Prominent electron-hole asymmetry in thermoelectric transport of LaCoO$_3$

TL;DR

This study reveals a strong electron-hole asymmetry in thermoelectric transport in LaCoO3, driven by spin-state blockade, with electron doping leading to insulating behavior and positive thermopower unlike hole doping.

Contribution

It demonstrates the impact of spin-state blockade on thermoelectric properties, highlighting the asymmetry between electron and hole doping in LaCoO3.

Findings

Electron-doped LaCoO3 remains insulating due to spin-state blockade.

Thermopower remains positive and large in electron-doped samples, unlike hole-doped ones.

Bipolar conduction involving mobile holes and immobile electrons causes the observed asymmetry.

Abstract

We have measured the electrical resistivity and the thermopower of the electron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to the hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$ = Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically suppressed. We find that, despite the electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly distinct from the hole-doped case where it comes close to zero with doping. This prominent electron-hole asymmetry seen in the thermopower originates from a bipolar conduction which consists of a slight amount of mobile holes and the main immobile electrons, demonstrating an impact of spin-state blockade on thermoelectric transport.