Polaron spectral properties in doped ZnO and SrTiO_{3} from first principles

Abstract

We reveal polaron signatures in the spectral function of n-doped SrTiO_{3} and ZnO through first-principles interacting Green's function calculations. In SrTiO_{3} we observe a clear replica band at 94 meV below the conduction band, which shows that the observed replica in recent angle-resolved photoemission spectroscopy experiment is an intrinsic feature from electron-phonon coupling in SrTiO_{3}. In contrast, we observe an elongated tail in the spectral function for ZnO but no well-separated replicas. By increasing the electron doping level, we identify kinks in the spectral function at phonon frequencies and a decreasing intensity of the tail structure. We find that the curvature of the conduction band bottom vanishes due to additional electron-phonon scattering channels enabled by increased occupied states at high-enough doping levels, beyond which the spectral function becomes a stronger quasiparticle one with a single peak structure. We further compare the spectral function computed from the Migdal-Dyson approach and the cumulant method, and show that the cumulant method can correctly reproduce the polaronic features observed in experiments.