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arXiv · 2508.00313

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

Mohamed Nawwar·Samuel Poage·Tobias Schwaigert·Maria N. Gastiasoro·Salva Salmani-Rezaie·Darrell G. Schlom·Kaveh Ahadi·Brandi L. Wooten·Joseph P. Heremans
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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.

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Mohamed Nawwar, Samuel Poage, Tobias Schwaigert, Maria N. Gastiasoro, Salva Salmani-Rezaie, Darrell G. Schlom, Kaveh Ahadi, Brandi L. Wooten, Joseph P. Heremans. 2025-08-01. Large phonon-drag thermopower polarity reversal in Ba-doped KTaO3. https://arxiv.org/abs/2508.00313

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