Origin of anomalous temperature dependence of the Nernst effect in narrow-gap semiconductors

Based on the Boltzmann transport theory, we study the origin of the anomalous temperature dependence of the Nernst coefficient (nu) due to the phonon-drag mechanism. For narrow-gap semiconductors, we find that there are two characteristic temperatures at which a noticeable peak structure appears in nu. Contrarily, the Seebeck coefficient (S) always has only one peak. While the breakdown of the Sondheimer cancellation due to the momentum dependence of the electron relaxation time is essential for the peak in nu at low T, the contribution of the valence band to the phonon-drag current is essential for the peak at higher T. By considering this mechanism, we successfully reproduce nu and S of FeSb2, for which a gigantic phonon-drag effect is observed experimentally.