Large transverse thermoelectric figure of merit in a topological Dirac semimetal

The Seebeck effect encounters a few fundamental constraints hindering its thermoelectric (TE) conversion efficiency. Most notably, there are the charge compensation of electrons and holes that diminishes this effect, and the Wiedemann-Franz (WF) law that makes independent optimization of the corresponding electrical and thermal conductivities impossible. Here, we demonstrate that in the topological Dirac semimetal Cd3As2 the Nernst effect, i.e., the transverse counterpart of the Seebeck effect, can generate a large TE figure of merit zNT. At room temperature, zNT ≈ 0.5 in a small field of 2 T and it significantly surmounts its longitudinal counterpart for any field. A large Nernst effect is genetically expected in topological semimetals, benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities. In this case, heat and charge transport are orthogonal, i.e., not intertwined by the WF law anymore. More importantly, further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport, but also the anomalous Nernst effect arising from a pronounced Berry curvature. A combination of the topologically trivial and nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.