  Relativistic nuclear response theory is formulated for the proton-neutron
pairing, or deuteron transfer, channel. The approach is based on the
meson-nucleon Lagrangian of Quantum Hadrodynamics (QHD) and advances the
relativistic field theory to connect consistently the high-energy scale of
heavy mesons, the medium-energy range of the pion and the low-energy domain of
emergent collective vibrations (phonons) in a parameter-free way. Mesons and
phonons build up the in-medium nucleon-nucleon interaction in spin-isospin
transfer channels, in particular, the phonon-exchange part takes care of the
leading-order retardation effects. In this framework, we explore $J^{\pi} =
0^+$ and $J^{\pi} = 1^+$ channels of the nuclear response to the proton-neutron
pair removal and addition in $^{56}$Ni and $^{100}$Sn with a special focus on
the lowest (soft) modes as precursors of deuteron condensate and candidates for
being the mediators of the proton-neutron pairing interaction.
