Small and large polarons in nickelates, manganites, and cuprates

TL;DR

This paper compares optical conductivities of various nickelates, manganites, and cuprates, revealing different polaron behaviors such as localization in some materials and large, more mobile polarons in cuprates at low temperatures.

Contribution

It identifies a novel behavior of polarons in cuprates, showing they are large and become more mobile at low temperatures, contrasting with small polaron localization in other materials.

Findings

Small polarons localize in LSNO and SLMO below transition temperature.

In cuprates, polarons are large and their binding energy decreases at low T.

Large polarons may enable coherent transport in cuprates.

Abstract

By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO), Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. While in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in those cuprates the polarons appear to be large, and at low T their binding energy decreases. This reflects into an increase of the polaron radius, which may trigger coherent transport.