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

Abstract

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 $fp\text{-shell}+0{g}_{9/2}+1{d}_{5/2}$ orbits with the truncation allowing $1\ensuremath{\hbar}\ensuremath{\omega}$ excitation of a neutron. The effective Hamiltonian consists of GXPF1Br for $fp$-shell orbits and ${V}_{\mathrm{MU}}$ 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 $N\ensuremath{\le}35$. The total energy surfaces present prolate deformations on the whole and indicate that the excitation of one neutron into the $0{g}_{9/2}$ orbit plays the role of enhancing prolate deformation. For 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 $9/{2}_{1}^{+}$ levels in going from $N=29$ to $N=35$ in odd-mass Cr, Fe, and Ni isotopes is explained by the Fermi surface approaching the $\ensuremath{\nu}0{g}_{9/2}$ orbit.