Microscopic structure of the low-lying negative-parity states in the proton-neutron symplectic model

The proton-neutron symplectic shell-model approach is applied to the description of the microscopic structure of the low-lying negative-parity states of the ${K}^{\pi }={0}_{1}^{-}$ and ${K}^{\pi }={1}_{1}^{-}$ bands in ¹⁵⁴Sm and ²³⁸U without the introduction of additional degrees of freedom that are inherent to other approaches to odd-parity nuclear states. A good description of the energy levels of the two bands under consideration, as well as the reproduction of some energy splitting quantities which are usually introduced in the literature as a measure of the octupole correlations, is obtained for these two nuclei. Additionally, the low-energy B(E1) transition strengths between the states of the ground band and ${K}^{\pi }={0}_{1}^{-}$ band for the two nuclei are calculated in the extended proton-neutron symplectic model and compared with experiment. The results obtained reproduce well the experimental data for the two nuclei under consideration without the use of an effective charge, which could be considered as a significant achievement of the present approach.