Strong energy–momentum dispersion of phonon-dressed carriers in the lightly doped band insulator SrTiO₃

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 λ ' ∼0.3 and an overall bandwidth renormalization suggesting an overall λ ' ∼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.