The potential energy surfaces of even-even $^{ 146-156}$Sm are investigated in the constrained reflection-asymmetric relativistic mean-field approach with parameter set PK1. It is shown that the critical-point candidate nucleus $^{ 152}$Sm marks the shape/phase transition not only from U(5) to SU(3) symmetry, but also from the octupole-deformed ground state in $^{ 150}$Sm to the quadrupole-deformed ground state in $^{ 154}$Sm. By including the octupole degree of freedom, an energy gap near the Fermi surface for single-particle levels in $^{ 152}$Sm with $\beta_2 = 0.14 \sim 0.26$ is found, and the important role of the octupole deformation driving pair $\nu 2f_{ 7/2}$ and $\nu 1i_{ 13/2}$ is demonstrated.
Categories: nucl-th