High-spin states in Yb156 and structure evolutions at large angular momenta in even-A Yb isotopes

High-spin states of Yb156 have been studied via the Sm144(O16,4n)Yb156 fusion-evaporation reaction at beam energy 102 MeV. The positive-parity yrast band and negative-parity cascade have been extended up to higher-spin states, respectively. In the present work, the negative-parity sequence above the 25- state was found to be irregular and fragment into many parallel branches. This pattern may related to the excitation from the nucleon in the Z=64,N=82 core. The characteristics of alignment plot and E-GOS curve for the positive-parity yrast sequence in Yb156 indicate that this nucleus may undergo an evolution from quasivibrational to quasirotational structure with increasing angular momentum. Based on a systematic summary of the available experimental alignments for the even-A Yb156,158,160,162,164 isotopes, the structural evolutions induced by the increase in angular momentum, as well as by the change in neutron numbers, in these even-A Yb isotopes have been discussed in comparison with the cranked Woods-Saxon-Strutinsky calculations by means of total-Routhian-surface (TRS) methods.