Nature of Low-Energy Dipole Strength in Nuclei: The Case of a Resonance at Particle Threshold inP208b

A high-resolution $(\gamma ,{\gamma }^{\prime })$ study of the electric dipole response in ${}^{\mathrm{208}}\mathrm{P}\mathrm{b}$ at the S-DALINAC reveals a resonance structure centered around the neutron emission threshold. Microscopic quasiparticle phonon model calculations in realistic model spaces including the coupling to complex configurations are able to describe the data in great detail. The resonance is shown to result from surface density oscillations of the neutron skin relative to an approximately isospin-saturated core. It also forms an integral part of a toroidal $E1$ mode representing an example of vortex collective motion in nuclei.