A New Classification for Axis Dependent Conduction Polarity*

Axis dependent thermopower polarity has been observed in metals. It is usually explained as a multi-band effect with mobility variation of two carrier. Yet the signature of a multi-band system is a large Nernst coefficient, because the Nernst coefficient in multiband systems has a term that expresses the difference between the thermopowers of the two bands. We observe the anisotropic thermopower in single-crystal NaSn₂As₂, with very small Nernst coefficient, ruling out the two-band explanation. Here we propose a new classification, which is a single-band induced anisotropy, due to a particular shape of the Fermi surface. This model successfully explains the anisotropic behavior of NaSn₂As₂. Re₄Si₇ is revisited, which has a positive thermopower along a, b direction and negative thermopower along c-direction. Four coefficients (thermopower, resistivity, Hall coefficient and Nernst coefficient) along 3-orientations are measured from 100 K to 400 K. The Hall coefficients along the three axes are showing the same sign and similar concentration, while the Nernst coefficients are negligibly small compared to the thermopower. This again suggests that the thermopower anisotropy of Re₄Si₇ should be a single-band effect.