Strong energy-momentum dispersion of phonon-dressed carriers in the lightly doped band insulator SrTiO$_3$

TL;DR

This study uses ARPES to analyze electron-phonon interactions in lightly doped SrTiO$_3$, revealing moderate coupling strength and contrasting with the stronger effects observed in doped Mott insulators, challenging assumptions about dielectric constant implications.

Contribution

First ARPES investigation of lightly doped SrTiO$_3$ showing moderate electron-phonon coupling, contrasting with doped Mott insulators and questioning the role of dielectric constant.

Findings

Moderate electron-phonon coupling with λ'~0.3-0.7.

No evidence of large pseudogap or small polaron phenomena.

Large dielectric constant does not necessarily imply strong el-ph coupling.

Abstract

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 how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is the perovskite SrTiO$_3$ (STO), well known for its giant dielectric constant of 10000 at low temperature, exceeding that of La$_2$CuO$_4$ 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 SrTiO$_3$. Comparing to lightly doped Mott insulators, we find the signatures of only moderate electron-phonon coupling: a dispersion anomaly associated with the low frequency optical phonon with a $\lambda'\sim0.3$ and an overall bandwidth renormalization suggesting an overall $\lambda'\sim0.7$ coming from the higher frequency phonons. Further, 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.