Electrical transport in the ferromagnetic state of manganites: Small-polaron metallic conduction at low temperatures

TL;DR

This study demonstrates that small-polaron coherent motion, involving a soft optical phonon mode, governs low-temperature metallic conduction in the ferromagnetic state of manganite thin films and polycrystals.

Contribution

It provides experimental evidence linking small-polaron metallic conduction to a soft optical phonon mode in manganites, supported by resistivity and specific heat measurements.

Findings

Resistivity fits a small-polaron model below 100 K

Specific heat indicates existence of a soft optical phonon mode

Evidence for small-polaron metallic conduction in manganites

Abstract

We report measurements of the resistivity in the ferromagnetic state of epitaxial thin films of La_{1-x}Ca_{x}MnO_{3} and the low temperature specific heat of a polycrystalline La_{0.8}Ca_{0.2}MnO_{3}. The resistivity below 100 K can be well fitted by \rho - \rho_{o} = E \omega_{s}/sinh^{2}(\hbar \omega_{s}/2k_{B}T) with \hbar \omega_{s}/k_{B} \simeq 100 K and E being a constant. Such behavior is consistent with small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode. The specific heat data also suggest the existence of such a phonon mode. The present results thus provide evidence for small-polaron metallic conduction in the ferromagnetic state of manganites.