Skyrme Random-Phase-Approximation description of lowest $K^π=2^+_γ$ states in axially deformed nuclei
The lowest quadrupole $γ$-vibrational $K^π=2^+$ states in axially deformed rare-earth (Nd, Sm, Gd, Dy, Er, Yb, Hf, W) and actinide (U) nuclei are systematically investigated within the separable random-phase-approximation (SRPA) based on the Skyrme functional. The energies $E_γ$ and reduced transition probabilities $B(E2)$ of $2^+_γ$-states are calculated with the Skyrme forces SV-bas and SkM$^*$. The energies of two-quasiparticle configurations forming the SRPA basis are corrected by using the pairing blocking effect. This results in a systematic downshift of $E_γ$ by 0.3-0.5 MeV and thus in a better agreement with the experiment, especially in Sm, Gd, Dy, Hf, and W regions. For other isotopic chains, a noticeable overestimation of $E_γ$ and too weak collectivity of $2^+_γ$-states still persist. It is shown that domains of nuclei with a low and high $2^+_γ$ -collectivity are related with the structure of the lowest 2-quasiparticle states and conservation of the Nilsson selection rules. The description of $2^+_γ$ states with SV-bas and SkM$^*$ is similar in light rare-earth nuclei but deviates in heavier nuclei. However SV-bas much better reproduces the quadrupole deformation and energy of the isoscalar giant quadrupole resonance. The accuracy of SRPA is justified by comparison with exact RPA. The calculations suggest that a further development of the self-consistent calculation schemes is needed for a systematic satisfactory description of the $2^+_γ$ states.