Return of Collective Rotation in {sup 157}Er and {sup 158}Er at Ultrahigh Spin

A new frontier of discrete-line {gamma}-ray spectroscopy at ultrahigh spin has been opened in the rare-earth nuclei {sup 157,158}Er. Four rotational structures, displaying high moments of inertia, have been identified, which extend up to spin {approx}65({Dirac_h}/2{pi}) and bypass the band-terminating states in these nuclei which occur at {approx}45({Dirac_h}/2{pi}). Cranked Nilsson-Strutinsky calculations suggest that these structures arise from well-deformed triaxial configurations that lie in a valley of favored shell energy which also includes the triaxial strongly deformed bands in {sup 161-167}Lu.