  We have analyzed two mechanisms proposed recently for superconductivity in
doped SrTiO3 - the plasmon-mediated pairing and the potential of an
instantaneous attraction between two electrons that owes its origin to the
exchange by high-frequency optical phonons. The first approach seems to be
self-consistent, but in a limited range of the dielectric constant. As to the
direct instantaneous interaction between two electrons,it was hypothesized as
due to the exchange by the band LO phonons. The latter supposition was
incorrect. In the current paper, it shows that the contributions into the
pairing matrix element possessing the structure of a net attraction come about
only as the result of disorder. Indeed, the experiment revealed the mobility
edge in doped SrTiO3. Doped electrons occupying states with the energy below
the mobility edge are localized on dopants thereby become strongly coupled with
the lattice vibrations. The emergence of the localized phonon modes attached to
defect is one of the results of such electron-lattice coupling. For two
electrons in the conduction band the virtual exchange by the quantum of such
local mode manifests itself as the effective attraction. The total contribution
into the pairing matrix element from these processes, increasing with
concentration can overwhelm the Coulomb repulsion screened by the band LO
phonons. That is, the role previously ascribed to the band LO phonons actually
belongs to the localized phonons.
