  Temperature gradient in a ferromagnetic conductor may generate a spontaneous
transverse voltage drop in the direction perpendicular to both magnetization
and heat current. This anomalous Nernst effect (ANE) has been considered to be
proportional to the magnetization, and thus observed only in ferromagnets,
while recent theories indicate that ANE provides a measure of the Berry
curvature at the Fermi energy $E_{\rm F}$. Here we report the observation of a
large ANE at zero field in the chiral antiferromagnet Mn$_3$Sn. Despite a very
small magnetization $\sim 0.002$ $\mu_{\rm B}/$Mn, the transverse Seebeck
coefficient at zero field is $\sim 0.35~\mu$V/K at room temperature and reaches
$\sim 0.6~\mu$V/K at 200 K, comparable with the maximum value known for a
ferromagnetic metal. Our first-principles calculation reveals that the large
ANE comes from a significantly enhanced Berry curvature associated with the
Weyl points nearby $E_{\rm F}$. The ANE is geometrically convenient for the
thermoelectric power generation, as it enables a lateral configuration of the
modules to efficiently cover the heat source. Our observation of the large ANE
in an antiferromagnet paves a way to develop a new class of thermoelectric
material using topological magnets to fabricate an efficient, densely
integrated thermopile.
