Core breaking and possible magnetic rotation in the semimagic nucleus 90 Zr

The semimagic nucleus 90Zr, with Z = 40 and N = 50, is investigated in terms of large scale shell model calculations. A logical agreement is obtained between the available experimental data and predicted values. The calculated results indicate that the low-lying states are primarily dominated by the proton excitations from the fp orbitals across the Z = 38 or 40 subshell into the high-j \begin{document}$1g_{9/2}$\end{document} orbital. For the higher-spin states of 90Zr, the breaking of the N = 50 core plays a crucial role, and the contribution of different orbitals to each state are discussed in this article. The evolution from neutron core excitations to proton excitations is systematically studied along the neighboring N = 50 isotones. Furthermore, the strong \begin{document}$\Delta I$\end{document} = 1 sequence demonstrates an abrupt backbend attributed to the alignment of the valence nucleons in fp proton orbitals and is proposed to have a \begin{document}$\pi(fp)^{-2}(1g_{9/2})^{2} \otimes $\end{document} \begin{document}$ \nu(1g_{9/2})^{-1}(2d_{5/2}/1g_{7/2})^{1}$\end{document} configuration before the backbend, based on the shell model calculations. The properties of this sequence before the backbend indicate a general agreement with the fingerprints of magnetic rotation; hence, the sequence with the \begin{document}$\pi(fp)^{-2}(1g_{9/2})^{2} \otimes \nu(1g_{9/2})^{-1}(2d_{5/2}/1g_{7/2})^{1}$\end{document} configuration is suggested as a magnetic rotational band arising from shears mechanism.