Relativistic QRPA description of low-lying dipole strength in neutron-rich nuclei

The relativistic quasi-particle random phase approximation (RQRPA) is applied in the analysis of the evolution of isovector dipole response in nuclei with large neutron excess. In neutron-rich nuclei the dipole response is characterized by the fragmentation of the strength distribution and its spreading into the low-energy region. In light nuclei the onset of dipole strength in the low-energy region is due to non-resonant single-particle excitations of the loosely bound neutrons. In heavier neutron-rich nuclei low-lying dipole states appear which display a more distributed structure of the RQRPA amplitudes. For these nuclei a single collective dipole state is identified in the low-energy region, and an analysis of the corresponding transition densities and velocity distributions reveals the dynamics of the pygmy resonance