Quadrupole Moments of Collective Structures up to Spin $sim$ $65hbar$ in $^{157}$Er and $^{158}$Er: A Challenge for Understanding Triaxiality in Nuclei

The transition quadrupole moments, $Q_{rm t}$, of four weakly populated collective bands up to spin $sim$ $65hbar$ in $^{157,158}$Er have been measured to be ${sim}11 {rm eb}$ demonstrating that these sequences are associated with large deformations. However, the data are inconsistent with calculated values from cranked Nilsson-Strutinsky calculations that predict the lowest energy triaxial shape to be associated with rotation about the short principal axis. The data appear to favor either a stable triaxial shape rotating about the intermediate axis or, alternatively, a triaxial shape with larger deformation rotating about the short axis. These new results challenge the present understanding of triaxiality in nuclei.