Breakdown of the lattice polaron picture in La0.7Ca0.3MnO3 single crystals

TL;DR

This paper investigates the breakdown of the polaronic model in La0.7Ca0.3MnO3 single crystals around the magnetic transition temperature, revealing deviations from polaronic behavior linked to metallic fluctuations.

Contribution

It demonstrates the limits of the polaron picture in describing transport properties near the magnetic transition in La0.7Ca0.3MnO3.

Findings

Polaronic behavior holds above 1.4Tc

Deviations from polaronic activation energies below 1.4Tc

Presence of metallic ferromagnetic fluctuations affects Hall effect

Abstract

When heated through the magnetic transition at Tc, La0.7Ca0.3MnO3 changes from a band metal to a polaronic insulator. The Hall constant R_H, through its activated behavior and sign anomaly, provides key evidence for polaronic behavior. We use R_H and the Hall mobility to demonstrate the breakdown of the polaron phase. Above 1.4Tc, the polaron picture holds in detail, while below, the activation energies of both R_H and the mobility deviate strongly from their polaronic values. These changes reflect the presence of metallic, ferromagnetic fluctuations, in the volume of which the Hall effect develops additional contributions tied to quantal phases.