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

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 re gion the thermopower is a square-root function of temperature S proportiona l to root T, and at low temperatures T < 25 K the hopping contribution to t he Seebeck coefficient dominates. A temperature increase induces a transiti on 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 ac count the higher-order derivatives of the density of states. It is establis hed 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 contributio n and an anomalous contribution presumably due to phonon drag. A model is p roposed which gives a quantitative description of the temperature dependenc e of the hopping thermopower on the basis of a single setup parameters char acterizing the density of localized states. (C) 1998 American Institute of Physics. [S0021-3640(98)00423- X].