Microscopic origin of reflection-asymmetric nuclear shapes

The existence of nuclei with stable reflection-asymmetric ground-state (g.s.) shapes has been supported by rich experimental evidence. Theoretically, a recent survey systematically predicts the regions of pear-liked shapes in the regions where the doublets of opposite parity shells with $\Delta\ell=\Delta j=3$ can be found. In our work, we investigate even-even Ba, Ra, U and Yb isotopes in the framework of the Skyrme-Hartree-Fock-Bogoliubov theory. We study neutron-proton, neutron-neutron, and proton-proton multipole interaction energies and analyze their role in the onset of reflection-asymmetric deformations. We demonstrate that reflection-asymmetric deformations are driven by the neutron-proton part of the nuclear interaction energy of odd multipolarity. We also show that the small reflection-asymmetric deformation energies result from strong cancellations between even- and odd- multipolarity components of the nuclear binding energy. Therefore, high-multipolarity components, especially $\lambda = 5$, are crucial for the appearance of stable reflection-asymmetric deformations.