Much progress has been made recently in the study of the effects
of electron–phonon (el–ph) coupling in doped insulators using angle-resolved
photoemission (ARPES), yielding evidence for the dominant role of el–ph
interactions in underdoped cuprates. As these studies have been limited to doped
Mott insulators, the important question arises as to how this compares with
doped band insulators where similar el–ph couplings should be at work. The
archetypical case is that of perovskite SrTiO3 (STO), well known for its giant
dielectric constant of 10 000 at low temperatures, exceeding that of La2CuO4 by a
factor of 500. Based on this fact, it has been suggested that doped STO should be
the archetypical bipolaron superconductor. Here we report an ARPES study from
high-quality surfaces of lightly doped STO. In comparison to lightly doped Mott
insulators, we find the signatures of only moderate el–ph coupling; a dispersion
anomaly associated with the low-frequency optical phonon with a 0 0.3 and
an overall bandwidth renormalization suggesting an overall 0 0.7 coming
from the higher frequency phonons. Furthermore, we find no clear signatures of
the large pseudogap or small-polaron phenomena. These findings demonstrate
that a large dielectric constant itself is not a good indicator of el–ph coupling
and highlight the unusually strong effects of the el–ph coupling in doped
Mott insulators.
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