Spin entropy as the likely source of enhanced thermopower in $\rm\bf Na_xCo_2O_4

TL;DR

This paper investigates how spin entropy contributes to the enhanced thermopower in Na_xCo_2O_4, providing evidence that strong electron correlations suppress spin entropy and boost thermoelectric efficiency.

Contribution

It presents experimental evidence linking spin entropy suppression to strong electron correlations in Na_xCo_2O_4, advancing understanding of thermopower enhancement mechanisms.

Findings

Spin entropy plays a significant role in thermopower.

Strong electron correlations suppress spin entropy.

Evidence supports the correlation between spin entropy suppression and thermoelectric performance.

Abstract

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This Peltier effect underlies all thermoelectric refrigerators. The upsurge in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical interest in how electron-electron interaction may enhance the thermopower $Q$ of materials such as the transition-metal oxides \cite{Mahan,Beni,Kotliar,Chaikin}. An important factor in this enhancement is the electronic spin entropy, which is predicted \cite{Chaikin,Kwak,KwakChaikin} to dominate the entropy current. Here we report evidence for such suppression in the layered oxide $\rm Na_xCo_2O_4$, and present evidence that it is a strong-correlation effect.