"Pudding mold" band drives large thermopower in Na$_x$CoO$_2$

TL;DR

This study identifies the unique 'pudding mold' shape of the $a_{1g}$ band in Na$_x$CoO$_2$ as key to its large thermopower and conductivity, offering new insights for thermoelectric material design.

Contribution

It reveals that the peculiar band shape, not just density of states or effective mass, is crucial for large thermopower in Na$_x$CoO$_2$, providing a new design principle.

Findings

The 'pudding mold' band shape enhances thermopower and conductivity.

Band shape, not just DOS or effective mass, influences thermoelectric properties.

Provides a new principle for designing thermoelectric materials.

Abstract

In the present study, we pin down the origin of the coexistence of the large thermopower and the large conductivity in Na$_x$CoO$_2$. It is revealed that not just the density of states (DOS), the effective mass, nor the band width, but the peculiar {\it shape} of the $a_{1g}$ band referred to as the "pudding mold" type, which consists of a dispersive portion and a somewhat flat portion, is playing an important role in this phenomenon. The present study provides a new guiding principle for designing good thermoelectric materials.