  The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn
with the (a,a'g) coincidence method at E=136 MeV. The comparison with results
of photon-scattering experiments reveals a splitting into two components with
different structure: one group of states which is excited in (a,a'g) as well as
in (g,g') reactions and a group of states at higher energies which is only
excited in (g,g') reactions. Calculations with the self-consistent relativistic
quasiparticle time-blocking approximation and the quasiparticle phonon model
are in qualitative agreement with the experimental results and predict a
low-lying isoscalar component dominated by neutron-skin oscillations and a
higher-lying more isovector component on the tail of the giant dipole
resonance.
