Band Inversion-Driven High Valley Degeneracy

Michael Toriyama, Northwestern University

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Thermoelectric materials can convert thermal energy to electrical energy and vice versa, enabling low-carbon alternatives for cooling and waste heat recovery. Yet, the low power conversion efficiencies of existing thermoelectric materials remain a challenge. One method to improve the thermoelectric performance of a material is by tuning the valley degeneracy, which we can control by modifying the degree to which the bands are inverted. We generalize this concept of "band inversion-driven high valley degeneracy" and derive simple rules for when inverted-band materials exhibit high thermoelectric performance. Using a combination of k.p perturbation theory and Density Functional Theory calculations, we show that electronic bands must generally be inverted to a critical degree for a material to possess high valley degeneracy. We apply this rule to discover potentially high-performing thermoelectric materials within the ABX chemical space of materials. We find that NaCaBi (space group: P63/mmc) is a promising candidate with a degeneracy of 6 for both the conduction and valence bands. Through detailed Boltzmann transport theory-based calculations, we find that the material can reach zT between 0.4 and 0.8 at 300 K. Band inversion is therefore a rational descriptor for identifying high-performing thermoelectric materials.

1M.Y. Toriyama, et al., “Tuning valley degeneracy with band inversion,” J. Mater. Chem. A 10, 1588 (2022).

Abstract Author(s): Michael Y. Toriyama, G. Jeffrey Snyder