Large Electron-Phonon Drag and Asymmetry in the Thermopower of the Topological Semimetal θ-Phase Tantalum Nitride

Using a first principles theoretical approach [1], we study the coupled electron-phonon transport and intrinsic thermopower of the topological semimetal, θ-phase tantalum nitride ([θ-TaN). We find that large electron-phonon drag enhancements of the thermopower are possible even around room temperature, with magnitudes that can exceed 500 μV K^-1. However, the large thermopower only occurs just above the chemical potential of θ-TaN, while below it, the thermopower becomes small and shows no drag enhancement. This striking thermopower asymmetry is the result of of an unusual interplay between electron and phonon properties in θ-TaN and the rapid variation of the former about the chemical potential. Specifically, the large drag enhancements of the thermopower are a consequence of contributions involving high frequency acoustic phonons, which are negligible in most materials. The small thermopower and apparent disappearance of the drag stems from two features. First, the steep linear electronic dispersions extending below one of the topological nodal points contribute to an extraordinarily high electrical conductivity that suppresses the thermopower. Second, a hole current generates through electron-phonon interactions oppositely directed phonon heat currents from low and high frequency phonons of nearly equal magnitude, which quenches the drag contributions.

This talk is in memory of Natalio Mingo. The last part of this presentation will be devoted to reflections on some of Natalio's many contributions to the thermal sciences that I was fortunate to participate in.

[1] N. H. Protik, C. Li, M. Pruneda, D. Broido, P. Ordejón, npj Comput. Mater. 8, 28 (2022).