  A new theoretical approach to spin-isospin excitations in open-shell nuclei
is presented. The developed method is based on the relativistic meson-exchange
nuclear Lagrangian of Quantum Hadrodynamics and extends the response theory for
superfluid nuclear systems beyond relativistic quasiparticle random phase
approximation in the proton-neutron channel (pn-RQRPA). The coupling between
quasiparticle degrees of freedom and collective vibrations (phonons) introduces
a time-dependent effective interaction, in addition to the exchange of pion and
$\rho$-meson taken into account without retardation. The time-dependent
contributions are treated in the resonant time-blocking approximation, in
analogy to previously developed relativistic quasiparticle time blocking
approximation (RQTBA) in the neutral (non-isospin-flip) channel. The new method
is called proton-neutron RQTBA (pn-RQTBA) and applied to Gamow-Teller resonance
in a chain of neutron-rich Nickel isotopes $^{68-78}$Ni. A strong fragmentation
of the resonance along with quenching of the strength, as compared to pn-RQRPA,
is obtained. Based on the calculated strength distribution, beta-decay
half-lives of the considered isotopes are computed and compared to pn-RQRPA
half-lives and to experimental data. It is shown that a considerable
improvement of the half-life description is obtained in pn-RQTBA because of the
spreading effects, which bring the lifetimes to a very good quantitative
agreement with data.
