Large phonon-drag thermopower polarity reversal in Ba-doped KTaO3

TL;DR

This paper demonstrates a polarity reversal in phonon-drag thermopower in Ba-doped KTaO3 thin films caused by electron-phonon Umklapp scattering, revealing new insights into thermoelectric behavior in oxides.

Contribution

It reports the first observation of thermopower polarity reversal in Ba-doped KTaO3 due to electron-phonon Umklapp scattering, advancing understanding of thermoelectric phenomena in oxides.

Findings

Polarity reversal occurs around 80 K in heavily doped samples.

Lightly doped samples show only negative thermopower down to 2 K.

Electron-phonon Umklapp scattering satisfies the reversal condition.

Abstract

This study reports the observation of phonon-drag thermopower polarity reversal in Ba-doped KTaO3 thin films, mediated by electron-phonon Umklapp scattering. Epitaxial films with distinct carrier concentrations (3.7 x 10^20 cm^-3 and 4.9 x 10^19 cm^-3) were grown via molecular-beam epitaxy. In heavily doped samples, where the Fermi surface spans 80% of the Brillouin zone, the Umklapp condition is satisfied, reversing electron momentum. This manifests as a sign-reversal in the thermopower around 80 K upon cooling despite the sample having only n-type carriers. On the other hand, the lightly doped sample (4.9 x 10^19 cm^-3) exhibits only a negative thermopower down to 2 K. These results advance the understanding of Umklapp electron-phonon drag in oxides and highlight KTaO3's potential for engineering unconventional thermoelectric materials.