Mode-coupling and the pygmy dipole resonance in a relativistic two-phonon model

A two-phonon version of the relativistic quasiparticle time blocking approximation (RQTBA-2) represents a new class of many-body models for nuclear structure calculations based on the covariant energy density functional. As a fully consistent extension of the relativistic quasiparticle random phase approximation (RQRPA), the two-phonon RQTBA implies a fragmentation of nuclear states over two-quasiparticle and two-phonon configurations. This leads, in particular, to a splitting-out of the lowest 1$^-$ state as a member of the two-phonon $[2^+\otimes3^-]$ quintuplet from the RQRPA pygmy dipole mode, thus establishing a physical mixing between these three modes. The inclusion of the two-phonon configurations in the model space allows to describe the positions and the reduced transition probabilities of the lowest 1$^-$ states in isotopes $^{116,120}$Sn as well as the low-energy fraction of the dipole strength without any adjustment procedures. The model is also applied to the low-lying dipole strength in neutron-rich $^{68,70,72}$Ni isotopes. Recent experimental data for $^{68}$Ni are reproduced fairly well.