Large-scale shell-model calculation of unnatural parity high-spin states in neutron-rich Cr and Fe isotopes*

We have investigated unnatural-parity high-spin states in neutron-rich Cr and Fe isotopes with large-scale shell-model calculation. This shell-model calculation has been carried out within the model space of the $fp$-shell + $0g_{9/2}$ +$1d_{5/2}$ orbits with the truncation of $1\hbar\omega$ excitation of a neutron. The effective Hamiltonian consists of GXPF1Br for the $fp$-shell orbits and $V_{\mbox{\scriptsize MU}}$ with the modification for the other parts. This shell-model calculation has described and predicted the energy levels of both natural and unnatural parity states up to the high spin in Cr and Fe isotopes with $N\le35$. The total energy surfaces have presented the overall prolate deformation and indicated that the excitation into a $\nu0g_{9/2}$ orbit plays the roles of inducing the prolate deformation for the unnatural parity states in these nuclei. It has been found that the excitation energy of $9/2_{1}^{+}$ dropping down with the increase of neutrons in neutron-rich odd-mass Cr and Fe isotopes is linked to the Fermi surface approaching the neutron shell orbits.