Mode Coupling and the Pygmy Dipole Resonance in a Relativistic Two-Phonon Model

A new class of many-body models, based on covariant density functional theory for excited states, is presented. It allows a parameter free description of the fragmentation of nuclear states induced by mode coupling of two-quasiparticle and two-phonon configurations. As compared to earlier methods it provides a consistent and parameter free theory of the fine structure of nuclear resonances. The method is applied very successfully to investigate the newly discovered low-lying dipole excitations in Sn and Ni isotopes with large neutron excess.