Thermopower in the hopping conductivity region: Transition from Mott’s to Zvyagin’s formula

Abstract

Zvyagin’s theoretical calculation for the thermopower due to electron hops is confirmed experimentally. It is shown for a-GaSb that in the Mott-law region the thermopower is a square-root function of temperature $$\sqrt T $$ , and at low temperatures T<25 K the hopping contribution to the Seebeck coefficient dominates. A temperature increase induces a transition to conductivity due to hops between nearest centers. The thermopower in this regime is described by the Mott formula modified so as to take into account the higher-order derivatives of the density of states. It is established that in a-GaSb the thermopower at temperatures 4.2 K <T<300 K can be represented as a superposition of two contributions: a hopping contribution and an anomalous contribution presumably due to phonon drag. A model is proposed which gives a quantitative description of the temperature dependence of the hopping thermopower on the basis of a single setup parameters characterizing the density of localized states.