Effects of electronic correlations and disorder on the thermopower of NaxCoO2

TL;DR

This study investigates how local Coulomb interactions and disorder influence the thermopower of NaxCoO2, revealing that their combined effects significantly enhance thermoelectric performance by creating a particle-hole imbalance.

Contribution

The paper demonstrates that the interplay of Coulomb interaction and disorder substantially increases thermopower, highlighting the importance of combined effects in thermoelectric materials.

Findings

Disorder alone increases resistivity and reduces thermopower.

Coulomb interaction alone causes minor changes.

Combined effects lead to a large positive thermopower.

Abstract

For the thermoelectric properties of NaxCoO2, we analyze the effect of local Coulomb interaction and (disordered) potential differences for Co-sites with adjacent Na-ion or vacancy. The disorder potential alone increases the resistivity and reduces the thermopower, while the Coulomb interaction alone leads only to minor changes compared to the one-particle picture of the local density approximation. Only combined, these two terms give rise to a substantial increase of the thermopower: the number of (quasi-)electrons around the Fermi level is much more suppressed than that of the (quasi-)holes. Hence, there is a particle-hole imbalance acting in the same direction as a similar imbalance for the group velocities. Together, this interplay results in a large positive thermopower. Introducing a thermoelectric spectral density, we located the energies and momenta regions most relevant for the thermopower and changes thereof.