Crossover from itinerant to localized states in the thermoelectric oxide [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

TL;DR

This study investigates the transport properties of a layered cobaltite, revealing a crossover from itinerant to localized electronic states with temperature, which enhances thermoelectric performance.

Contribution

It provides detailed analysis of in-plane anisotropy and identifies a temperature-driven crossover in conduction mechanisms in Ca3Co4O9.

Findings

In-plane anisotropy observed in resistivity and thermopower.

Crossover from itinerant to localized conduction states with increasing temperature.

High-temperature resistivity becomes temperature-independent.

Abstract

The layered cobaltite [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], often expressed as the approximate formula Ca$_3$Co$_4$O$_9$, is a promising candidate for efficient oxide thermoelectrics but an origin of its unusual thermoelectric transport is still in debate. Here we investigate \textit{in-plane} anisotropy of the transport properties in a broad temperature range to examine the detailed conduction mechanism. The in-plane anisotropy between $a$ and $b$ axes is clearly observed both in the resistivity and the thermopower, which is qualitatively understood with a simple band structure of the triangular lattice of Co ions derived from the angle-resolved photoemission spectroscopy experiments. On the other hand, at high temperatures, the anisotropy becomes smaller and the resistivity shows a temperature-independent behavior, both of which indicate a hopping conduction of localized carriers. Thus the present observations reveal a crossover from low-temperature itinerant to high-temperature localized states, signifying both characters for the enhanced thermopower.