  Systematic investigations of the electric dipole (E1) modes of excitation are
performed using the canonical-basis time-dependent Hartree-Fock-Bogoliubov
(Cb-TDHFB) theory. The Cb-TDHFB is able to describe dynamical pairing
correlations in excited states of nuclear systems. We apply the method to the
real-time calculation of linear response in even-even nuclei with Skyrme
functionals. Effects of shell structure, neutron skin, deformation, and neutron
chemical potential (separation energy) are studied in a systematic way. This
reveals a number of characteristic features of the low-energy E1 modes. We also
find a universal behavior in the low-energy E1 modes for heavy neutron-rich
isotopes, which suggests the emergence of decoupled E1 peaks beyond N = 82.
