Alternating-parity doublets of even-even Ba isotopes

In this paper we study the structure of even-even Ba isotopes with neutron number $N=84--90$ within the framework of nucleon-pair approximation of the shell model. We use a phenomenological Hamiltonian, which includes single-particle energies, monopole, quadrupole, and octupole pairing energies, as well as quadrupole-quadrupole and octupole-octupole interactions. Our calculated energy levels and $B(E2)$ values are in good agreement with experimental data. The alternating parity doublets are well reproduced by considering F^- nucleon pairs with spin-three and negative parity, which demonstrates the importance of F^- pair in octupole deformation. The octupole-octupole interaction is found to favor stronger $B(E3)$ transitions and suppress $B(E2)$ transitions in the low-lying states of nuclei in these even-even Ba isotopes.