The Essential Interactions in Oxides and Spectral Weight Transfer in Doped Manganites

TL;DR

This paper demonstrates that strong Fröhlich electron-phonon interactions lead to small polaron formation in manganites and other charge-transfer insulators, explaining optical conductivity behavior and challenging recent alternative theories.

Contribution

It provides a quantitative calculation of the Fröhlich interaction strength and supports the polaronic model over magnetic interaction explanations in manganites.

Findings

Fröhlich interaction is much stronger than magnetic interactions in these materials.

Carriers are small (bi)polarons across all temperatures and doping levels.

Optical conductivity data aligns with bipolaron theory.

Abstract

We calculate the value of the Fr\"ohlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction is much stronger than any relevant magnetic interaction. A polaron shift due to the Fr\"ohlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. An opposite conclusion, recently suggested in the literature, is shown to be incorrect. The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory.