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

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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 + 0g₉/₂ + 1d₅/₂ orbits with the truncation allowing 1ℏω excitation of a neutron. The effective Hamiltonian consists of GXPF1Br for fp-shell orbits and V 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≤35. The total energy surfaces present the prolate deformations on the whole and indicate that the excitation of one neutron into the 0g₉/₂ 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 9/2₁⁺ levels in going from N=29 to N=35 in odd-mass Cr and Fe isotopes is explained by the Fermi surface approaching the ν0g₉/₂ orbit.