Large-scale shell-model calculations for unnatural-parity high-spin states in neutron-rich Cr and Fe isotopes

We investigate unnatural-parity high-spin states in neutron-rich Cr and Fe isotopes using large-scale shell-model calculations. These shell-model calculations are carried out within the model space of $fp$-shell + $0g_{9\mathrm{/}2}$

• $1d_{5\mathrm{/}2}$ orbits with the truncation allowing $1\mathrm{\hslash }\omega$ excitation of a neutron. The effective Hamiltonian consists of GXPF1Br for $fp$-shell orbits and $V_{\mathrm{M}\mathrm{U}}$ with a modification for the other parts. The present shell-model calculations can describe and predict the energy levels of both natural- and unnatural-parity states up to the high-spin states in Cr and Fe isotopes with $N\le 35$. The total energy surfaces present the prolate deformations on the whole and indicate that the excitation of one neutron into the $0g_{9\mathrm{/}2}$ orbit plays the role of enhancing the prolate deformation. For the positive(unnatural)-parity states in odd-mass Cr and Fe isotopes, their energy levels and prolate deformations indicate the decoupling limit of the particle-plus-rotor model. The sharp drop of the $9\mathrm{/}{2}_1^{+}$ levels in going from $N=29$ to $N=35$ in odd-mass Cr and Fe isotopes is explained by the Fermi surface approaching the $\nu 0g_{9\mathrm{/}2}$ orbit.