Anomalous rotational alignment in NÄZ nuclei and residual neutron-proton interaction

Recent experiments have demonstrated that the rotational alignment for the N5Z nuclei in the mass-80 region is considerably delayed as compared to the neighboring NÞZ nuclei. We investigate whether this observation can be understood by a known component of nuclear residual interactions. It is shown that the quadrupole-pairing interaction, which explains many of the delays known in rare-earth nuclei, does not produce the substantial delay observed for these N5Z nuclei. However, the residual neutron-proton interaction, which is conjectured to be relevant for N5Z nuclei, is shown to be quite important in explaining the new experimental data.