  We study theoretically the electric dipole transitions of even Ni isotopes at
low energies, using the self-consistent quasi-particle random-phase
approximation (RPA) with the D1S Gogny interaction and a continuum-RPA model
with the SLy4 Skyrme force. We analyze isoscalar states, isovector states, and
the dipole polarizability. We define a reference value for the polarizability,
to remove a trivial dependence on the mass number. We compare our results with
data and other calculations, with a focus on collective states, shell effects,
and threshold transitions.
  Our results support the presence of a strong isoscalar transition, with
little or moderate E1 strength, as a universal feature of ordinary nuclei. In
moderately neutron-rich Ni isotopes, namely 68Ni and neighboring isotopes, this
transition is found bimodal due to couplings with surface neutrons. An adequate
treatment of the continuum states appears essential for describing
suprathreshold E1 strength, especially beyond 68Ni. Very exotic isotopes (N>50)
are found highly polarizable, with practically all their E1 strength in the
continuum. The dipole polarizability and the neutron skin thickness are
influenced by shell structure in different ways, so they can appear
anticorrelated. A comparison with existing results for lighter (Ca) and heavier
(Sn) nuclei suggests that the so-called pygmy dipole strength is influenced
strongly by shell effects and that, partly for that reason, its isospin
structure depends on the mass region.
