The nature of $E1$ low-energy strength (LES), often denoted as a “pygmy dipole resonance”, is analyzed within the random-phase approximation (RPA) in ${}^{208}$Pb using Skyrme forces in a fully self-consistent manner. A first overview is given by the strength functions for the dipole, compressional, and toroidal operators. More detailed insight is gained by averaged transition densities and currents where the latter provide a very illustrative flow pattern. The analysis reveals clear isoscalar toroidal flow in the low-energy bin 6.0–8.8 MeV of the LES and a mixed isoscalar/isovector toroidal/compression flow in the higher bin 8.8–10.5 MeV. Thus the modes covered by LES embrace both vortical and irrotational motion. The simple collective picture of the LES as a “pygmy” mode (oscillations of the neutron excess against the nuclear core) is not confirmed.

• Received 10 December 2012

DOI:https://doi.org/10.1103/PhysRevC.87.024305