Toroidal, compression, and vortical dipole strengths in $^{144-154}$Sm: Skyrme-RPA exploration of deformation effect

TL;DR

This study uses Skyrme-RPA calculations to analyze how nuclear deformation affects toroidal, compressional, and vortical dipole modes in samarium isotopes, revealing significant strength redistribution and mode splitting.

Contribution

It provides a comparative analysis of dipole strengths in spherical and deformed samarium isotopes, highlighting the impact of deformation and effective mass on excitation modes.

Findings

Deformation causes significant strength redistribution and mode splitting.

Low-energy strength increases with neutron number, overlapping multiple modes.

Skyrme forces with larger effective mass better match experimental data.

Abstract

A comparative analysis of toroidal, compressional and vortical dipole strengths in the spherical $^{144}$Sm and the deformed $^{154}$Sm is performed within the random-phase-approximation using a set of different Skyrme forces. Isoscalar (T=0), isovector (T=1), and electromagnetic excitation channels are considered. The role of the nuclear convection $j_{\text{con}}$ and magnetization $j_{\text{mag}}$ currents is inspected. It is shown that the deformation leads to an appreciable redistribution of the strengths and causes a spectacular deformation splitting (exceeding 5 MeV) of the isoscalar compressional mode. In $^{154}$Sm, the $\mu$=0 and $\mu$=1 branches of the mode form well separated resonances. When stepping from $^{144}$Sm to $^{154}$Sm, we observe an increase of the toroidal, compression and vortical contributions in the low-energy region (often called pygmy resonance). The strength in this region seems to be an overlap of various excitation modes. The energy centroids of the strengths depend significantly on the isoscalar effective mass $m_0$. Skyrme forces with a large $m_0$ (typically $m_0/m \approx 0.8 - 1$) seem to be more suitable for description of experimental data for the isoscalar giant dipole resonance.