Hopping conduction in FeSi. I. The Hall, Seebeck, and Nernst effects due to hopping conduction in the top and bottom impurity Hubbard bands

FeSi is known as a narrow-gap semiconductor showing peculiar temperature dependence of transport properties, which evoked debate for over 50 years. In this study, it is shown that the peculiar temperature dependence of the electrical conductivity σ, the Hall coefficient RH, the Hall mobility μH, the Seebeck coefficient S, and the Nernst coefficient Q of FeSi can be well explained in a model that includes the conduction and the valence band with parabolic dispersions together with the top and bottom impurity Hubbard bands. In particular, the coincidence of the hump of σ(T), the maximum of S(T), the minimum of μH(T), and the maximum of Q(T) can be attributed to the contribution from hopping conduction in the top impurity Hubbard band.