  Spin-isospin response in exotic nuclear systems is investigated. It is found
that in some nuclei excitations with pionic quantum numbers (0-, 1+, 2-, ...)
appear at very low energies with large transition probabilities, which is an
indication of the vicinity of the onset of pion condensation. As an example, 2-
components of the spin-dipole resonance in 78-Ni and 132-Sn are considered. The
existence of such modes points out to the necessity of taking into account
their coupling to other elementary modes of excitation, e.g.
single-quasiparticle ones. This coupling is introduced in the theory for the
first time. Thereby, pion-exchange contribution to the nucleon-nucleon
interaction is included in the relativistic framework beyond the Hartree-Fock
approximation. Namely, classes of Feynman diagrams are selected according to
their significance for nuclear spectroscopic characteristics, such as
single-particle energies and strength functions, and included into the
nucleonic self-energy in all orders of meson-exchange. As an illustration, the
impact of these new contributions on the single-particle energies of 100-Sn is
discussed.
