Gyromagnetic ratios and octupole collectivity in the structure of the ^90-96Zr isotopes

Shell model calculations have been performed for low-excitation states in the Zr isotopes between ⁹⁰Zr and ⁹⁶Zr with an emphasis on the g factors and electromagnetic decay rates for the lowest 2⁺ and 3^- states. Overall the 2⁺ states are reasonably well described. In contrast, the 3^- states present a puzzle because the measured g factors imply a single-particle configuration whereas the experimental E3 transition rates imply collective structures that cannot be explained by shell model calculations. A consistent description of the 3 ^- states in ⁹⁰Zr and ⁹⁶Zr is sought in terms of coupling between the single-particle structure and a collective octupole vibration.