Coexistence of localized and itinerant carriers near Tc in calcium-doped manganites

TL;DR

This paper investigates the coexistence of localized polaronic and itinerant band carriers in calcium-doped manganites near the Curie temperature, using an effective medium model to explain electrical resistivity and thermoelectric data.

Contribution

It introduces a mean-field model that accounts for coexisting polaronic and band-electron phases, successfully explaining resistivity and thermoelectric measurements in manganites.

Findings

Polaronic distortions persist in the ferromagnetic phase.

A low percolation threshold explains the resistivity data.

The model reproduces magnetization and susceptibility features.

Abstract

We explore the possibility that polaronic distortions in the paramagnetic phase of La$_{0.67}$Ca$_{0.33}$MnO$_3$ manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting field- and temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the two-component system, and find that a limit with low percolation threshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility.