  Thermoelectric (TE) conversion in conducting materials is of eminent
importance for providing renewable energy and solid-state cooling. Although
traditionally, the Seebeck effect plays a key role for the TE figure of merit
zST, it encounters fundamental constraints hindering its conversion efficiency.
Most notably, there are the charge compensation of electrons and holes that
diminishes this effect, and the intertwinement of the corresponding electrical
and thermal conductivities through the Wiedemann-Franz (WF) law which makes
their independent optimization in zST impossible. Here, we demonstrate that in
the 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; it significantly surmounts its
longitudinal counterpart zST for any field and further increases upon warming.
A large Nernst effect is generically 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 former topologically trivial and the latter nontrivial
advantages promises to open a new avenue towards high-efficient transverse
thermoelectricity.
