Scaling behavior in thermoelectric misfit cobalt oxides

TL;DR

This study explores the thermoelectric and thermodynamic properties of a misfit cobalt oxide, revealing a universal scaling behavior of thermopower linked to spin entropy and electronic correlations, with implications for thermoelectric efficiency.

Contribution

It demonstrates a universal low-temperature thermopower scaling with electronic specific heat, highlighting the role of spin entropy in misfit cobalt oxides.

Findings

Large negative magnetothermopower scales with magnetic field and temperature.

Enhanced electronic specific heat indicates strong electronic correlations.

Universal behavior of thermopower slope as a function of electronic specific heat.

Abstract

We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$. A large negative magnetothermopower is found to scale with both magnetic field and temperature revealing a significant spin entropy contribution to thermoelectric properties giving rise to a constant S$_0\approx$ 60 $\mu$V K$^{-1}$ equal to the high temperature asymptotic value of the spin 1/2 entropy. Low temperature specific heat measurements allow us to determine an enhanced electronic part with $\gamma\approx$ 50 mJ (mol K$^{2}$)$^{-1}$ attesting of strong correlations. Thereby, a critical comparison between [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$, other cobaltites as well as other materials reveals a universal behavior of the thermopower low temperature slope as a function of $\gamma$ testifying thus a purely electronic origin. This potentially generic scaling behavior suggests here that the high room temperature value of the thermopower in misfit cobalt oxides results from the addition of a spin entropy contribution to an enlarged electronic one.