quasi-$\gamma$ band. The present approach allows to describe the observed intraband and interband quadrupole collectivity. The results obtained in present work suggest a different interpretation of the fundamental question concerning the nature of low-energy vibrations and the emergence of deformation and collectivity in the weakly deformed atomic nuclei.