  Weyl semimetals expand research on topologically protected transport by
adding bulk Berry monopoles with linearly dispersing electronic states and
topologically robust, gapless surface Fermi arcs terminating on bulk node
projections. Here, we show how the Nernst effect, combining entropy with charge
transport, gives a unique signature for the presence of Dirac bands. The Nernst
thermopower of NbP (maximum of 800 microV K-1 at 9 T, 109 K) exceeds its
conventional thermopower by a hundredfold and is significantly larger than the
thermopower of traditional thermoelectric materials. The Nernst effect has a
pronounced maximum near T_M=90 +/- 20 K=mu_0/kB (mu_0 is chemical potential at
T=0 K). A self-consistent theory without adjustable parameters shows that this
results from electrochemical potential pinning to the Weyl point energy at
T>=TM, driven by charge neutrality and Dirac band symmetry. Temperature and
field dependences of the Nernst effect, an even function of the charge
polarity, result from the intrinsically bipolar nature of the Weyl fermions.
Through this study, we offer an understanding of the temperature dependence of
the position of the electrochemical potential vis-a-vis the Weyl point, and we
show a direct connection between topology and the Nernst effect, a potentially
robust experimental tool for investigating topological states and the chiral
anomaly.
