Isomers and oblate rotation in Pt isotopes: Delineating the limit for collectivity at high spins

Rotation-aligned isomeric states and associated oblate collective sequences are established in even Pt isotopes. Reduced E2 transition probabilities for the deexcitation of the 12+ isomers indicate an abrupt and unexpected quenching of oblate collectivity around neutron number N=120N=120. Structure and shape evolution at high spin in the heaviest stable isotopes is found to be markedly different from observations in the lighter ones.