Atomic nuclei are complex systems which can exhibit a large variety of phenomena where single-nucleon
and emergent collective degrees of freedom are deeply intertwined. The nuclear structure method
which I will present is based on the relativistic meson-nucleon Lagrangian of quantum hadrodynamics and
nuclear field theory. This approach naturally connects the intermediate-energy scales of mesons to the low-energy
domain of nucleons and their collective motion, and provides a consistent framework for the
description of nuclear excitations.
Recently, we have extended this method to the description of isospin-transfer modes in open-shell nuclei,
which have various applications in nuclear and particle physics as well as astrophysics. In the charge-exchange
channel, the coupling between single-nucleon degrees of freedom and collective vibrations
generates a time-dependent proton-neutron effective interaction, in addition to the static pion and rho-meson
exchange, and introduces complex configurations of nucleons which induce fragmentation and spreading of
the resonances. Such effects are important to describe the observed quenching of the transition strength and
have a great impact on the computing of beta-decay rates that govern the r-process nucleosynthesis. I will
show some recent results of calculations for spin-isospin excitations and weak decays in medium-mass
nuclei, and will address some new developments.