Systematics in the thermopower of electron-doped layered manganites

We report on a comparative study of the transport properties of lightly La-doped layered CaO (CaMnO3)(m) (m=1, 2, and infinity) and similar Mn3+/Mn4+ ratios. In contrast to the large and rather nonsystematic variations of the resistivity rho(T) for samples with the same doping and various in, the absolute thermopower (S) exhibits remarkable systematics. In the paramagnetic (PM) state, S varies linearly with temperature and its extrapolation to T=0 has a finite intercept. The intercept and the slope of these lines vary systematically with in. In particular, within the experimental error, the inverse slopes of the straight lines increase linearly with 1/m, which in its turn varies Linearly with the average number of Mn-Mn near neighbors. The implications of this simple systematics are discussed using a model developed by Cutler and Mott [Phys. Rev. 181, 1336 (1968)] for transport by activated hopping of carriers obeying nondegenerate statistics. This model (or a possible extension into the semidegenerate regime) leads also to a simple, qualitative interpretation of S(T) of these materials on crossing the Neel temperature (T-N) from the PM state and below T-N.