Collective structures up to spin $\sim$ $65\hbar$ in the $N = 90$ isotones $^{158}$Er and $^{157}$Ho

Stable asymmetric or triaxial shapes are a longstanding prediction of theory [1]. In the present work, a third collective band in $^{158}$Er at spins beyond band termination was found in addition to the two previously reported ones [2]. The measured transition quadrupole moments ($Q_{\rm t}$ $\sim$ 10$-$11eb) have suggested that these bands possess a triaxial strongly deformed shape, based on comparisons with CNS calculations and with TAC calculations using the SHF model [3]. Three new collective bands with similar characters, tentatively assigned to $^{157}$Ho, will also be presented. [1] A. Bohr and B. R. Mottelson, Nuclear Structure, vol. II (1975); [2] E. S. Paul $\it{et~al.}$, Phys. Rev. Lett. 98, 012501 (2007); [3] Y. Shi $\it{et~al.}$, in preparation.