  p x n-type materials refer to materials with a p-type Seebeck coefficient in
one direction and a n-type coefficient in the orthogonal direction. This type
of materials allows for a transverse thermoelectric response, which is highly
desirable for energy applications. Here, we report the observation of p x
n-type behavior in TaIrTe4, a type-II Weyl semimetal, with an in-plane
thermopower anisotropy S$_{xx}$-S$_{yy}$ reaches a maximum value 40${\mu}$V/K
at 200K. Intriguingly. we found that such a p x n-type behavior is absent in
the similar compound NbIrTe4. The presence and absence of p x n-type behavior
in these two materials are consistent with density functional theory
calculations, which further predict that the thermopower anisotropy in both
compounds can be enhanced up to 130${\mu}$V/K by electron doping. Such a strong
thermopower anisotropy originates from the presence of both p-type and n-type
carriers, each with high mobility in one direction. These results suggest that
although type-II Weyl semimetal phase does not guarantee the existence of p x
n-type behavior, its unique band structure provides the ingredient to engineer
and optimize this phenomenon.
