On the isoscalar-isovector splitting of pygmy dipole structures

The electric dipole response of $^{140}$Ce is investigated using the fully consistent relativistic quasiparticle random phase approximation. By analyzing the isospin structure of the E1 response, it is shown that the low-energy (pygmy) strength separates into two segments with different isospin character. The more pronounced pygmy structure at lower energy is composed of predominantly isoscalar states with surface-peaked transition densities. At somewhat higher energy the calculated E1 strength is primarily of isovector character, as expected for the low-energy tail of the giant dipole resonance. The results are in qualitative agreement with those obtained in recent $(\gamma,\gamma')$ and $(\alpha,\alpha'\gamma)$ experiments, and provide a simple explanation for the splitting of low-energy E1 strength into two groups of states with different isospin structure and radial dependence of the corresponding transition densities.