Dipole response in neutron-rich nuclei with new Skyrme interactions

We investigate the isoscalar and isovector $E1$ response of neutron-rich nuclei, within a semiclassical transport model employing effective interactions for the nuclear mean field. In particular, we adopt the recently introduced SAMi-J Skyrme interactions, whose parameters are specifically tuned to improve the description of spin-isospin properties of nuclei. Our analysis evidences a relevant degree of isoscalar-isovector mixing of the collective excitations developing in neutron-rich systems. Focusing on the low-lying strength emerging in the isovector response, we show that this energy region essentially corresponds to the excitation of isoscalar-like modes, which also contribute to the isovector response owing to their mixed character. Considering effective interactions which mostly differ in the isovector channels, we observe that these mixing effects increase with the slope $L$ of the symmetry energy at saturation density, leading to a larger strength in the low-energy region of the isovector response. This result appears connected to the increase, with $L$, of the neutron-proton asymmetry at the surface of the considered nuclei, i.e., to the neutron skin thickness.

Figure 1

The symmetry energy versus reduced density $\tilde{\rho }=\frac{\rho }{{\rho }_0}$ for the EoS without (a) and with (b) momentum-dependent terms.

Figure 2

The neutron (full lines) and proton (dashed lines) density profiles of ${}^{132}\mathrm{Sn}$ for the SAMi-J27, SAMi-J31, and SAMi-J35 parametrizations.

Figure 3

The dipole oscillations (left panels) and corresponding strength (right panels) for ${}^{132}\mathrm{Sn}$ and the SAMi-J31 interaction. Panels from (a) to (d) represent the results obtained with the initial IS perturbation and panels from (e) to (h) show the results obtained with the initial IV perturbation.

Figure 4

The strength functions versus excitation energy for ${}^{68}\mathrm{Ni}$ with SAMi-J27, SAMi-J31, and SAMi-J35 interactions. Panel (a) is for the initial IS perturbation and panel (b) is for the initial IV perturbation.

Figure 5

Similar to Fig. 4 but for ${}^{132}\mathrm{Sn}$.

Figure 6

Similar to Fig. 4 but for ${}^{208}\mathrm{Pb}$.

Figure 7

Similar to Fig. 5, but for the MI interactions.

Figure 8

The percentage of the EWSR exhausted by the PDR region (see text) for the three mass regions with MI and MD interactions and IV initial perturbation. The solid circles, squares, and triangles refer to the results for MI asysoft, asystiff, and asysuperstiff, interactions, respectively. The open circles, squares, and triangles refer to SAMi-J27, SAMi-J31, and SAMi-J35 and the diamonds refer to the experimental results [13], respectively.

Figure 9

The transition densities versus $r$ in the low-energy excitation region, for the IS initial perturbation, for ${}^{68}\mathrm{Ni}$ with SAMi-J27, SAMi-J31, and SAMi-J35 interactions. Full lines are for protons, dashed lines for neutrons. The energy of the excitation mode considered is indicated in each panel.

Figure 10

Similar to Fig. 9 but for ${}^{132}\mathrm{Sn}$.

Figure 11

Similar to Fig. 9 but for ${}^{208}\mathrm{Pb}$.

Figure 12

Same as Fig. 11 but with a reduced scale on the vertical axis.

Figure 13

The transition densities versus $r$ with different excitation energies with IS initial perturbation for ${}^{132}\mathrm{Sn}$ with SAMi-J31 interaction. Full lines are for protons, dashed lines for neutrons.

Figure 14

Similar to Fig. 13 but with IV initial perturbation.

Figure 15

Transition densities, as obtained considering an initial IS [(a) and (b)] or IV [(c) and (d)] perturbation, as a function of the excitation energy $E$, for ${}^{132}\mathrm{Sn}$ and the SAMi-J31 interaction, at two radial distances: $r=1.75\text{fm}$ and $r=4.75\text{fm}$.